CHEESE IN THE HOUSEHOLD 362-381
Food value of cheese, 334; Digestibility of cheese, 335;
Cheese flavor, 336; Relation to health, 337; Cheese
poisoning, 338; Proper place in the diet, 339; Care of
cheese, 340; Food value and price, 341; Methods and
recipes for using cheese, 342.
THE BOOK OF CHEESE
CHAPTER I
_GENERAL STATEMENT ON CHEESE_
Cheese is a solid or semi-solid protein food product manufactured from
milk. Its solidity depends on the curdling or coagulation of part or all
of the protein and the expulsion of the watery part or whey. The
coagulum or curd so formed incloses part of the milk-serum (technically
whey) or watery portion of the milk, part of the salts, part or all of
the fat, and an aliquot part of the milk-sugar. The loss in manufacture
includes a small fraction of the protein and fat, the larger proportion
of the water, salts and milk-sugar.
+1. Nature of cheese.+--Milk of itself is an exceedingly perishable
product. Cheese preserves the most important nutrient parts of the milk
in condition for consumption over a much longer period. The duration of
this period and the ripening and other changes taking place depend very
closely on the composition of the freshly made cheese. There is an
intimate relation between the water, fat, protein and salt-content of
the newly made cheese and the ripening processes which produce the
particular flavors of the product when it is ready for the consumer.
This relation is essentially biological. A cheese containing 60 to 75
per cent of water, as in "cottage cheese" (the sour-milk cheese so
widely made in the homes), must be eaten or lost in a very few days.
Spoilage is very rapid. In contrast to this, the Italian Parmesan, with
30 to 32 per cent of water, requires two to three years for proper
ripening.
The cheeses made from soured skim-milk probably represent the most
ancient forms of cheese-making. Their origin is lost in antiquity. The
makers of Roquefort cheese cite passages from Pliny which they think
refer to an early form of that product. It is certain that cheese in
some form has been familiar to man throughout historic times. The
technical literature of cheese-making is, however, essentially recent.
The older literature may be cited to follow the historical changes in
details of practice.
+2. Cheese-making as an art+ has been developed to high stages of
perfection in widely separate localities. The best known varieties of
cheese bear the geographical names of the places of their origin. The
practices of making and handling such cheeses have been developed in
intimate relation to climate, local conditions and the habits of the
people. So close has been this adjustment in some cases, that the
removal of expert makers of such cheeses to new regions has resulted in
total failure to transplant the industry.
+3. Cheese-making as a science+ has been a comparatively recent
development. It has been partly a natural outgrowth of the desire of
emigrant peoples to carry with them the arts of their ancestral home,
partly the desire to manufacture at home the good things met in foreign
travel. Its development has been largely coincident with the development
of the agricultural school and the science of dairy biology. Even now
we have but a limited knowledge of a few of the 500 or more varieties of
cheese named in the literature. It is desirable to bring together the
knowledge of underlying principles as far as they are known.
No technical description of a cheese-handling process can replace
experience. Descriptions of appearances and textures of curd in terms
definite enough to be understood by beginners have been found to be
impossible. It is possible, however, to lay down principles and
essentials of practice which are common to the industry and form the
foundation for intelligent work. Cheese-making will be a science only as
we depart from the mere repetition of a routine or rule-of-thumb
practice and understand the underlying principles.
+4. Problems in cheese-making.+--Any understanding of these problems
calls for a working knowledge of the very complex series of factors
involved. These include the chemical composition of the milk, the nature
of rennet and character of its action under the conditions met in
cheese-making, the nature of the micro-organisms in milk, and the
methods of controlling them, their relation to acidity and to the
ripening of the cheese. To these scientific demands must be added
acquaintance with the technique of the whole milk industry, from its
production and handling on the farm through the multiplicity of details
of factory installation and organization, to those intangible factors
concerned with the texture, body, odor and taste of the varied products
made from it. Some of these factors can be adequately described; others
have thus far been handed on from worker to worker but have baffled
every effort at standardization or definition.
+5. History.+--The recorded history of the common varieties of cheese is
only fragmentary. Practices at one time merely local in origin followed
the lines of emigration. Records of processes of manufacture were not
kept. The continuance of a particular practice depended on the skill and
memory of the emigrant, who called his cheese after the place of origin.
Other names of the same kind were applied by the makers for selling
purposes. The widely known names were thus almost all originally
geographical. Some of them, such as Gorgonzola, are used for cheeses not
now made at the places whose names they bear. Naturally, this method of
development has produced national groups of cheeses which have many
common characteristics but differ in detail. The English cheeses form a
typical group of this kind.
Emigration to America carried English practices across the Atlantic. The
story of cheese-making in America has been so closely linked with the
development of the American Cheddar process that the historical aspects
of the industry in this country are considered under that head in
Chapter VIII.
CHAPTER II
_THE MILK IN ITS RELATION TO CHEESE_
The opaque whitish liquid, secreted by the mammary glands of female
mammals for the nourishment of their young, is known as milk. The milk
of the cow is the kind commonly used for cheese-making in America.
+6. Factors affecting the quality.+--The process of cheese-making begins
with drawing the milk from the udder. The care and treatment the milk
receives, while being drawn, and its subsequent handling, have a decided
influence on its qualities. The process of cheese-making is varied
according to the qualities of the milk. There are five factors that
influence the quality of the milk for cheese-making: (1) its chemical
composition; (2) the flavor of feed eaten by the cow; (3) the absorption
of flavors and odors from the atmosphere; (4) the health of the cow; (5)
the bacteria present. The first factor is dependent on the breed and
individuality of the cow. The other four factors are almost entirely
within the control of man. Of these factors, number five is of the most
importance, and is the one most frequently neglected.
+7. Chemical composition.+--The high, low and average composition of
milk is approximately as follows:
TABLE I
COMPOSITION OF MILK
--------+-------+------+--------+-------+---------+-----
| WATER | FAT | CASEIN | SUGAR | ALBUMIN | ASH
| PER | PER | PER | PER | PER | PER
| CENT | CENT | CENT | CENT | CENT | CENT
--------+-------+------+--------+-------+---------+-----
High | 88.90 | 5.50 | 3.00 | 5.00 | .72 | .73
Low | 85.05 | 3.00 | 2.10 | 4.60 | .70 | .70
Average | 87.47 | 3.80 | 2.50 | 4.80 | .71 | .72
--------+-------+------+--------+-------+---------+-----
+8. Factors causing variation in composition.+--The composition of cow's
milk varies according to several factors. The composition of the milk of
different breeds differs to such a degree that whole series of factories
are found with lower or higher figures than these averages on account of
dominant presence of particular kinds of cattle.
The following table shows the usual effect of breed on fat and total
solids of milk:
TABLE II
THE USUAL EFFECT OF BREED OF COWS ON FAT AND TOTAL
SOLIDS OF MILK
------------------+------------------------
| AVERAGES
BREED OF COWS +----------+-------------
| Fat | Total Solids
| Per Cent | Per Cent
------------------+----------+-------------
Jersey | 5.62 | 14.74
Guernsey | 5.34 | 14.70
Shorthorn | 4.17 | 13.41
Ayrshire | 3.61 | 12.72
Holstein-Friesian | 3.30 | 11.89
------------------+----------+-------------
The figures[1] in Tables I and II are compiled and averaged from a large
number of analyses made at different agricultural experiment stations.
This variation not only affects the fat, but all constituents of the
milk. While there is a difference in the composition of the milk from
cows of different breeds, there is almost as wide variation in the
composition of the milk from single cows[2] of the same breed. With the
same cow the stage of lactation causes a wide variation in the
composition of the milk.[3] As the period of lactation advances, the
milk increases in percentage of fat and other solids.
+9. Milk constituents.+--From the standpoint of the cheese-maker, the
significant constituents of milk are water, fat, casein, milk-sugar,
albumin, ash and enzymes. These will be discussed separately.
+10. Water.+--The retention of the solids and the elimination of the
water are among the chief considerations in cheese-making. Water forms
84 to 89 per cent of milk. Cheese-making calls for the reduction of this
percentage to that typical of the particular variety of cheese desired
with the least possible loss of milk solids. This final percentage
varies from 30 to 70 per cent with the variety of cheese. The water has
two uses in the cheese: (1) It imparts smoothness and mellowness to the
body of the cheese; (2) it furnishes suitable conditions for the action
of the ripening agents. To some extent the water may supplement or even
replace fat in its effect on the texture of the cheese. If the cheese is
properly made, the water present is in such combination as to give no
suggestion of a wet or "leaky" product.
+11. Fat.+--Fat is present in the milk in the form of suspended small
transparent globules (as an emulsion). These globules vary in size with
the breed and individuality of the cow and in color from a very light
yellow to a deep yellow shade as sought in butter. Milk with small fat
globules is preferred for cheese-making, because these are not so easily
lost in the process. Milk-fat is made up of several different compounds
called glycerids,[4] which are formed by the union of an organic acid
with glycerine as a base.
Fat is important in cheese-making for two reasons: (1) Its influence on
the yield of cheese; (2) its effect on the quality of the cheese. Many
of the details of cheese-making processes have been developed to prevent
the loss of fat in manufacture. The yield of cheese is almost directly
in proportion to the amount of fat in the milk; nevertheless, because
the solids not fat do not increase exactly in proportion to the fat, the
cheese yield is not exactly in proportion to the fat. The fat, however,
is a good index of the cheese-producing power of the milk.
+12. Casein.+--Cheese-making is possible because of the peculiar
properties of casein. This is the fundamental substance of cheese-making
because it has the capacity to coagulate or curdle under the action of
acid and rennet enzymes. Casein is an extremely complex organic
compound.[5] Authorities disagree regarding its exact composition, but
it contains varying amounts of carbon, oxygen, nitrogen, hydrogen,
phosphorus and sulfur, and it usually is combined with some form
of lime or calcium phosphate. It belongs to the general class of
nitrogen-containing compounds called proteins. It is present in milk in
the form of extremely minute gelatinous particles in suspension. Casein
is insoluble in water and dilute acids. The acids, when added, cause a
heavy, white, more or less flocculent precipitate. Rennet (Chapter III)
causes the casein to coagulate (curdle), forming a jelly-like mass
called curd, which is the basis of manufacture in most types of cheese.
In the formation of this coagulum (curd), the fat is imprisoned and
held. The casein compounds in the curd hold the moisture and give
firmness and solidity of body to the cheese. Casein contains the protein
materials in which important ripening changes take place. These changes
render the casein more soluble, and are thought to be the source of
certain characteristic cheese flavors.
+13. Milk-sugar.+--Milk-sugar (lactose) is present in solution in the
watery part of the milk. It forms on the average about 5 per cent of
cow's milk. Since it is in solution, cheese retains the aliquot part of
the total represented by the water-content of the cheese, plus any part
of the sugar which has entered into combination with the milk solids
during the souring process. The larger part of the lactose passes off
with the whey. Lactose[6] is attacked by the lactic-acid bacteria and by
them is changed to lactic acid. Cheeses in which this souring process
goes on quickly, soon contain a large enough percentage of acid to check
the rotting of the cheese by decay organisms. Without this souring, most
varieties of cheese will begin to spoil quickly. For each variety there
is a proper balance between the souring, which interrupts the growth of
many kinds of putrefactive bacteria, and the development of the forms
which are essential to proper ripening.
+14. Albumin.+--This is a form of protein which is in solution in the
milk. Albumin forms about 0.7 per cent of cow's milk. It is not
coagulated by rennet. Most rennet cheeses, therefore, retain only that
portion of the total albumin held in solution in the water retained, as
in the case of milk-sugar. Albumin is coagulated by heat, forming a film
or membrane upon the surface. There are certain kinds of cheese, such
as Ricotte, made by the recovery of albumin by heating.
+15. Ash.+--The ash or mineral constituents make up about 0.7 per cent
of cow's milk. This total includes very small amounts of a great many
substances. The exact form of some of the substances is still unknown.
Of these salts, the calcium or lime and phosphorus salts are most
important in cheese-making. They are partially or completely
precipitated by pasteurization. After such precipitation rennet fails to
act[7] or acts very slowly; hence pasteurized milk cannot be used for
making rennet cheese unless the lost salts are replaced, or the
condition of the casein is changed by the addition of some substance,
before curdling is attempted.
+16. Enzymes.+--Milk also contains enzymes. These are chemical ferments
secreted by the udder. They have the power to produce changes in organic
compounds without themselves undergoing any change. Minute amounts of
several enzymes are found in milk as follows: Diastase, galactase,
lipase, catalase, peroxidase and reductase. Just what part they play in
cheese-making is not definitely known.
+17. The flavor of feeds eaten by the cow.+--Undesirable flavors in the
milk are due many times to the use of feed with very pronounced flavors.
The most common of these feeds are onions, garlic, turnips, cabbage,
decayed ensilage, various weeds and the like. These undesirable flavors
reach the milk because the substances are volatile and are able to pass
through the tissues of the animal. While feed containing these flavors
is being digested, these volatile substances are not only present in
the milk, but in all the tissues of the animal. By the time the process
of digestion is completed, the volatile flavors have largely passed
away. Therefore, if the times of milking and feeding are properly
regulated, a dairy-man may feed considerable quantities of
strong-flavored products, such as turnip, cabbage and others, without
any appreciable effect on the flavor of the milk. To accomplish this
successfully, the cows should be fed immediately before or immediately
after milking, preferably after milking. This allows time for the
digestive process to take place and for the volatile substances to
disappear. If, however, milking is performed three or four hours after
feeding, these volatile substances are present in the milk and flavor
it.[8]
In the case of those plants which grow wild in the pasture, and to which
the cows have continued access, it is more difficult to prevent bad
flavor in the milk. The cows may be allowed to graze for a short time
only, and that immediately after milking, without affecting the flavor
of the milk. This will make it necessary to supplement the pasture with
dry feed, or to have another pasture where these undesirable plants do
not grow.
Undesirable flavors are usually noticeable in the milk when the cows are
turned out to pasture for the first time in the spring; and when they
are pastured on rank fall feed, such as second growth clover.
+18. Absorption of odors.+--Milk, especially when warm, possesses a
remarkable ability to absorb and retain odors from the surrounding
atmosphere.[9] For this reason, the milk should be handled only in
places free from such odor. Some of the common sources of these
undesirable odors are bad-smelling stables, strong-smelling feeds in the
stable, dirty cows, aërating milk near hog-pens, barn-yards and swill
barrels. The only way to prevent these undesirable flavors and odors is
not to expose the milk to them. The safest policy is to remove the
source of the odor.
+19. Effect of condition of the cow.+--Any factor which affects the cow
is reflected in the composition and physiological character of the milk.
(1) Colostrum. Milk secreted just before or just after parturition is
different in physical properties and chemical composition from that
secreted at any other time during the lactation period. This milk is
known as colostrum. It is considered unfit for human food, either as
milk or in products manufactured from the milk. Most states[10] consider
colostrum adulterated milk, and prohibit the sale of the product for
fifteen days preceding and for five days after parturition. (2) Disease.
When disease is detected in the cow, the milk should at once be
discarded as human food. Some diseases are common both to the cow and to
man, such as tuberculosis, foot-and-mouth disease. If such diseases are
present in the cow, the milk may act as a carrier to man. Digestive
disorders of any sort in the cow are frequently accompanied by
undesirable flavors in the milk. These are not thought to be due to the
feed, but to the abnormal condition of the cow. When the normal
condition is restored, these undesirable flavors disappear.
+20. Bacteria in the milk.+--Bacteria are microscopic unicellular
plants, without chlorophyll. Besides bacteria, there are other forms of
the lower orders of plants found in milk, such as yeasts and molds.
While the bacteria are normally the more important, frequently yeasts
and molds produce significant changes in milk and other dairy products.
Bacteria are very widely distributed throughout nature. They are so
small that they may easily float in the air or on particles of dust.
Many groups of bacteria are so resistant to adverse conditions of growth
that they may be present in a dormant or spore stage, and, therefore,
not be easily recognized; when suitable environments for growth are
again produced, development begins at once. They are found in all
surface water, in the earth and upon all organic matter. There are a
great many different groups of bacteria; some are beneficial, and some
are harmful. As they are so small, it is difficult to differentiate
between the beneficial and harmful kinds, except by the results
produced, or by a careful study in an especially equipped laboratory.
The bacteria multiply very rapidly. This is brought about by fission;
that is, the cell-walls are drawn in at one place around the cell, and
when the walls unite at the center, the cell is divided. There are then
two bacteria. In some cases, division takes place in twenty to thirty
minutes. Like other plants, they are very sensitive to food supply, to
temperature and to moisture, as conditions of growth. Inasmuch as the
bacteria are plant cells, they must absorb their food from materials in
solution. They may live on solid substances, but the food elements must
be rendered soluble before they can be used. Most bacteria prefer a
neutral or slightly acid medium for growth, rather than an alkaline
reaction. Ordinary milk makes a very favorable medium for the growth of
bacteria, because it is an adequate and easily available food supply.
In milk, certain groups of bacteria are commonly present, but many
others which happen to get into it live and multiply rapidly. A
favorable temperature is very necessary for such organisms to multiply.
There is a range of temperature, more or less wide, at which each group
of bacteria grows and multiplies with the greatest rapidity. This range
varies with the different groups, but most of them find temperatures
between 75° F. and 95° F. the most favorable for growth. Excessive heat
kills the bacteria. Low temperatures stop growth, but kill few if any
bacteria. Temperatures of 50° F. and lower retard the growth of most
forms of bacteria found commonly in milk. Many forms will slowly
develop, however, below 50° and some growth will occur down to the
freezing point. Milk held at 50° F. or lower will remain in good
condition long enough to be handled without injury to quality until
received in the cheese factory. In the place of seeds, some groups of
bacteria form spores. The spores are exceedingly resistant to
unfavorable conditions of growth, such as heat, cold, drying, food
supply and even chemical agents. This property makes it difficult to
destroy such bacteria.
+21. Groups of bacteria in milk.+--Milk when first drawn usually shows
an amphoteric reaction; that is, it will give the acid and alkaline
reactions with litmus paper. Under normal conditions, milk soon begins
to undergo changes, due to the bacteria. Changes produced in this way
are called "fermentations"; the agents causing them, "ferments."
Normally the acid fermentation takes place first, and later other
fermentations or changes begin, which, after a time, so decompose the
milk that it will not be suitable for cheese-making or human
consumption.
The following grouping of the organisms in milk is based on their
effects on the milk itself[11]:
I. Acid-producing types.
II. Peptonizing types.
III. Inert types.
IV. Alkali-producing types.
V. Butyric fermenting types.
Each type of bacteria produces more or less specific changes in the
milk. As a general rule, the predominance of one of these types is an
aid in the interpretation of the quality of the product at the time of
analysis, such as the age, the temperature at which it has been held,
the conditions under which it was produced and, in some cases, the
general source of the contamination. The reaction due to certain
bacteria is utilized in the manufacture and handling of dairy products;
other groups have deleterious effects. (See Fig. 2.)
[Illustration: FIG. 2.--Effect of different fermentations of milk: _U_,
Curd pitted with gas holes; _G_ and _O_, gassy curds which float; _K_,
smooth, solid desirable curd.]
+22. Acid fermentation of milk.+--By far the most common and important
fermentation taking place in milk is due to the action of the lactic
acid-forming bacteria on the milk-sugar or lactose. The bacteria that
bring about this fermentation may be divided into several groups on the
basis of their morphology, proteolytic activity, gas production,
temperature adaptation and production of substances other than lactic
acid. The larger number of organisms producing lactic acid in milk also
produce other organic acids in greater or less abundance. Inasmuch as
lactic acid is the principal substance produced, they are called lactic
acid organisms. This group contains different kinds of organisms which
may be subdivided into small groups as follows:
(a) _Bacterium lactis-acidi_ group.
(b) _Bacterium colon-aërogenes_ group.
(c) Acid peptonizing group.
(d) _Bacillus bulgaricus_ group.
(e) Acid cocci or weak acid-producing group.
+23. Bacterium lactis-acidi group.+--There are many strains or varieties
in this group which are closely related in their activities. They are
universally present in milk and are commonly the greatest causal agent
in its souring. They are widely distributed in nature. At a temperature
of 65° F. to 95° F., these bacteria grow and multiply very rapidly; at
70° F. (approximately 20° C.) these forms usually outgrow all others.
The total amount of acid produced in milk by these organisms varies from
0.6 of one per cent to 1 per cent acid calculated as pure lactic acid.
These forms coagulate milk to a smooth curd of uniform consistency. In
addition to the lactic acid, there are produced traces of acetic,
succinic, formic and proprionic acids, traces of certain alcohols,
aldehydes and esters. Substances other than lactic acid are not produced
by organisms of this group to such an extent as to impart undesirable
flavors to the milk. The action of this group on the milk proteins is
very slight. They produce no visible sign of peptonization. The _B.
lactis-acidi_ group of organisms are essential to the production of the
initial acidity necessary in most types of cheese. The practical culture
and utilization of them for this purpose under factory conditions are
discussed in Chapter IV, entitled "Lactic Starters."
+24. Colon-aërogenes group.+--This group takes its name from a typical
species, _Bacterium coli communis_, which is a normal inhabitant of the
intestines of man and animals, and from _Bacterium coli aerogenes_,
which is similar in many respects to _B. coli communis_. The initial
presence of these bacteria in milk is indicative of fecal contamination
or unclean conditions of production. These organisms, however, grow and
develop in milk very rapidly at high temperatures of handling. The total
acidity produced by these forms is less than that by the _Bacterium
lactis-acidi_ group. Of the acid produced, less than 30 per cent is
lactic acid; the other acids are formic, acetic, proprionic and
succinic. The large percentage of these acids, with comparatively large
amounts of certain alcohols, aldehydes and esters, invariably impart
undesirable flavors and odors to the milk. Members of this group
uniformly ferment the lactose with the production of the gases, carbon
dioxide and hydrogen. The milk is coagulated into a lumpy curd,
containing gas pockets.
+25. Acid peptonizing group.+--These are often associated with colon
organisms. The group includes those bacteria which coagulate milk with
an acid curd and subsequently partly digest it. They grow and multiply
rapidly at a temperature between 65° and 98° F. They impart undesirable
flavors and odors to the milk, which appear to be due to the formation
of acids other than lactic acid, and to action on the milk proteins.
+26. Bacillus bulgaricus group.+--These organisms grow best at a
temperature of 105° to 115° F. They will develop at lower temperatures,
but not so rapidly. They survive heating to 135° F. without loss of
vigor, as occurs in Swiss cheese-making. They produce from 1 to 4 per
cent of acid in milk, which is practically all lactic acid. They do not
produce gas. They impart no undesirable flavors to the milk.
+27. Acid cocci or weak acid-producers.+--This group of organisms is not
very well defined. It consists mostly of coccus forms, commonly found
in the air and in the udder. Their presence in the milk may indicate
direct udder contamination. These are regarded as of little importance,
unless in very large number, and they have been only partially studied.
They produce little or no lactic acid, and small amounts of acetic,
proprionic, butyric and caproic acids. These forms rarely create enough
acid to coagulate milk.
+28. Peptonizing organisms.+--This group includes all bacteria which
have a peptonizing effect on the milk. It includes the acid peptonizing
organisms, although they are of primary importance in the acid type of
bacteria, because the acid-producing power is greater than the
peptonizing power. Some of the specific organisms in this class are
_Bacillus subtilis_, _Bacterium prodigiosus_ and _Bacterium
liquefaciens_. These are commonly found in soil water and in fecal
material. The presence of these organisms denotes contamination from
such sources.
+29. Inert types.+--As the name indicates, these are organisms not known
to have an appreciable effect on milk. The ordinary tests fail to
connect them with important processes; hence they appear to feed upon,
but not to affect the milk in any serious way. Milk ordinarily contains
more or less of these organisms, but no particular significance is
attached to their presence.
+30. Alkali-producing bacteria.+--This group of organisms has only
recently been studied in relation to its action on milk. Investigators
still disagree as to the usual percentage in the normal milk flora.
Their presence in milk has been considered to be relatively unimportant.
+31. Butyric fermenting types.+--Organisms causing butyric fermentation
may be present in the milk, but seldom become active, because they are
commonly anaërobic and so will not develop in milk kept under ordinary
conditions, and the rapid growth of the lactic acid-forming bacteria
prevents their growth. These organisms act on the milk-fat, decomposing
it. Butyric acid fermentations are more common in old butter and cheese.
In these, the fermentation causes a rancid flavor.
+32. Molds and yeasts.+--The cattle feed and the air of the barn always
contain considerable numbers of yeasts and mold spores. Yeasts have been
found by Hastings[12] to cause an objectionable fermentation in
Wisconsin cheese. No further study of this group as factors in
cheese-handling has been reported. Mold spores, especially those of the
blue or green molds (Penicillum sp.) and the black molds (Mucors), are
always abundant in milk. These spores are carried into all cheeses made
from unpasteurized milk, in numbers sufficient to cover the cheeses
with mold if they are permitted to grow. Pasteurization[13] kills
most of them. The border-line series commonly referred to as the
streptothrix-actinomyces group are also very abundant in all forage and
are carried in large numbers into all milk and its products.
+33. Bacterial contamination of milk.+--When drawn from the cow, milk is
seldom if ever sterile. Organisms usually work their way from the tip of
the teat into the udder and multiply there. The fore milk usually
contains more organisms than does that drawn later. Most of the
bacterial contamination of the milk is due to the handling after it is
drawn from the cow.
+34. Germicidal effect of milk.+--Authorities agree that when a
bacterial examination of the milk is made, hour by hour, beginning as
soon as it is drawn from the cow, there is no increase in the number of
organisms for a period of several hours at first, but an actual
reduction not infrequently takes place. This is called the
"germicidal"[14] property of milk. The lower the temperature of the
milk, the longer and less pronounced is the germicidal action; the
higher the temperature, the shorter and more pronounced is this action.
This is explained as either: (1) a period of selection within which
types of bacteria entering by accident and unadapted for growth die off;
or (2) an actual weak antiseptic power in the milk-serum itself; or (3)
the forming of clusters by the bacteria and so reducing the count.
In working on a small scale or on an experimental basis, this property
at times introduces a factor of difficulty or error which is not to be
lost sight of in the selection of the milk for such purposes.
+35. Sources and control of bacteria in milk.+--Most of the bacterial
infection of milk is due to lack of care in handling. Some of the common
sources[15] of contamination are: the air in the stable; the cow's body;
the milker; the utensils; the method of handling the milk after it is
drawn from the cow; unclean cheese factory conditions.
Since bacteria cause various kinds of fermentation, not only in the
milk but in the products manufactured from it, the question of their
control is of prime importance. There are two ways in which the
bacterial growth in milk used for cheese-making may be controlled: (1)
prevention of infection; (2) the retardation of their development when
present. The former is accomplished by strict cleanliness, the latter by
adequate cooling.
+36. The cow.+--The body of the cow may be a source of bacterial
contamination. Bacteria adhere to the hair of the animal, and to the
scales of the skin, and during the process of milking these are very
liable to fall into the milk. To prevent this, the cow should be curried
to remove all loose material and hair. Just before milking, the udder
and flank should be wiped with a damp cloth; this removes some of the
material, and causes the remainder to adhere to the cow.
+37. Stable air.+--If the air of the stable is not clean, it will be a
source of contamination. Particles of dust floating in the air carry
more or less bacteria, and these fall into the milk during the process
of milking. To keep the stable air free from dust at milking time, all
operations which stir up dust, such as feeding, brushing the cows,
cleaning the floor, should be practiced after milking or long enough
before so that the dust will have settled. It is a good plan to close
the doors and to sprinkle the floor just before milking.
+38. The milker+ himself may be a source of contamination. He should be
clean and wear clean clothing. The hands should not be wet with milk
during milking.
[Illustration: FIG. 3.--Types of small-top milk pails.]
+39. Utensils.+--The utensils are an important source of bacterial
contamination. The bacteria lodge in the seams and corners unless these
are well-flushed with solder. From these seams they are not easily
removed. When fresh warm milk is placed into such utensils, the bacteria
begin to grow and multiply. All utensils with which milk comes in
contact should first be rinsed with cold water and then thoroughly
washed and finally scalded with boiling water, and drained or blown
absolutely dry. They should then be placed in an atmosphere free from
dust until wanted for use again. If an aërator is used, this should be
operated in pure air, free from odors and dust. One of the greatest
sources of bacterial contamination of cheese milk is the use of the
milk-cans to return whey to the farms for pig feed. Frequently, sour
whey is left in the cans until ready to feed. These cans are then not
properly washed and scalded. The practice of pasteurizing the whey at
the cheese factory is a great help in preventing this source of
infection and the spreading of disease.
The use of a small-top milk pail[16] is to be especially recommended in
preventing bacterial contamination. Because of the small opening,
bacteria cannot easily fall into the milk in as large numbers as when
the whole top of the pail is open. (See Fig. 3.)
If a milking machine[17] is used, great care must be exercised to see
that all parts that come in contact with the milk are cleaned after each
milking, and then put in a clean place until ready to use again.
+40. The factory.+--Another source of contamination is the cheese
factory itself. The cheese-maker should keep his factory in the cleanest
condition possible, not only because of the effect on the milk itself,
but as a stimulus for the producers to follow his example. All doors and
windows in the factory should be screened to keep out flies.
+41. The control of bacteria.+--If, in spite of preventive measures,
bacteria get into the milk, their growth can be retarded by controlling
the temperature. If the temperature of the milk, as soon as drawn, can
be reduced below that at which the bacteria grow and multiply rapidly,
it will retard their development. In general, all milk should be cooled
to 50° F. or below. In cooling the milk, it should not be exposed to
dust or odors. One of the best methods of cooling is to set the can
containing the milk into a tub of cold running water, and then stir. If
running water is not available, cold well-water[18] may be used, but the
water should be changed several times. If the milk is not stirred during
the cooling process, it will not cool so rapidly, because the layer of
milk next the can will become cold and act as an insulator to the
remainder in the center of the can.
One way to destroy many of the bacteria in milk is by pasteurization.
This consists in heating the milk to such a degree that the bacteria are
killed, and then quickly cooling it. After pasteurization, the milk is
so changed that some kinds of cheese cannot be made successfully.
+42. Fermentation test.+--When a cheese-maker is having trouble with gas
in his cheese, or bad flavors, he can generally locate the source of
difficulty. This can be done by making a small amount of cheese from
each patron's milk, called a fermentation test.[19] Pint or quart fruit
jars or milk bottles make suitable containers. They should be thoroughly
washed and scalded, to be sure they are clean and sterile, and then
covered to prevent contamination. As the milk is delivered to the
factory, a sample is taken of each patron's milk. The best way to secure
the sample is to dip the sterile jar in the can of milk as delivered and
fill two-thirds full of milk.
The jars are then set in water at 110° F. to bring the temperature of
the milk to 98° F. The jar should be kept covered. A sink or wash-tub
makes a convenient place in which to keep the jars. When the temperature
of the milk is 98° F., ten drops of rennet extract or pepsin is added to
each jar. A uniform temperature of 98° F. should be maintained in the
jars. This will necessitate the addition of warm water occasionally to
the water surrounding the jars. When the milk is coagulated, the curd is
broken up with a sterile knife. Precaution should be taken to sterilize
the knife after using it in one jar before putting it into another. The
best way to do this is to hold the knife for a minute in a pail of
boiling water, after taking it out of each jar. The same precaution
should be observed with the thermometer. Unless care is taken,
contamination is liable to be carried from one jar to the other. After
cutting, the whey is poured off. The temperature should be kept at 98°
F. so that the organisms will have a suitable temperature for growth.
The whey should be poured from the jars occasionally, usually about
every half hour.
As the fermentation takes place, different odors will be noticed in
different jars. In ten to twelve hours the jar should be finally
examined for odors and the curd taken out and cut to examine it for gas
pockets. By this means, bad flavors and gas in the cheese can be traced
to their sources.
[Illustration: FIG. 4.--A gang sediment tester, one tester removed.]
[Illustration: FIG. 5.--A single sediment tester.]
+43. The sediment test.+--The presence of solid material or dirt in the
milk is always accompanied by bacterial contamination. By means of the
sediment test, the amount of solid material can be determined. The test
consists of filtering the milk through a layer of cotton; the foreign
material is left on the cotton filter. Various devices for filtering the
milk have been manufactured. (Figs. 4 and 5.) In order to be able to
compare the filters from the different dairy-men's milk, the same amount
of each patron's milk is filtered, usually about a pint. These tests are
usually made once or twice a month at the factory and the filters placed
on a card where the dairy-men can see them. Much improvement in the
quality of the milk has been accomplished by the use of the sediment
test. The purpose of this test may be and often is defeated by the use
of efficient strainers. Milk produced in an unclean way may be rendered
nearly free from sediment if carefully strained. It must be remembered
that the strainer takes out only the undissolved substances and that
bacteria and soluble materials which constitute a very large part of the
filth pass through with the milk.
CHAPTER III
_COAGULATING MATERIALS_
At the present time, two substances are used to coagulate milk for
cheese-making,--rennet extract and commercial pepsin.[20] Many
substances will coagulate milk, such as acids and other chemicals.
Enzymes in certain plants will also coagulate it.
The curing or ripening of the cheese seems to depend on the physical and
chemical properties of the curd, on the activity of certain organisms
and on enzymes produced by them or in the milk. Rennet extract and
pepsin are the only known substances which will produce curd of such
character as will permit the desired ripening changes to take place.
Until recently, rennet extract was principally used to coagulate the
milk, but because of the scarcity, pepsin is now being substituted.
+44. Ferments.+--Many of the common changes taking place in milk are due
to fermentations. The souring of milk is one of the most familiar cases
of fermentation. The important change taking place is the formation of
lactic acid from the milk-sugar. The change is brought about by certain
living organisms, namely, the lactic acid-forming bacteria. Another
familiar case of fermentation is the coagulation of milk by rennet
extract or pepsin. In this case, the change is produced by a chemical
substance, not a living organism. Fermentation may be defined as a
chemical change of an organic compound through the action of living
organisms or of chemical agents.
There are two general classes of ferments: (1) living organisms, or
organized ferments; (2) chemical, or unorganized ferments. Organized
ferments are living microorganisms, capable, as a result of their
growth, of causing the changes. Unorganized ferments are chemical
substances or ferments without life, capable of causing marked changes
in many complex organic compounds, while the enzymes themselves undergo
little or no change. These unorganized ferments are such as rennin,
pepsin, trypsin, ptyalin. The rennet and pepsin must, therefore, be very
thoroughly mixed into the milk to insure complete and uniform results,
because they act by contact, and theoretically, if they could be
recovered, might be used over and over again. Practically, the amount
used is so small a percentage that recovery would be impractical even if
possible.
+45. Nature of rennet.+--Two enzymes or ferments are found in rennet
extract, rennin and pepsin. They are prepared from the secreting areas
of living membranes of the stomachs of mammalian young. For
rennet-making, these stomachs are most valuable if taken before the
young have received any other feed than milk. Rennin at this stage
appears to predominate over pepsin which is already secreted to some
extent. With the inclusion of other feed, the secretion of pepsin comes
to predominate. Rennin has never been separated entirely from pepsin.
Both of these enzymes are secreted by digestive glands in the same area,
perhaps even by the same glands. They are so closely related that many
workers have regarded them as identical. In practical work the
effectiveness of rennet preparations has been greatest when stomachs
which have digested feed other than milk are excluded. The differences,
therefore, however difficult to define, appear to be important in the
commercial preparation of rennet.
It was the practice until a few years ago for each cheese-maker to
prepare his own rennet extract. Each patron was supposed to supply so
many rennets. Now commercial rennet extract and pepsin are on the
market; however, some Swiss cheese-makers prefer to make their own
rennet extract. For sheep's and goat's milk cheese, some makers hold
that rennet made from kid or lamb stomachs is best for handling the milk
of the respective species. The objection to the cheese-maker preparing
his own rennet extract is that it varies in strength from batch to batch
and is liable to spoil quickly. Taints and bad odors and flavors develop
in it and so taint the cheese.
+46. Preparation of rennet extract.+--This extract may be manufactured
commercially from digestive stomachs of calves, pigs or sheep. An animal
is given a full meal just before slaughtering; this stimulates a large
flow of the digestive juices, containing the desired enzymes.
The stomach is taken from the animal, cleaned, commonly inflated and
dried. It may be held in the dry condition until needed for use. Such
stomachs are usually spoken of as "rennets" in the trade. Such old
rennets may be seen to-day hanging from the rafters of some of the older
cheese factories. When wanted for use, rennets are placed in oak barrels
and covered with water. Before placing them in the barrel, they are cut
open so that the water may have easy access. Salt is usually added to
the water at the rate of 3 to 5 per cent. They are stirred and pounded
in this solution from five to seven days. At the end of this time, they
are wrung through a clothes-wringer to remove the liquid. The rennets
are put back into a fresh solution of salt and water, the object being
to obtain all the digestive juices possible. They are usually soaked
from four to six weeks. At the end of this time, most of the digestive
juices will have been removed. The liquid portion is passed through a
filter made of straw, charcoal and sand. When clean, an excess of salt
is added to preserve it.
Such extracts cannot be sterilized by heat because the necessary
temperature would destroy the enzyme. Effective disinfectants cannot be
used in food products. The extract, therefore, should be kept cool to
retard bacterial growth. The extract is kept in wooden barrels, stone
jugs or yellow glass bottles to protect it from light, which is able to
destroy its activity. Rennet extract should be clear, with a clean salty
taste and a distinct rennet flavor. There should be no cloudy appearance
and no muddy sediment in properly preserved rennet. Rennet extract is on
the market in the form of a liquid and a powder, the former being much
more common. The commercial forms of rennet have the advantage in the
skill used in their preparation and standardization. The combined
product from large numbers of stomachs may not be as effective a
preparation as the most skillfully produced sample from the very
choicest single stomach, but it gives a uniformity of result which
improves the average product greatly.
+47. Pepsin.+--Pepsin is on the market in several commercial forms, as a
liquid, scale pepsin and in a granular form known as spongy pepsin. Some
commercial concerns put out a preparation which is a mixture of rennet
extract and commercial pepsin.
+48. Chemistry of curdling.+--The chemistry of casein[21] and of curd
formation under the influence of acid and rennet extract and pepsin has
been the subject of many years' research. While many points remain
unsettled, the general considerations together with a large mass of
accepted facts may be presented and some of the unsolved problems
pointed out as left for future researches.
Casein is a white amorphous powder, practically insoluble in water. It
is an acid and as such readily dissolves in solutions of the hydroxides
or the carbonates of alkalies and alkaline earths by forming soluble
salts.
Pure casein salt solutions and fresh milk do not coagulate on boiling,
but in the presence of free acid coagulation may take place below the
boiling temperature. The coagulum formed in the case of milk includes
fat and calcium phosphate. The slight pellicle which coats over milk
when it is warmed is of the same composition.
+49. Use of acid.+--A commonly accepted explanation of the precipitation
of casein by acids is that the casein is held in solution by chemical
union with a base (lime in the case of milk); that added acid removes
the base, allowing the insoluble casein to precipitate; and that excess
of acid unites with casein, forming a compound which is more or less
readily soluble.
+50. Robertson's theory.+--According to Robertson's conception, in a
soluble solution of a protein or its salt, the molecules of the protein
unite with each other to a certain extent, in this way forming polymers.
The reaction is reversible, and the point of equilibrium between the
compound and its polymeric modification varies under the influence of
whatever condition affects the concentration of the protein ions.
Addition of water, or of acid, alkali or salt, or the application of
heat has such an effect, and consequently alters the relative number of
heavier molecule-complexes. Robertson's experiments give evidence that
one of the effects of increase of temperature on a solution of casein is
a shifting of the equilibrium in the direction of the higher complexes.
He explains coagulation as being a result of these molecular aggregates
becoming so large as to assume the properties of matter in mass and to
become practically an unstable suspension and then a precipitate. The
acid curd then is casein or some combination of casein with the
precipitant acid.
+51. Rennet curd.+--Rennet extract and pepsin coagulation differs from
coagulation by acids, and cannot be looked on as a simple removal of the
base from a caseinate. The presence of soluble calcium salts (or other
alkaline earth salts) seems to be essential, and the precipitate formed
is not casein or a casein salt, but a salt of a slightly different
nucleoalbumin called "paracasein." Many writers, following Halliburton,
call this modification produced by rennin the "casein" and that from
which it is derived, "caseinogen." Foster and a few others have used the
term "tyrein" for the rennet clot.
A number of investigations have been made on the conditions essential or
favorable to formation of the coagulum, especially with regard to the
effects of the degree of acidity and of conditions affecting the amount
of calcium present, either as free soluble salt or bound to the casein.
Soluble salts of calcium, barium and strontium favor or hasten
coagulation, while salts of ammonium, sodium and potassium retard or
prevent coagulation.
The bulk of the coagulum from milk is a calcium paracaseinate, but it
carries down with it calcium phosphate and fat, both of which bodies
have been helped to remain in their state of suspension in milk by the
presence of the casein salt. Lindet (1912) has concluded that about
one-half of the phosphorus contained in the rennet curd is in the form
of phosphate of lime (probably tricalcic), the other half being
organically combined phosphoric acid.
+52. Hammarsten's theory.+--According to Hammarsten (1877, 1896), whose
view has been commonly held, the distinctive effect of the ferment is
not precipitation but the transformation of casein into paracasein. This
is evidenced by the fact that if rennet be allowed to act on solutions
free from lime salts no precipitate occurs; but there is an invisible
alteration of the casein, for now, even if the ferment be destroyed by
boiling the solution, addition of lime salts will cause immediate
coagulation. (See also Spiro, 1906.) Hence the process of rennet
coagulation is a two-phase process; the first phase is the
transformation of casein by rennin, the second is the visible
coagulation caused by lime salts.
Furthermore, if the purest casein and the purest rennin were used,
Hammarsten always found after coagulation that the filtrate contained
very small amounts of a protein. This protein he designated as the "whey
protein."
In accordance with these observations, Hammarsten (1911) explains the
rennin action "as a cleavage process, in which the chief mass of the
casein, sometimes more than 90 per cent, is split off as paracasein, a
body closely related to casein, and in the presence of sufficient
amounts of lime salts the paracasein-lime precipitates out while the
proteose-like substance (whey-protein) remains in solution."
By continued action of rennin on paracasein, a further transformation
has been found in several cases (Petry, 1906; Van Herwerden, 1907; Van
Dam, 1909), but perhaps due to a contamination of the rennin with
pepsin, or to the identity of these two enzymes. The action which forms
paracasein and whey-protein takes place in a short time (Hammarsten,
1896; Schmidt-Nielson, 1906). The composition and solubilities of
paracasein have received considerable attention. (See Loevenhart, 1904;
Kikkoji, 1909; Van Slyke and Bosworth, 1912.) It is more readily
digested by pepsin-hydrochloric acid than is casein (Hosl, 1910).
+53. Duclaux theory.+--Duclaux (1884) and Loevenhart (1904) and others
do not accept Hammarsten's theory; but to most workers it seems
probable, at least, that the action of the rennin is to cause a cleavage
of casein with formation of paracasein. However, the chemical and
physical differences observed between casein and paracasein appear to be
so slight that Loevenhart and some others think that they are only
physical, perhaps differences in the size of the colloid or solution
aggregates. Loevenhart conceives of a large part of the work of the
rennet (or of the acid, in acid and heat coagulation) as being a freeing
of the calcium to make it available for precipitation. Some think that
the aggregates of paracasein are larger than those of casein, but there
is more evidence of their being smaller, which idea corresponds with the
findings of Bosworth, though he looks on the change as a true cleavage.
+54. Bang's theory.+--Another description of the precipitation is given
by Bang (1911), who studied the progress of the coagulation process by
means of interruptions at definite intervals. His observations confirm
the idea that rennin causes the formation of paracasein, and that the
calcium salt serves only for the precipitation of the paracasein; the
rennin has to do also with the mobilizing of lime salts. According to
Bang, before coagulation occurs, paracaseins with constantly greater
affinity for calcium phosphate are produced. These take up increasing
amounts of calcium phosphate, until finally the combination formed can
no longer remain in solution.
+55. Bosworth's theory.+--By a very recent work of L. L. Van Slyke and
A. W. Bosworth (Van Slyke and Bosworth, 1912, 1913; and Bosworth and Van
Slyke, 1913), in which ash-free casein and paracasein were compared as
to their elementary composition, and as to the salts they form with
bases, and the properties of these salts, it is indicated that the two
compounds are alike in percentage composition and in combining
equivalent, the paracasein molecule being one-half of the casein
molecule. Moreover, Bosworth (1913) has shown that, if the rennin
cleavage be carried out under conditions which avoid autohydrolysis, no
other protein is formed; also that, if the calcium caseinate present be
one containing four equivalents of calcium, the paracaseinate does not
precipitate, save in the presence of a soluble calcium salt, while, if
the calcium caseinate be one of two equivalents of base, rennin does
cause immediate coagulation. Bosworth concludes that the rennin action
is a cleavage (probably hydrolytic) of a molecule of caseinate into two
molecules of paracaseinate, the coagulation being a secondary effect due
to a change in solubilities, dicalcium paracaseinate being soluble in
pure water but not in water containing more than a trace of calcium
salt, and the monocalcium caseinate being insoluble in water. The alkali
paracaseinates, as well as caseinates, are soluble. This explanation
seems to promise to harmonize the observations with regard to acidity
and the effects of the presence of soluble salts. This theory
represents, therefore, many years of continuous work at the New York
Experiment Station centered primarily on American Cheddar cheese.
Disputed points remain for further study but these workers have
contributed much toward a clear description of the chemical constitution
of casein as affected by rennet action and bacterial activity.
The investigations of these authors and of Hart with regard to the
changes which the paracasein, the calcium and the phosphorus undergo
during the ripening of cheese (Van Slyke and Hart, 1902, 1905; Van Slyke
and Bosworth, 1907, 1913; Bosworth, 1907) contributed to this
interpretation.
BANG, IVAR, Ueber die chemische Vorgang bei der
Milchgerinnung durch Lab, Skand. Arch. Physiol. 25, pages
105-144; through Jahresb. u. d. Fortsch. d. Thierchem.
41, pages 221-222, 1911.
BOSWORTH, A. W., The action of rennin on casein, N. Y.
Exp. Sta. Tech. Bul. 31, 1913.
BOSWORTH, A. W., Chemical studies of Camembert cheese, N.
Y. Exp. Sta. Tech. Bul. 5, 1907.
BOSWORTH, A. W., and L. L. VAN SLYKE, Preparation and
composition of basic calcium caseinate and paracaseinate,
Jour. Biol. Chem. Vol. 14, pages 207-210, 1913.
DUCLAUX, ÉMILE, Action de la présure sur le lait, Compt.
Rend. Acad. Sci. 98, pages 526-528, 1884.
HAMMARSTEN, OLOF, Zur Kenntnis des Caseins und der
Wirkung des Labfermentes, Nova. Acta Regiae Soc. Sci.
Upsaliensis in Memoriam Quattuor Saec. ab Univ.,
Upsaliensi Peractorum, 1877.
HAMMARSTEN, OLOF, Ueber das Verhalten des Paracaseins zu
dem Labenzyme, Zeit. physiol. Chem. 22, pages 103-126,
1896.
HAMMARSTEN, OLOF, A text book of physiological chemistry,
from the author's 7th German edition, 1911.
HOSL, J., Unterschiede in der tryptischen und peptischen
Spaltung des Caseins, Paracaseins und des
Paracaseinkalkes aus Kuh- und Ziegenmilch, Inaug. Diss.
Bern., 31 pp., 1910.
KIKKOJI, T., Beitrage zur Kenntniss des Caseins und
Paracaseins, Zeit. physiol. Chem. No. 61, pages 130-146,
1909.
LINDET, L., Solubilité des albuminoides du lait dans les
éléments du sérum; rétrogradation de leur solubilité sous
l'influence du chlorure, Bul. Soc. Chim. (ser. 4) 13,
pages 929-935.
LINDET, L., Sur les éléments mineraux contenus dans la
caseine du lait, Rep. Eighth Internat. Congr. of Applied
Chem. 19, 199-207, 1912.
LOEVENHART, A. S., Ueber die Gerinnung der Milch, Zeit.
physiol. Chem. 41, pages 177-205, 1904.
PETRY, EUGEN, Ueber die Einwirkung des Labferments auf
Kasein, Beitrage z. Chem. Physiol. u. Path. 8, pages
339-364, 1906.
ROBERTSON, T. BRAILSFORD, On the influence of temperature
upon the solubility of casein in alkaline solutions,
Jour. Biol. Chem. 5, pages 147-154, 1908.
SCHMIDT-NIELSON, SIGVAL, Zur Kenntnis des Kaseins und der
Labgerinnung, Upsala läkaref. Förh. (N. F.) No. 11,
Suppl.
Hammarsten Festschrift No. XV, 1-26; through Jahresb. u.
d. Fortschr. d. Thierchem. No. 36, pages 255-256, 1906.
SPIRO, K., Beeinflussung und Natur des Labungsvorganges,
Beitrage z. Chem. Physiol. u. Path. 8, pages 365-369,
1906.
VAN DAM, W., Ueber die Wirkung des Labs Auf.
Paracaseinkalks, Zeit. physiol. Chem. No. 61, pages
147-163, 1909.
VAN HERWERDEN, M., Beitrag zur Kenntnis der Labwirkung
auf Casein, Zeit. physiol. Chem. 52, pages 184-206, 1907.
VAN SLYKE, L. L., and A. W. BOSWORTH, I. Some of the
first chemical changes in Cheddar cheese. II. The acidity
of the water extract of Cheddar cheese, N. Y. Exp. Sta.
Tech. Bul. 4, 1907.
VAN SLYKE, L. L., and A. W. BOSWORTH, Composition and
properties of some casein and paracasein compounds and
their relations to cheese, N. Y. Exp. Sta. Tech. Bul. 26,
1912.
VAN SLYKE, L. L., and A. W. BOSWORTH, Method of preparing
ash-free casein and paracasein, Jour. Biol. Chem. Vol.
14, pages 203-206, 1913.
VAN SLYKE, L. L., and A. W. BOSWORTH, Preparation and
composition of unsaturated or acid caseinates and
paracaseinates, _Ibid._ Vol. 14, pages 211-225, 1913.
VAN SLYKE, L. L., and A. W. BOSWORTH, Valency of
molecules and molecular weights of casein and paracasein,
_Ibid._ Vol. 14, pages 227-230, 1913.
VAN SLYKE, L. L., and A. W. BOSWORTH, Composition and
properties of the brine-soluble compounds in cheese,
Jour. Biol. Chem. 14, pages 231-236, 1913.
VAN SLYKE, L. L., and E. B. HART, A study of some of the
salts formed by casein and paracasein with acids; their
relations to American Cheddar cheese, N. Y. Exp. Sta.
Bul. 214, 1902.
VAN SLYKE, L. L., and E. B. HART, Casein and paracasein
in some of their relations to bases and acids, American
Chem. Jour. 33, pages 461-996, 1905.
VAN SLYKE, L. L., and E. B. HART, Some of the relations
of casein and paracasein to bases and acids, and their
application to Cheddar cheese, N. Y. Exp. Sta. Bul. 261,
1905.
CHAPTER IV
_LACTIC STARTERS_
Acidity in cheese-making arises almost exclusively from the lactic acid
produced from the fermentation of milk-sugar (lactose) by bacteria.
Hydrochloric acid is used in the Wisconsin[22] process of making
pasteurized milk cheese and sometimes for making skimmed-milk curd for
baking purposes. It is regularly used in precipitating casein not for
food but for manufacturing purposes.
+56. Acidifying organisms.+--Many species of bacteria have been shown to
possess the power to produce lactic acid by fermenting lactose. In
practice, however, the cheese-maker seeks to control this fermentation
by the actual introduction of the desired organisms and by the
production of conditions which will insure this dominance through
natural selection. For this purpose the initial souring for most types
of cheeses is produced by some variety of the species originally
described by Esten[23] and commonly referred to as _Bacterium
lactis-acidi_, but variously named as _B. acidi-lactici_, _Streptococcus
lacticus_, _B. guntheri_ by different authors. Organisms of this series
dominate all other species in milk which is incubated at 70° F. They
produce a smooth solid mass without a sign of gas holes and without the
separation of whey from the curd, and develop in milk a maximum acidity
of about 0.90 of one per cent when titrated as lactic acid. (For
titration see Chapter V.) This species is usually present in small
numbers in fresh milk. There are many varieties or strains of the
species with differing rates of activity and measurable differences in
acid produced but with approximately the same qualitative characters.
Most commercial starters for cheese- and butter-making belong to this
group of species, although special mixtures with other organisms are
prepared for special purposes. In addition to this group, most varieties
of cheese contain some members of the colon-aërogenes group. When the
milk is in proper condition, the activity of this group should be held
in check by the early and rapid development of acid. Free development of
members of this group usually shows itself in the presence of gas holes
in the curd.
+57. Starter.+--The practice of using pure cultures in cheese-making has
brought about the development of factory methods of producing day by day
cultures of the organisms desired, in quantities sufficient to inoculate
the total quantity of milk used in manufacture. For this purpose milk is
mostly used and the product is known as "starter." For cheese-making
purposes, a starter is a substance used in the manufacture of dairy
products having a predominance of lactic acid-forming microorganisms in
an active state. There are two general classes of starter: (1) Natural
starter; (2) commercial starter.
+58. Natural starter.+--Milk, or other similar substance, which has
become sour or in which large numbers of lactic acid-forming organisms
are present, is called a natural starter when used in the manufacture of
dairy products. To secure clean-flavored milk, the cheese-maker usually
selects the milk of some producer who usually brings good milk and
allows it to sour naturally for use the next day. There is often a
variation from day to day in the milk delivered by the same producer, so
that the cheese-maker is not certain of a uniform quality in his
fundamental material. While the lactic acid-forming organisms are
developing, other organisms may also be present in numbers sufficient to
produce bad flavors. If a starter has any objectionable flavor, it
should not be used. Natural starters very commonly develop objectionable
flavors which at first are very difficult to recognize. When natural
starters with objectionable but not easily recognizable odors are used,
the effect may be seen on the cheese. Milk, sour whey and buttermilk are
materials commonly used as natural starter. A common difficulty in
skimmed-milk cheese is caused by the use of buttermilk as a starter.
+59. Commercial starter or pure cultures.+--The alternative practice
consists in the introduction of pure cultures of known strains of lactic
bacteria into special milk to make the starter. Since these cultures
must be prepared by a bacteriologist, commercial laboratories have
developed a large business in their production. Many such commercial
brands are manufactured under trade-marked names. Some of these cultures
represent races of lactic bacteria cultivated and cared for efficiently,
hence uniformly valuable over long periods of time. Others carelessly
produced are worthless, or even a peril to the user.
These organisms are usually shipped in small quantities in bottles of
liquid or powder, or in capsules of uniform size. The contents may be
either the culture medium upon which the organisms grew or inert
substance designed merely to hold the bacteria in inactive form. In
either solid or liquid form, the producer of the culture should
guarantee its activity up to a plainly stated date.
It is the problem[24] of the cheese-maker or butter-maker to increase
this small amount of lactic acid-forming organisms to such numbers and
in such active condition that it may be used in the factory; while being
built up, these organisms must be kept pure. The usual practice is to
allow them to develop in some material, usually whole milk or
skimmed-milk; dissolved milk powder may be used in the place of milk.
+60. Manufacturer's directions.+--The manufacturer usually sends
directions with his starter preparation, telling how it should be used
to secure the best result. These directions apply to average conditions
and must be varied to suit the individual instances so that a good
starter will be the result. The directions usually state the amount of
milk necessary for the first inoculation. It is usually a small amount,
one or two quarts. After the specific amount has been selected, this
milk should be pasteurized.
+61. Selecting milk.+--The milk for use in starter-making should be
selected with very much care. Only clean-flavored sweet milk, free from
undesirable micro-organisms, should be used in the preparation of
starter. The milk is ordinarily chosen from a producer whose milk is
usually in good condition. The quality of the milk can be determined by
the use of the fermentation test. (See Chapter II.) It is better to
choose only the morning's milk for the making of starter, because the
bacteria have not had so much opportunity to develop. In no case should
the mixed milk be used in the preparation of starter, as this eliminates
all opportunity for selection. The flavor of the starter will be the
same as that of the milk from which it is made.
+62. Pasteurization+ is the process of heating to a high temperature for
a given length of time and quickly cooling. It kills most of the
micro-organisms in the milk. In other words, it makes a clean seed-bed
for the pure culture. The temperatures of pasteurization recommended for
starter-making differ with the authority. A temperature of 180° F. for
thirty minutes or longer seems to be very satisfactory, since under
these conditions nearly all the micro-organisms in the milk are killed.
+63. Containers.+--Various kinds of containers may be used for
starter-making. One-quart glass fruit jars or milk bottles make very
satisfactory containers, because the condition of the starter may be
seen at any time. They are also easily cleaned. They have the
disadvantage, however, of being easily broken, if the temperature is
suddenly changed, or if severely jarred. Tin containers may also be
used. Such containers are not easily broken, but they are harder to
clean and must be opened to examine the contents; hence the liability of
contamination is very much greater.
This small amount of milk may be pasteurized by placing the container in
water heated to the desired temperature. A very satisfactory arrangement
is to cut of a barrel, and place a steam pipe in it. The barrel can then
be filled partly full of water and heated by steam. The bottles of milk
to be pasteurized are hung in the water in the barrel. Two or three
more bottles should be prepared than it is expected will be used as some
of the bottles are liable to be broken while cooling or heating. The
bottles should be filled about two-thirds full. This leaves room enough
to add the mother starter and later to break up the starter to examine
it. It is desirable not to have the milk or starter touch the cover
since contaminations are more likely. It is a good plan when
pasteurizing to have one bottle as a check. This may be filled with
water and left open and the thermometer placed in it. A uniform
temperature may be obtained by shaking the bottles.
+64. Adding cultures.+--After being pasteurized, the milk should be
cooled to a temperature of 80° F. This is a suitable temperature for the
development of the lactic acid-forming organisms. The commercial or pure
culture should now be added to the milk at the rate specified in the
directions. Care should be exercised in opening bottles not to put the
covers in an unclean place. A sterile dipper is a good place to put
them. After the culture has been added to the milk, it should be mixed
thoroughly by shaking the bottle. This should be repeated every fifteen
or twenty minutes for four or five times. This is done to make certain
that the culture is thoroughly mixed with the milk. The milk should be
placed in a room or incubator as near 80° F. as possible, in order to
have a uniform temperature for the growth of the organisms. The bacteria
in the pure culture are more or less dormant so that a somewhat higher
temperature than the ordinary is necessary to stimulate their activity.
This milk should be coagulated in eighteen to twenty-four hours,
depending largely on the uniformity of the temperature during
incubation.
+65. Cleanliness.+--To produce a good starter, great care should be
exercised that all utensils coming in contact with the milk are sterile.
After the milk is in the container in which the starter is made, it
should be kept covered as continuously as possible. Thermometers should
not be put into it to ascertain the temperature. When examining the
starter, do not dip into it, but pour out, as this prevents
contamination. The cover, when removed from the container, should be put
in a sterile place in such way that the dirt will not stick to it and
later get into the starter.
+66. "Mother" starter or startoline.+--The thickened sour milk obtained
by inoculating the sweet pasteurized milk with pure culture of lactic
acid-forming bacteria is known as "mother starter" or "startoline."
+67. Examining starter.+--This starter should be examined carefully as
to physical properties, odor and taste. The coagulation should be
smooth, free from whey and gassy pockets or bubbles. Sometimes the first
few inoculations from a new culture will show signs of gas, but,
usually, this will quickly disappear, and not injure the starter. It
should have a clean sour cream odor and clean, mild, acid flavor. After
breaking up it should be thick and creamy, entirely free from lumps.
This starter may have an objectionable flavor, due to the media in which
the organisms were growing when shipped. In such cases it is necessary
to carry the starter one or two propagations to overcome the flavor, to
enliven the micro-organisms and to secure the quantity desired.
+68. Second day's propagation.+--For the second day, the milk for the
starter is selected as on the first day. It is pasteurized, and this
time cooled to 70° F. The milk is cooled a trifle colder the second day
than the first, because the organisms have become more active and hence
do not require as high a temperature to grow. Instead of inoculating
with powder, as was done the first day, the mother starter prepared the
first day is used. This requires only a very small amount, perhaps a
tablespoonful to a quart bottle. It should be thoroughly mixed with the
milk. This starter may have the flavor of the media used in the
laboratory culture, therefore may need to be carried one or two days
more to eliminate it. After the flavor has become normal, the mother
starter is ready for commercial use.
[Illustration: FIG. 6.--An improved starter-can.]
+69. Preparation of larger amount of starter.+--The first thing to
determine is the quantity of starter required. As much milk should be
carefully chosen as the amount of starter desired. This milk should then
be pasteurized. An improved starter-can (Fig. 6) may be used in the
pasteurization of the milk and the making of starter, or a milk-can
(Fig. 7) placed in a tub of water in which there is a steam pipe. The
former requires mechanical power to operate the agitator, but the latter
can be used where mechanical power is not available. In the latter the
milk and starter are stirred by hand. This is the kind of apparatus more
often found in cheese factories.
[Illustration: FIG. 7.--A simple device for the preparation of starter.]
If possible, this milk should be pasteurized to 180° F. for thirty
minutes; this kills most of the bacteria and spores. The milk should be
cooled to 60°-65° F., the temperature of incubation. This temperature
may be varied with conditions, so that the starter will be ready for use
at the desired time. The higher the temperature, the less time is
required to ripen the starter.
+70. Amount of mother starter to use.+--The mother starter prepared the
day before is now used to inoculate the starter milk. The amount to use
will depend on:
1. Temperature of milk when mother starter is added;
2. Average temperature at which the milk will be kept during the
ripening period;
3. Time allowed for starter to ripen before it is to be used;
4. Vigor and acidity of the mother starter added. There is a very wide
range as to the amount of mother starter required, from 0.5 of one per
cent to 10 per cent being used under different conditions.
Some operators prefer to add the mother starter while the milk is at a
temperature of about 90° F., before it has been cooled to the incubating
temperature. This reduces the amount of mother starter necessary.
If an even incubating temperature can be maintained, it will require
less mother starter than if the temperature goes down.
If the ripening period is short, it will require a larger amount of
mother starter, than if the ripening period is longer. If the starter
has a low acidity or weak body indicating that organisms are of low
vitality, it will require more mother starter.
+71. Qualities.+--The starter, when ready to use, may or may not be
coagulated; a good idea of the quality of the starter may be gained by
the condition of the coagulation. The coagulation should be jelly- or
custard-like, close and smooth, entirely free from gas pockets and
should not be wheyed off.
When broken up, the starter should be of a smooth creamy texture and
entirely free from lumpiness or wateriness. It should have a slightly
pronounced acid aroma. The starter should be free from taints and all
undesirable flavors; the flavor should be a clean, mild acid taste.
+72. How to carry the mother starter.+--Some mother starter must be
carried from day to day to inoculate the large starter. This may be
carried or made in several ways:
1. Independently: By this method a mother starter is made and carried
entirely separately from the large starter. It requires more time and
work, but is by far the best method. With a good mother starter, there
is not so much danger of the larger starter becoming poor in quality.
2. Mother starter may be made by dipping pasteurized milk from that
prepared for the large starter with sterile jars and then inoculating
these jars separately. By this method, if the milk selected for the
large starter is poor, the mother starter for the next day will be the
same. It is very difficult by this method to carry a uniform, high
quality mother starter.
There is danger that the container used for the mother starter may not
be sterile, and there is also danger of contamination in transferring
the milk.
3. Another practice is to hold over some of the large starter used
to-day for mother starter. This is by far the easiest method. By this
practice, there is no certainty of the quality of the starter, because
there is little or no control of the mother starter. If the large
starter is for some reason not good, there is no separate reserve of
mother starter on which to rely.
+73. Starter score-cards.+--The use of a score-card tends to analyze the
observations in such a way as to emphasize all the characteristics
desired in the starter. Such an analysis seeks to minimize the personal
factor and produce a standardization of the quality. The score-card
finally reduces the qualities of the starter to a numerical basis for
ease of comparison. Many score-cards have been proposed but the one
preferred by the authors is that used by the Dairy Department of the New
York State College of Agriculture, which is as follows:
CORNELL SCORE-CARD
Flavor 50 Clean, desirable acid.
Aroma 20 Clean, agreeable acid. No undesirable aroma.
Acidity 20 0.6 per cent-0.8 per cent.
Body 10 Before breaking up: jelly-like, close,
absence of gas holes. No free
whey. After breaking up:
smooth, creamy, free from granules
or flakes.
The qualities mentioned in this score-card can be quickly and easily
determined by examining and tasting the starter and by making an acid
test of a sample. The acid test is conducted as with milk (see Chapter
II) except the starter must be rinsed out of the pipette with pure
water. Some starter score-cards call for a bacterial examination and
counting of the starter organisms. This takes a considerable period of
time and is not entirely necessary. The physical properties and acid
test are closely correlated with the presence of the desired organisms.
+74. Use of starter.+--If all milk could be clean and sweet and the only
fermentation from it were the clean acid type, a starter would be
useless. Such milk is hard to obtain; therefore, a starter is used to
overcome the bad fermentation. This improves the flavor, body and
texture of the cheese. The common contaminations which the starter will
tend to correct are:
1. Gas-producing bacteria.
2. Yeasts.
3. Bad flavors or taints.
The length of time a starter may be carried depends on the accuracy and
carefulness of the maker. This calls for scrupulous attention to the
temperature, the selection of milk and keeping out contaminations. The
maker must remember that a starter is not merely milk, but milk full of
a multitude of tiny plants, very sensitive to food, temperature, clean
surroundings and the quantity of their own acid.
NEW YORK STATE COLLEGE OF AGRICULTURE AT CORNELL UNIVERSITY
+STARTER LOT-CARD+ +Department of Dairy Industry.+
==================================================================
Day and Date_____________________
+MILK:+
Kind________________ % fat_____% solids not fat_____
Flavor__________________________________
Amount of milk____________ Hours old______________
+PASTEURIZATION:+
Method______________________________________________
Milk when received: Temperature_____ °
Acidity_____ %
Heating: Turning on heat__________ APM.
Desired temp. reached__________ APM.
Turning off heat__________ APM.
Length of time at desired temp.__________
Beginning to cool__________
Cooled__________ APM; to__________ °
Acidity: After pasteurization__________
When inoculated________________
+INOCULATION:+
Time__________ Temperature__________
Amount__________ lbs. __________%
+INCUBATION:+
Temperature__________ Time__________
+MOTHER STARTER USED:+
Source_____________________________________ % used_______________
Times propagated___________________________ Acidity______________
Amount used________________________________ Appearance___________
Flavor_______________________________
Comments__________________________________________________________
+STARTER:+
Time of examining__________
Temperature________________
+SCORE-CARD:+
Flavor__________50| Clean, desirable acid.
|
Aroma___________20| Clean, agreeable acid. No undesirable aroma.
|
Acidity_________20| 0.6%-0.75%.
|
Body____________10| Before breaking up: jelly-like, close,
| absence of gas holes. No free whey.
____| ___After breaking up: smooth, creamy, free from
| granules or flakes.
Total_________ 100|
-----------------------------------
The above is a tentative score-card.
+COMMENTS:+____________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
+Work and observation by+______________________________________
+75. The amount of starter to use+ depends on the amount of acid desired
in the milk for any particular kind of cheese. The great abuse of
starter is the practice of using too much. It is better and safer to add
starter a little at a time and several times than to add too much at
once. When starter is added to milk for cheese-making, it should be
strained to remove any lumps; otherwise an uneven color is likely to
result.
+76. Starter lot-card.+--For certain dairy operations, a permanent
record is desired. This is especially true in the making of starter and
certain varieties of cheese. A lot-card not only serves as a record but
also points out the succeeding steps of the operation. This latter is
especially useful for beginners and students. Page 53 shows a desirable
lot-card to be used when making starter. Each operation has been
referred to the page in the text where it is discussed. This makes this
particular lot-card an index to the whole process of starter-making as
here treated.
CHAPTER V
_CURD-MAKING_
Aside from the purely sour-milk cheeses, the coagulum or curd resulting
from rennet action is the basis of cheese-making. The finished cheese,
whatever its final condition, is primarily dependent on a particular
chemical composition and fairly definite physical characters in the
freshly made curd mass. These characters are determined by a series of
factors under control of the cheese-maker. Assuming the milk to be
normal in character, success depends on the use of a proper combination
of these factors. The possible variations in each factor together with
their number makes an almost infinite series of such combinations
possible. The essential steps in the process are, therefore, presented
as underlying all cheese-making. The special adaptations of each factor
are considered in the discussion of the varieties group by group.
These factors follow:
_A._ The coagulation group:
1. Fat-content of the milk.
2. The acidity of the milk.
3. The temperature of renneting.
4. The effective quantity of rennet.
5. Curdling period or the time allowed for rennet action.
_B._ The handling group:
6. Cutting or breaking the curd.
7. Heating (cooking) or not heating.
8. Draining (including pressing, grinding and putting into hoops
or forms).
+77. The composition of the milk.+--The fat percentage in the milk in
the cheese-vat should be known to the cheese-maker and be strictly under
his control. The fat tester and the separator make this clearly
possible. He can go further. Milk from particular herds whose quality is
a matter of record from the routine test of each patron's milk may be
selected and brought together for the manufacturer of cheese of special
quality. Control of casein or lactose, on the contrary, is not nearly so
practicable. The purchase of milk on the fat test has become so well
established in most dairy territories, as to insure the presence and
constant use of the tester. A fat test of the mixed product in the
cheese-vat in connection with established tables thus insures an
accurate knowledge of the materials which go into each day's cheese. For
some varieties of cheese, whole milk should always be used. For other
varieties, the addition or removal of fat is regularly recognized as
part of the making process. The presence of added fat or the removal of
fat affects the texture of the product and the details of the process of
making.
+78. Cheese color.+--An alkaline solution of annatto is usually used as
a cheese color. This colors both casein and fat in contrast to butter
color which is an oil solution of the dye and mixes only with the fat.
Cheese color is added to the milk in making some varieties of cheese,
and not for others. When lactic starter is used, the color should be
added after the starter and just before the addition of the rennet. The
amount is determined by the color desired in the cheese. The usual
amount varies from one to four ounces to each thousand pounds of milk.
Before adding, the color should be diluted in either milk or water,
preferably water. It should then be mixed thoroughly with the milk.
+79. The acidity factor.+--Milk as drawn shows a measurable acidity when
titrated to phenolphthalein with normal sodium hydroxide. This figure
varies with the composition of milk. Casein itself gives a weakly acid
reaction with this indicator. Calculated as lactic acid, this initial
acidity varies within fairly wide limits, records being found from 0.12
to 0.21 of one per cent or even more widely apart. Commonly, however,
such titration shows 0.14 to 0.17 per cent. Some forms of cheese
(Limburger, Swiss, Brie) are made from absolutely fresh milk. Acidity
from bacterial activity is important as a factor in the making of most
types of cheese and probably in the ripening of all types.
Increasing the acidity of the milk hastens rennet action and within
limits produces increased firmness of the curd. If carried too high,
acidity causes a grainy or sandy curd. Normally fresh milk is
sufficiently acid in reaction when tested to phenolphthalein to permit
rennet to act, but the rate of action increases rapidly with the
development of acid. Increase of acidity may be accomplished: (_a_) by
the addition of acid as has been done by Sammis[25] and Bruhn in
pasteurized milk for Cheddar cheese; or (_b_) by the development of acid
through the activity of lactic organisms, which is the usual way. For
renneting, the acidity necessary for particular cheeses runs from that
of absolutely fresh milk still warm (as in French Brie, Limburger,
Swiss, Gorgonzola) through series calling for increase of acidity,
hundredth by hundredth per cent calculated as lactic acid. This ranges
from 0.17 to 0.20 per cent as is variously used in American factory
Cheddar to about 0.25 to 0.28 per cent as obtained by adding acid in
Sammis' method. This method is discussed under the heading "Cheddar
Cheese from Pasteurized Milk" (p. 229) since it requires special
apparatus and has not thus far been used with other types of cheese. For
the development of acidity by the action of bacteria, lactic starter is
almost universally used. This may be added in very small quantities and
the acidity secured by closely watching its development or by adding
starter in amount sufficient to obtain the required acidity at once. In
either case, the cheese-maker needs to know the rate of action of the
culture to insure the proper control of the process. The amount of acid
already present when the rennet is added affects not only the texture of
the curd as first found, but within limits indicates also the rate at
which further acidity may be expected to develop.
A series of experiments in making Roquefort were tabulated to show the
rate of acidification from various initial points. In the graphs (Fig.
8) the curves for acid development are parallel after the determination
reaches 0.30 per cent. These experiments were made at a temperature 80°
to 84° F. Milk at the lowest acidities tried developed titratable acid
very slowly. A period of several hours was required to produce
sufficient acid to affect the curd texture. When the acid reached 0.25
per cent by titration, the further rise was rapid and all the lines
became almost straight and parallel after the titration reached 0.30
per cent. If this rapid souring occurred after the completion of the
cheese-making process, the texture of the experimental cheese was not
measurably affected. In those cases, however, in which 0.30 per cent was
reached before the cheese reached its final form in the hoop, the
texture of the ripened cheese was entirely different from that desired
for this variety under experiment. These curves apply directly to but
one cheese process in which a particular combination of acidity, rennet
and time is used to obtain a very delicately balanced result. In other
varieties it is equally important to obtain exactly the adjustment of
these factors which will bring the desired result.
[Illustration: FIG. 8.--The acidification of Roquefort cheese.]
+80. Acidity of milk when received.+--If proper care has been taken,
milk should be delivered to the factory fresh, clean and without the
development of acid. If the milk has not been handled properly, the
early stages of souring or some other unfavorable fermentation will have
developed. Such milk may develop too much acid, or gas, or any one of
several objectionable flavors during the making and ripening of the
cheese. Some cheese-makers become very expert in detecting the first
traces of objectionable qualities, but most makers are dependent on
standardized tests to determine whether milk shall be accepted or
rejected, and when accepted to determine the rate at which it may be
expected to respond during the cheese-making process.
Various tests have been devised to determine the amount of acid present
in milk. There are two tests commonly used in cheese-factories. One is
known as the "acid test" and the other the "rennet test."
+81. The acid test+[26] is made by titrating a known amount of milk
(Fig. 9) against an alkali solution of known strength, using
phenolphthalein as an indicator. The object of the indicator is to tell
the condition of the milk, whether it is acid, alkaline or neutral. The
indicator does not change in an acid solution but turns pink when the
solution is or becomes alkaline. To make the test, a known quantity of
the material to be tested is placed in a white cup, and to this several
drops of indicator are added. As an indicator, a 1 per cent solution of
phenolphthalein in 95 per cent alcohol is commonly used. As an alkali
solution, sodium hydroxide (NaOH) is used in the standardized strength
usually either tenth (N/10) normal or twentieth (N/20) normal. This
solution should be obtained in some one of the standardized forms
commercially prepared. The alkali is added, drop by drop, from a
graduated burette until a faint pink color appears. This shows that the
acid in the milk has been neutralized by the alkali. The amount of
alkali that has been used can be determined from the burette. Knowing
the amount of milk and alkali solution used, it is easy to calculate the
amount of acid in the substance tested. The results are usually
expressed either as percentages of lactic acid or preferably as cubic
centimeters of normal alkali required to neutralize 100 or 1000 c.c. of
milk. This kind of test is on the market under different names, such as
Mann's, Publow's, Farrington's and Marschall's.
[Illustration: _FIG. 9._--An acid tester.]
+82. Rennet tests.+--Several rennet tests have been devised, but the one
most widely used is the Marschall (Fig. 10). This consists of a 1 c.c.
pipette to measure the rennet extract, a small bottle in which to dilute
the extract, a special cup to hold the milk and a spatula to mix the
milk with the rennet extract. This cup has on the inside from top to
bottom a scale graduated from 0 at the top to 10 at the bottom. There is
a hole in the bottom to allow the milk to run out.
[Illustration: FIG. 10.--Marschall rennet test.]
+83. Marschall rennet test.+--To make a Marschall rennet test, 1 c.c. of
rennet extract is measured, with the 1 c.c. pipette, and placed in the
bottle. Care should be exercised to rinse out the pipette. The bottle is
then filled to the mark with cold water. After the milk has been heated
to the setting temperature, 84°-86° F., the cup is filled with milk and
set on the edge of the vat so that the milk running out through the hole
in the bottom of the cup will flow into the vat. Just as the surface of
the milk reaches the 0 mark on the cup, the diluted rennet extract is
added and thoroughly mixed with the milk, using the small spatula to
stir it. The rennet and milk should be mixed until it has run down at
least one-half space on the scale in the cup. As the rennet begins to
coagulate the milk, it runs slower from the hole in the bottom of the
cup, until it finally stops. When it stops, the point on the scale
indicated by the surface of the coagulated milk is noted. The test is
recorded by the number of spaces the surface of the milk lowers from the
time the rennet is added until it is coagulated. This test depends on
three factors: the strength of the rennet extract, the temperature of
the milk, the acidity of the milk. The more acid, the quicker the milk
will coagulate. To measure any one of these factors, the other two must
be constant. The variable factor is the acidity of the milk. This test
will not indicate the percentage of acid in the milk, but is simply a
comparative test to be used from day to day; for example, if the rennet
test to-day shows three spaces, and the operator makes that milk into
cheese and the process seems to be normal, it shows that for good
results in this factory, milk should be ripened to show three spaces
every day. If the next day the milk showed four spaces, it should be
allowed to ripen more until it shows three spaces. If it shows only two
spaces, this indicates that the milk has too much acid development or is
over-ripe. A cheese-maker will have to determine at what point to set
his milk, because the test will vary from one factory to another.
+84. Comparison of acid and rennet test.+--Each of these tests has its
advantages and disadvantages. The advantage of the acid test is that it
can be made as well of warm as cold milk. This is of great importance in
determining whether the milk delivered by any patron is too ripe to be
received. The acidity of other materials, such as whey and starter, can
be determined as well as that of milk. The disadvantages are that it is
difficult to get the alkali solution of the proper strength and the
solution is liable to deteriorate on standing. It requires a careful
exact operator to make the test.
The advantages of the rennet test are that it is easy to make, and it
requires no materials that are hard to replace. The disadvantage is that
the milk must be warmed to the same temperature before a comparative
test can be made. The size of the outlet in cups varies. It does not
indicate the percentage of acid present in the milk. It is simply a
comparative test. To obtain the best result, both tests should be used
in conjunction.
+85. Control of acid.+--The control of acidity in curd and cheese is
dependent on the control of the moisture or water-content. The control
of both factors is very important in relation to the quality[27] of the
cheese. Often acidity is spoken of when moisture is really intended, and
vice versa. The close relation between the moisture and acidity is due
to the presence of the milk-sugar in solution in the milk-serum which
becomes the whey of cheese-making. Water or moisture in cheese consists
of the remnant of this whey which is not expelled in the making process.
During manufacture and the ripening process, the milk-sugar is changed
to lactic acid. A cheese may be sweet when first made and after a time
become sour because it contains too much moisture in the form of whey.
Excess of whey carries excess of milk-sugar from which fermentation
produces intense acidity.
Various tests have been devised to determine the amount of acid
developed at the different stages of manufacture. These tests are
described on page 61. By the use of such tests, the development of acid
during the manufacturing process can be very accurately determined.
There is no quick, accurate test to determine the amount of moisture in
the curd. The cheese-maker has to rely on his own judgment, guided
largely by the appearance, feeling and condition of the curd.
After the rennet extract has been added, all control of the acid
development is lost. The cheese-maker can determine rather accurately
how fast the acid will develop during the ripening of the milk. This
shows the importance of the proper ripening. The amount of acid
developed during the different stages of the manufacturing process can
be approximately followed with the various acid tests. The manufacturing
process should then be varied to obtain the proper relation between the
moisture and the acid present. The only time that the acidity may be
controlled is when the milk is being ripened. If too much acid is
developed before the rennet is added, there is apt to be too much acid
at each stage of the manufacturing process. This is liable to hurry the
cheese-making process and to cause a loss, both in quality and quantity
of cheese, and may cause a high acid or sour cheese. If sufficient acid
is not developed at the time the rennet is added or if the milk is not
sufficiently ripened, the acid is liable not to develop fast enough so
that there will not be sufficient at each step in the cheese-making
process. Such a cheese is called "sweet." There are several conditions
which will cause an over-development of acid. Such a cheese is called
"acidy" or "sour." These factors are within the control of the
cheese-maker, hence should be avoided. A sour cheese shows lack of skill
and care on the part of the cheese-maker.
_Conditions causing an acidy or sour cheese:_
Receiving sour or high acid milk at the cheese-factory.
Use of too much starter.
Ripening the milk too much before the rennet is added.
Removing the whey before the curd is properly firmed, hence leaving
it with too much moisture.
Development of too much acid in the whey before the whey is removed.
Improper relationship between the moisture and acidity at the time
of removing the whey.
_Conditions causing deficient acid:_
Adding the rennet before sufficient acid has developed.
Not using sufficient starter.
Not developing sufficient acid in the whey.
+86. Acidity and rennet action.+--The rennet extract acts only in an
acid medium. The greater the acid development, within certain limits,
the faster the action of the rennet. If enough acid has developed to
cause a coagulation of the casein, the rennet will not coagulate the
milk. This is one reason why Cheddar cheese cannot be made from sour
milk.
+87. Acidity and expulsion of the whey.+--The contraction of the curd
and expulsion of the whey are so closely related that they may be
treated under the same heading. The more acid, the faster the whey
separates from the curd, other conditions being uniform. The relation of
acidity and firmness of the curd to temperature of the curd is another
important factor in the successful manufacture of cheese. The higher the
acidity, the faster the temperature of the curd can be raised without
any harmful effects. If the temperature is raised too fast in relation
to the acidity, the film surrounding each piece of curd will become
toughened so that the moisture will not be able to escape. When this
condition exists, the curd will feel firm but when the pieces are
broken open the inside is found to be very soft. This results in a large
loss later or may cause a sour cheese. It usually causes an uneven
texture and color in the cheese.
+88. Acidity in relation to cheese flavor.+--Just what part the acid
plays in the development of cheese flavor is not known. If a certain
amount of acid is not present, the characteristic cheese flavor does not
develop. If too much acid is developed, it gives the cheese a sour
flavor which is unpleasant. If sufficient acid is not developed, the
other undesirable factors seem to be more active, causing very
disagreeable flavor and may cause the cheese to putrefy. A cheese with a
low acid usually develops a very mild flavor, and if carried to
extremes, as in the case of some washed curd cheese, the true cheese
flavor never develops.
+89. Acidity in relation to body and texture of cheese.+--If a cheese is
to have a close, smooth, mellow, silky body and texture, a certain
amount of acid development is necessary. If too much acid is developed,
the body and texture will be dry, harsh, sandy, mealy, corky. If the
acid is not sufficient the cheese may be soft or weak bodied, and is
usually characterized by "Swiss curd holes," which are spaces of various
sizes usually more or less round and very shiny on the inside.
+90. Acidity in relation to cheese color.+--An over-development of
acidity affects the color of a cheese. If this development of acidity is
uniform throughout the cheese, it causes the color to become pale or
bleached. If this development is uneven, due to the uneven distribution
of moisture, the color will be bleached in spots, causing a mottled
effect.
+91. Control of moisture.+[28]--The cheese-maker must use skill and
judgment in regulating the amount of moisture in relation to the
firmness of the curd and the acid. Since there are no quick accurate
tests to determine the amount of moisture, this is left entirely to the
judgment of the operator. Certain methods of handling the curd reduce
the moisture-content, while others increase it. The cheese-maker must
decide how to handle the curd. If the curd becomes too dry, methods
should be employed to increase the moisture, and vice versa.
_Causes of excessive moisture:_
Cutting the curd coarse.
Cutting the curd after it has become too hard.
Setting the milk at a high temperature.
Use of excessive amount of rennet extract.
Low acid in the curd at the time of removing the whey.
Not stirring the curd with the hands as the last of the whey is
removed.
High piling of the curd during the cheddaring process.
Piling the curd too quickly after removing the whey.
Use of a small amount of salt.
Holding the curd at too low a temperature after the whey is removed.
Soaking the curd in water previous to salting.
Allowing the curd to remain in the whey too long so that it reabsorbs
the whey.
Heating the curd too rapidly.
_Causes of insufficient moisture:_
Cutting the curd too fine or breaking up the pieces with the rake into
too small pieces.
Cutting the curd too soft.
Stirring the curd too much by hand as the last of the whey is being
removed.
Developing high acid in the curd at the time of removing the whey.
Insufficient piling of the curd during the cheddaring process.
Using a large amount of salt.
High temperature and low humidity in the curing room.
+92. Relation of moisture to manufacture and quality.+--(1) _Flavor:_ If
the cheese contains too much moisture, it is likely to develop a sour or
acidy flavor. A cheese with a normally high moisture-content usually
ripens or develops a cheese flavor much faster than one with a lower
moisture-content, other conditions being uniform. A cheese with a high
moisture-content is much more liable, during the curing process, to
develop undesirable flavors than is one with a lower moisture-content.
(2) _Body and texture:_ A cheese containing too much moisture is very
soft and is difficult to hold in shape. Such a product breaks down very
rapidly and is usually pasty and sticky in texture. If too little
moisture is present, the cheese is very dry and hard, and cures or
ripens very slowly because of the lack of moisture together with
milk-sugar from which acid may be formed. Dry cheeses are usually harsh,
tough and rubbery in texture. Such cheeses also have poor rinds. (3)
_Color:_ If the ideal conditions exist, the moisture will be evenly
distributed throughout the cheese. The spots containing more moisture
will be lighter in color. If a cheese contains so much moisture that it
becomes "acidy," the effect is the same as when too much acid is
developed, that is, the color becomes pale from the action of the acid.
(4) _Finish:_ A cheese containing too much moisture is usually soft. A
good rind does not form. Such a cheese loses its shape very easily,
especially in a warm curing room. (5) _Quality:_ A cheese with a high
moisture-content is usually marketable for only a very short period.
Such a product usually develops flavor very quickly in comparison to a
dry cheese. It must be sold very soon because if held too long, the
flavor becomes so strong as to be undesirable, and objectionable flavors
are liable to develop. In some cases, such cheeses rot.
+93. Relation of moisture to acidity.+--From the preceding discussion,
it is evident that the relation between the moisture and acidity is very
close, in fact so intimate that in some cases it is difficult to
distinguish one from the other when the quality of the cheese is
considered. The proper relation of the moisture and the acidity
determines the quality of the resulting cheese. If too much acid is
developed during the manufacturing process, the product will be sour. If
too much moisture is retained in the form of whey, the cheese will be
sour. The less acid in the curd, the more moisture in the form of whey
may be retained in the curd without causing a sour cheese. The proper
relationship between the moisture and the acidity must be maintained or
a sour cheese will result.
The relation of the moisture to the acidity also has an influence on the
curing. If the cheese has a low development of acidity and a low
moisture-content, it will cure very slowly. The increasing of either the
acidity or moisture usually increases the rate of cheese ripening, other
factors being the same.
The relation of the acidity and the moisture is so important that it
cannot be neglected without injuring both the quality and quantity of
cheese. This knowledge can be obtained only by experience.
+94. Setting temperature.+--The temperature of renneting makes very much
difference in the texture of the product. The enzyme rennin is sensitive
to very slight changes in temperature. Below 70° F., its rate of action
is very slow. Beginning with approximately 20 per cent of its maximum
effectiveness at 70° F. (the curdling point for Neufchâtel), it has
risen to 65 per cent at 84° F., to 70 per cent at 86° F., as used in
Cheddar, to about 80 to 85 per cent at 90-94° F., as used in Limburger.
At 105° F. it reaches its maximum effective working rate to fall from
that efficiency to about 50 per cent at 120° F. Curdling at low
temperature lengthens the time required for the same amount of rennet to
curdle a given quantity of the same milk. The texture of curd produced
at temperatures between 70° F. and 84° F. is soft, jelly-like, friable
rather than rubbery. At 86° F. it begins to show toughening or rubbery
characters which become very marked at 90° F. to 94° F. as used in
Limburger. With the increased vigor of action as it passes its maximum
rate of action at 105° F., the texture tends to become loose, floccose
to granular. Aside from the Neufchâtel group, the working range of
temperatures for the renneting period runs from about 84° F. to about
94° F., a range of barely 10° F., or the use of 65 per cent to 80 or
possibly 85 per cent of the maximum efficiency of the rennet. Within
this range of temperature, the curd has the physical characters demanded
for making most varieties of cheese.
+95. Strength of coagulating materials.+--Rennet and pepsin preparations
vary in strength and in keeping quality. With a particular stock,
changes go on to such a degree that the last samples from a barrel of
rennet are much weaker than the earlier ones. Each sample, barrel, keg
or bottle should be tested before used. In continuous work the results
of each day's work furnish the guide for the next day's use of a
particular lot of rennet.
+96. Amount of coagulating materials to use.+--For most varieties of
cheese, sufficient rennet extract or pepsin is added to the milk to give
a firm curd in twenty-five to forty minutes. Of the ordinary commercial
rennet extract, this requires from two and one-half to four ounces to
one thousand pounds of milk. This gives a maximum of one part rennet for
each four to six thousand parts of milk. The great strength of the
rennet extract is thus clearly shown.
+97. Method of adding rennet.+--Before rennet is added to the milk, it
is diluted in about forty times its volume of cold water, which chills
the enzyme and retards its action until it can be thoroughly mixed with
the milk. If the material is added without such dilution, the
concentrated extract produces instant coagulation in the drops with
which it comes in contact, forming solid masses from which the enzyme
escapes only slowly to diffuse throughout the mass. Uniform coagulation
thus becomes impossible. After the rennet extract has been diluted with
cold water, it should be distributed the entire length of the vat in an
even stream from a pail. It should then be mixed with the milk by
stirring from top to bottom for about three to four minutes. For this
purpose, either a long-handled dipper or a wooden rake may be used. A
dipperful should be drawn from the gate and stirred into the vat,
otherwise the milk in the gate will fail to coagulate properly because
the rennet diffuses too slowly to reach and affect all the milk at that
point. The milk should be stirred on the top, preferably with the bottom
of a dipper, until signs of coagulation begin to appear. This stirring
keeps the cream from rising. There are various ways or signs to indicate
when the coagulation has gone to the stage at which the mix is about to
become thick: (1) The milk becomes lazy or thicker as the finger is
passed through it; (2) bubbles caused by moving the finger remain on the
milk longer, usually until one can count ten when ready to thicken.
If the milk is stirred too long or after it begins to thicken, the
result is a granular sort of curd, and there will be an abnormally large
loss of fat in the manufacturing process. The addition of the rennet and
subsequent stirring require the exercise of great care and constant
attention to details. The cheese-maker can do nothing else for those few
minutes. When through stirring, it is a good plan in cold weather to
cover the vat with a cloth as this will keep the surface of the curd
warm. In summer the same cover will keep out the flies.
_Causes of a delayed coagulation:_
(1) Weak rennet extract or too small an amount.
(2) Low temperatures due to inaccurate thermometers.
(3) Pasteurized milk.
(4) Presence of abnormal bacterial ferments.
(5) Presence of preservatives.
(6) Heavily watered milk.
(7) Use of badly rusted[29] cans.
(8) Milk containing small amounts of casein or calcium salts.
_Causes of uneven coagulation:_
(1) Uneven temperature of the mix in the vat, due to lack of
agitation.
(2) Uneven distribution of the rennet extract.
(3) Adding rennet to vat too soon after heating, while the sides and
bottom are still hot, causes curd to stick to sides and bottom
of the vat making cutting difficult.
(4) Sloshing after the milk begins to thicken breaks the curd and
causes it to whey off.
+98. The curdling period.+--The time allowed for rennet action also
affects the texture of the curd. The enzymes of rennet (rennin and
pepsin) do not cease acting with the thickening of the milk. In many
cheeses, the handling process begins as soon as the curd has become
solid enough to split cleanly before a finger thrust into it. If let
stand further, the same curd mass will continue to harden with the
progressive separation of whey; this shows first as drops ("sweating")
on its surface, which then increase in number and size until they run
together and form a sheet of whey. The limit of such action is difficult
to measure. The solidifying process ceases in a period of hours. The
further action of the enzymes is digestive in character and goes on
slowly. It requires a period of weeks or even months to accomplish
measurable results at the working temperatures in use in the trade.
Other ripening agents with more rapid action intervene to shape the
final result. It follows that the rennet factor in the ripening changes
found at the end of the period is almost negligible for most varieties
of cheese, although it appears to be measurable in some varieties.
+99. Cutting or breaking[30] the curd.+--As soon as curd is formed,
separation of whey begins upon the surface and perhaps around the sides
of the vessel. This is accompanied by shrinkage and hardening of the
mass. If the curd remains unbroken, the separation is extremely slow. In
cheese-making practice, such curd masses may be dipped at once into
hoops as in Camembert, dumped in mass into cloths for drainage as in
Neufchâtel or, as in the larger number of cheeses, cut or broken in some
characteristic manner. After the curd mass is firm, the rate at which
subsequent changes take place depends largely on the size of the
particles into which the curd is cut. The smaller the particles, the
quicker the water is expelled. Consequently the development of the
acidity and other changes take place more slowly. For this reason the
curd should be cut into pieces of uniform size. If the work is not
properly performed, the pieces of curd of various sizes will be at
different stages of development. The fine particles will be firm and
elastic while the larger particles are still soft and full of whey and
may be developing too much acid. The knives should be inserted into the
curd obliquely so that they will cut their way into the curd and not
break it. The horizontal knife is used lengthwise of the vat and cuts
the curd into layers of uniform thickness. The perpendicular knife then
is used lengthwise and crosswise of the vat. It first cuts the curd
into strips and then into cubes. The knives may have wire blades or
steel blades, some operators preferring one and some the other.
Whichever is used, the blades should be close enough together to give
the fineness of curd desired.
After the knife passes through, the cut faces quickly become covered
with a smooth coating, continuous over all exposed areas. This surface
has the appearance of a smooth elastic coating or film. This can be seen
by carefully breaking a piece in the hand. It is this film which holds
the fat within the pieces of curd. If the film is broken, some of the
fat globules are lost because the rennet extract acts only on the casein
and that in turn holds the fat. All the fat globules which come in
contact with the knives as they pass through the curd will be left
between the pieces of curd and will pass off in the whey. If care is
exercised in cutting, the loss of fat will be confined to what may be
called a mechanical loss. This is similar to the loss of the sawdust
when sawing a board. This loss in American Cheddar is about 0.3 per cent
and cannot be avoided. If it is greater than this, it is due to
negligence on the part of the cheese-maker or the poor condition of the
milk. The cutting of the curd into small pieces may be considered a
necessary evil. If the moisture could be expelled from the whole mass
without disturbing it, this fat loss could be prevented. The cutting,
breaking or turning should be done with the greatest care, that the loss
may be as small as possible.
+100. Curd knives.+--For cutting curd, special knives have been devised
(Fig. 11). They consist of series of parallel blades fixed in a frame to
make cuts equidistant. The blades run vertically in one, horizontally
in another. They are spaced according to the demands of the variety of
cheese to be made. Wires stretched in a frame take the place of blades
in some makes of curd knife.
[Illustration: FIG. 11.--Blade and wire curd knives: horizontal,
perpendicular, horizontal, perpendicular.]
+101. Heating or "cooking."+--Curdling by rennet has already been shown
to be markedly hastened by moderate heating. After the coagulum or curd
is formed, the making process may be completed without the application
of further heat, as in Neufchâtel, Camembert and related forms (Fig. 12)
and in some practices with Limburger. In other forms and especially in
the hard cheeses in which cutting of curd is a prominent part of the
process, the curd after being cut is reheated or "cooked." The cooking
process hastens the removal of the whey, thus shortening the time
required to reduce the water-content of the mass to the percentage most
favorable for the type of cheese desired. The process also produces
marked changes in the physical character of the curd mass. With the rise
in temperature the casein becomes elastic first, then approaches a
melting condition and assumes a tough, almost rubbery consistency. The
final texture is the result of the combination of the amount of rennet
added, the temperature, the acidity reached during the process, and the
final water-content of the mass.
[Illustration: FIG. 12.--The heat relation. See pages 77 and 87.]