=510. Composition Of Cheese.=—Pure cheese is made from whole milk by
precipitating the casein with rennet. The precipitated casein carries
down also the fat of the milk and a little lactose and whey remain
incorporated with the cheesy mass. The ingredients of cheese are
therefore those of the whole milk less the greater part of the whey,
_id est_, milk sugar, lactalbumin, globulin, soluble mineral matters
and water. In the conversion of the crude precipitate noted above into
the cheese of commerce, it is subjected to a ripening process which is
chiefly conditioned by bacterial action. It is not possible here to
enter into a discussion of methods of isolating and identifying the
bacteria which promote or retard the ripening process.[514] As a rule,
about a month is required for the curing process, before the cheeses
are ready for boxing and shipment. The most important changes during
ripening take place in the proteid matter, which is so altered as to
become more palatable and more digestible as a result of the bacterial
activity.
The percentage composition of the principal cheeses of commerce are
shown in the following table:[515]
Water, Casein, Fat, Sugar, Ash,
Per cent. Per cent. Per cent. Per cent. Per cent.
Cheddar 34.38 26.38 32.71 2.95 3.58
Cheshire 32.59 32.51 26.06 4.53 4.31
Stilton 30.35 28.85 35.39 1.59 3.83
Brie 50.35 17.18 25.12 1.94 5.41
Neufchatel 44.47 14.60 33.70 4.24 2.99
Roquefort 31.20 27.63 33.16 2.00 6.01
Edam 36.28 24.06 30.26 4.60 4.90
Swiss 35.80 24.44 37.40 2.36
Full cream, 38.60 25.35 30.25 2.03 4.07
(mean of 143 analyses)
It is evident that the composition of the cheese will vary with
the milk from which it is made and the manipulation to which it
is subjected. A good American green cheese made from milk of
the composition noted below will have the composition which is
appended.[516]
TABLE SHOWING MEAN COMPOSITION OF
MILK AND CHEESE MADE THEREFROM.
Milk. Cheese.
Per cent. water 87.38 36.70
” ” fat 3.73 34.18
” ” proteids 3.13 23.44
” ” sugar, ash etc. 5.76 5.68
From the above it is seen that in full milk cheese the ratio of fat to
casein is 1.46: 1, and to solids not fat 1.17: 1. This is a point of
some importance in judging the purity of a cheese. When the full milk
of a mixed herd is used the percentage of fat in a cheese will always
be considerably higher than that of casein.
=511. Manipulation of the Milk.=—When sweet milk is received at the
cheese factory, a starter of sour milk is added to it in order to
hasten its ripening. When it is thought that the proper degree of
acidity has been secured, it is subjected to a rennet test. In this
test 160 cubic centimeters of the milk are heated to 30° and mixed
with five cubic centimeters of the rennet solution made by diluting
five cubic centimeters of the rennet of commerce with fifty cubic
centimeters of water. The number of seconds required for the milk
to curdle is noted. The observation is facilitated by distributing
throughout the milk a few fine fragments of charcoal. The contents of
the vessel are given a circular motion and, at the moment of setting,
the movement of the black particles is suddenly arrested. If coloring
matter be added to the milk, it should be done before it becomes sour.
The quantity of rennet required is determined by the nature of the
cheese which it is desired to make. For a cheese to be rapidly cured,
enough rennet should be added to produce coagulation in from fifteen
to twenty minutes, and when slow curing is practiced in from thirty
to forty-five minutes. When the mass is solid so that it can be cut
with a knife, the temperature is raised to 37°, and it is tested on a
hot iron until it forms threads an eighth of an inch in length. This
test is made by applying an iron heated nearly to redness to the curd.
When the curd is in proper condition threads from a few millimeters to
two centimeters in length are formed, when the iron is withdrawn. The
longer threads indicate, but only to a limited extent, a higher degree
of acidity.[517] This test is usually made about two and one-half hours
from the time of coagulation. The whey is then drawn off through a
strainer and the curd is placed on racks with linen bottoms in order
that the residual whey may escape, the curd being stirred meanwhile.
In from fifteen to twenty minutes it can be cut into blocks eight or
ten inches square and turned over. This is repeated several times in
order to facilitate the escape of the whey. When the curd assumes a
stringy condition, it is run through a mill and cut into small bits and
is ready for salting, being cooled to 27° before the salt is added.
From two to three pounds of salt are used for each 100 pounds of curd.
The curd is then placed in the molds and pressed into the desired
form. The cheeses thus prepared are placed on shelves in the ripening
room and the rinds greased. They should be turned and rubbed every day
during the ripening, which takes place at a temperature of from 15° to
18°.[518]
=512. Official Methods of Analysis.=—The methods of cheese analysis
recommended by the Association of Official Agricultural Chemists
are provisional and are not binding on its members. They are as
follows:[519]
_Preparation of Sample._—Where the cheese can be cut, a narrow wedge
reaching from the edge to the center will more nearly represent the
average composition than any other sample. This should be chopped quite
fine, with care to avoid evaporation of water, and the several portions
for analysis taken from the mixed mass. When the sample is obtained
with a cheese trier, a plug perpendicular to the surface one-third of
the distance from the edge to the center of the cheese more nearly
represents the average composition than any other. The plug should
either reach entirely or half way through the cheese. For inspection
purposes the rind may be rejected, but for investigations where the
absolute quantity of fat in the cheese is required the rind should be
included in the sample. It is well, when admissible, to secure two or
three plugs on different sides of the cheese, and, after splitting them
lengthwise with a sharp knife, use portions of each for the different
determinations.
_Determination of Water._—From five to ten grams of cheese are placed
in thin slices in a weighed platinum or porcelain dish which contains
a small quantity of freshly ignited asbestos to absorb the fat. The
dish is heated in a water oven for ten hours and weighed; the loss in
weight is considered as water. If preferred, the dish may be placed in
a desiccator over concentrated sulfuric acid and dried to constant
weight. In some cases this may require as much as two months. The acid
should be renewed when the cheese has become nearly dry.
_Determination of Ether Extract._—Grind from five to ten grams of
cheese in a small mortar with about twice its weight of anhydrous
copper sulfate. The grinding should continue until the cheese is finely
pulverized and evenly distributed throughout the mass, which will have
a uniform light blue color. This mixture is transferred to a glass tube
having a strong filter paper, supported by a piece of muslin, tied over
one end. Put a little anhydrous copper sulfate into the tube next to
the filter before introducing the mixture containing the cheese. On top
of the mixture place a tuft of ignited asbestos, and place the tube in
a continuous extraction apparatus and treat with anhydrous ether for
fifteen hours. Dry the fat obtained at 100° to constant weight.
_Determination of Nitrogen._—The nitrogen is determined by the kjeldahl
method, using about two grams of cheese, and multiplying the percentage
of nitrogen found by 6.25 for proteid compounds.
_Determination of Ash._—The dry residue from the water determination
may be used for the ash determination. If the cheese be rich in fat,
the asbestos will be saturated therewith. This may be carefully ignited
and the fat allowed to burn, the asbestos acting as a wick. No extra
heat should be applied during this operation, as there is danger of
spurting. When the flame has died out, the burning may be completed in
a muffle at low redness. When desired, the salt may be determined in
the ash in the manner specified under butter (=498=).
_Determination of Other Constituents._—The sum of the percentages of
the different constituents, determined as above, subtracted from 100
will give the amount of organic acids, milk sugar etc., in the cheese.
=513. Process of Mueller.=—The process of Müller,[520] as modified by
Kruger,[521] is conducted as follows: About ten grams of a good average
sample of cheese are rubbed in a porcelain mortar with a mixture of
three parts of alcohol and one part of ether. After the mixed liquids
have been in contact with the cheese five or ten minutes they are
poured upon a weighed filter of from fifteen to sixteen centimeters
diameter, and this process is repeated from one to three times, after
which the contents of the mortar are brought upon the filter. The
filtrate is received in a weighed flask, the alcohol ether driven off
by evaporation and the residue dried. Since it is difficult to get
all the particles of cheese free from the mortar, it is advisable to
perform the above process in a weighed dish which can afterwards be
washed thoroughly with ether and alcohol and dried and the amount of
matter remaining thereon accounted for. The residue remaining in the
flask after drying is treated several times with pure warm ether,
and the residue also remaining upon the filter mentioned above is
completely extracted with ether. The dried residue obtained in this
way from the filter plus the residue in the flask which received the
filtrate, plus the amount left upon the dish in which the cheese was
originally rubbed up, constitute the total dry matter of the cheese
freed of fat. All the material soluble in ether should be collected
together, dried and weighed as fat.
By this process the cheesy mass is converted into a fine powder which
can be easily and completely freed from fat by ether, and can be dried
without becoming a gummy or horny mass.
For the estimation of the nitrogen, about three grams of the well
grated cheese are used and the nitrogen determined by moist combustion
with sulfuric acid.[522]
For the estimation of ash, about five grams are carbonized, extracted
with water, and the ash determined as described below.[523]
Char from two to three grams of the substance and burn to whiteness
at the lowest possible red heat. If a white ash cannot be obtained in
this manner, exhaust the charred mass with water, collect the insoluble
residue on a filter, burn, add this ash to the residue from the
evaporation of the aqueous extract and heat the whole to a low redness
till the ash is white.
=514. Separation of Fat from Cheese.=—It is often desirable to secure
a considerable quantity of the cheese fat for physical and chemical
examination without the necessity of effecting a complete quantitive
separation. In this laboratory this is accomplished by the method of
Henzold.[524] The cheese, in quantities of about 300 grams, is cut
into fragments about the size of a pea and treated with 700 cubic
centimeters of potash lye, which has previously been brought to a
temperature of about 20°. The strength of the lye should be such that
about fifty grams of the caustic potash are contained in each liter of
the solution.
The treatment is conveniently conducted in a wide neck flask and the
solution of the casein is promoted by vigorous shaking. After from five
to ten minutes, it will be found that the casein is dissolved and the
fat is found swimming upon the surface of the solution in the form of
lumps. The lumps of fat are collected in as large a mass as possible by
a gentle shaking to and fro. Cold water is poured into the flask until
the fat is driven up into the neck, whence it is removed by means of a
spoon.
The fat obtained in this way is washed a few times with as little cold
water as possible in order to remove the residue of potash lye which
it may contain. Experience shows that the fat by this treatment is
not perceptibly attacked by the potash lye. In a short time, by this
procedure, the fat is practically all separated and is then easily
prepared for chemical analysis by filtering and drying in the manner
already described (=283=). The fat may also be separated, but with less
convenience, by partially drying the sample, reducing it to a finely
divided state and applying any of the usual solvents. The solvent
is removed from the extract by evaporation and the residual fat is
filtered and prepared for examination as usual.
=515. Filled Cheese.=—The skim milk coming from the separators is
unfortunately too often used for cheese making. The abstracted fat
is replaced with a cheaper one, usually lard. These spurious cheeses
are found in nearly every market and are generally sold as genuine.
The purchasers only discover the fraud when the cheese is consumed.
Many of the States have forbidden by statute the manufacture and sale
of this fraudulent article. Imported cheeses may also be regarded
with suspicion, inasmuch as the method of preparing filled cheese is
well known and extensively practiced abroad. A mere determination of
the percentage of fat in the sample is not an index of the purity of
the cheese. It is necessary to extract the fat by one of the methods
already described and, after drying and filtering, to submit the
suspected fat to a microscopic and chemical examination. A low content
of volatile fat acid and the occurrence of crystalline forms foreign to
butter will furnish the data for a competent judgment.
When the reichert-meissl number falls below twenty-five the sample
may be regarded with suspicion. The detection of the characteristic
crystals of lard or tallow is reliable corroborating evidence (=308=).
It is stated by Kühn[525] that the margarin factory of Mohr, at
Bahrenfeld-Altona, has made for many years a perfect emulsion of fat
with skim milk. This product has been much used in the manufacture of
filled cheese which is often found upon the German market.
=516. Separation of the Nitrogenous Bodies in Cheese.=—The general
methods of separation already described for proteid bodies (=417-425=)
are also applicable to the different nitrogenous bodies present
in cheese, representing the residue of these bodies as originally
occurring in the milk, and also the products which are formed therefrom
during the period of ripening. For practical dietary and analytical
purposes, these bodies may be considered in three groups:
(_a_) The useless (from a nutrient point of view) nitrogenous bodies,
including ammonia, nitric acid, the phenylamido-propionic acids,
tyrosin, leucin and other amid bodies.
(_b_) The albumoses and peptones, products of fermentation soluble in
boiling water.
(_c_) The caseins and albuminates, insoluble in boiling water.
The group of bodies under (_a_), according to Stutzer, may be separated
from the groups (_b_) and (_c_) by means of phosphotungstic acid. For
this purpose a portion of an intimate mixture of fine sand and cheese
(100 cheese, 400 sand) corresponding to five grams of cheese, is
shaken for fifteen minutes with 150 cubic centimeters of water. After
remaining at rest for another fifteen minutes 100 cubic centimeters of
dilute sulfuric acid (one acid, three water) are added, followed by
treatment with the phosphotungstic acid as long as any precipitate is
produced. The mixture is thrown on a filter and the insoluble matters
washed with dilute sulfuric acid until the filtrate amounts to half
a liter. Of this quantity an aliquot part (200 cubic centimeters) is
used for the determination of nitrogen. From the quantity of nitrogen
found, that representing the ammonia, as determined in a separate
portion, is deducted and the remainder represents the nitrogen present
in the cheese as amids.[526]
_Albumoses and Peptones._—Albumoses and peptones are determined in
cheese by the following method:[527] A quantity of the sand mixture
already described, corresponding to five grams of the cheese, is
treated with about 100 cubic centimeters of water, heated to boiling,
and the clear liquid above the sand poured into a flask of half a liter
capacity. The extraction is continued with successive portions of water
in like manner until the volume of the extract is nearly half a liter.
When cold, the volume of the extract is completed to half a liter with
water, the liquor filtered, 200 cubic centimeters of the filtrate
treated with an equal volume of dilute sulfuric acid (one to three) and
phosphotungstic acid added until no further precipitate takes place.
The nitrogen is determined in the precipitate after filtration and
washing with dilute sulfuric acid.
_Casein and Albuminates._—The quantity of casein and albuminates in
cheese is calculated by subtracting from the total nitrogen that
corresponding to ammonia, amids, that in the indigestible residue
and that corresponding to the albumose and peptone. In three samples
of cheese, _viz._, camembert, swiss, and gervais, Stutzer found the
nitrogen, determined as above, distributed as follows:[528]
Camembert. Swiss. Gervais.
N as ammonia 13.0 3.7 1.6
N as amids 38.5 9.0 5.2
N as albumose peptone 30.5 8.6 15.5
N indigestible 4.0 2.4 8.6
N as casein, albuminates 14.0 76.3 69.1
_Ammoniacal Nitrogen._—The ammoniacal nitrogen is determined by mixing
a quantity of the sand-cheese corresponding to five grams of cheese,
with 200 cubic centimeters of water, adding an excess of barium
carbonate and collecting the ammonia by distillation in the usual way.
_Digestible Proteids._—The digestible proteids in cheese are determined
by the process of artificial digestion, which will be described in the
part of this volume treating of the nutritive value of foods.
These data show the remarkable changes which the proteids undergo where
the ripening is carried very far as in the camembert cheese.
=517. Koumiss.=—Fermented mare milk has long been a favorite beverage
in the East, where it is known as koumiss. In Europe and this country
cow milk is employed in the manufacture of fermented milk, although it
is less rich in lactose than mare milk. The process of manufacture is
simple, provided a suitable starter is at hand. A portion of a previous
brewing is the most convenient one, the fermentation being promoted by
the addition of a little yeast. After the process of fermentation is
finished the koumiss is placed in bottles and preserved in a horizontal
position in a cellar, where the temperature is not allowed to rise
above 12°.
=518. Determination of Carbon Dioxid.=—The carbon dioxid in koumiss is
conveniently estimated by connecting the bottle by means of a champagne
tap with a system of absorption bulbs.[529] The exit tube from the
koumiss bottle passes first into an erlenmeyer, which serves to break
and retain any bubbles that pass over. The water is next removed by
means of sulfuric acid. The koumiss bottle is placed in a bath of water
which is raised to the boiling point as the evolution of the gas is
accomplished. The arrangement of the apparatus is shown in Fig. 113. At
the end of the operation any residual carbon dioxid in the apparatus
is removed by aspiration after removing the tap and connecting it
with a soda-lime tube to hold the carbon dioxid in the air. A large
balance suited to weighing the koumiss bottle is required for this
determination. The carbon dioxid may also be determined, but less
accurately, by loss of weight in the koumiss bottle after adding weight
of water retained in the apparatus.
=519. Acidity.=—Although koumiss may contain a trace of acetic acid,
it is best to determine the acid as lactic. The clarification is
most easily accomplished by mixing the koumiss with an equal volume
of ninety-five per cent alcohol, shaking and filtering. The first
filtrate will usually be found clear. If not it is refiltered. In an
aliquot part of the filtrate the acidity is determined by titration
with tenth-normal sodium hydroxid solution, using phenolphthalein as
indicator. The necessary corrections for dilution and volume of the
precipitated casein are to be made. A linen filter may be used when
paper is found too slow.
[Illustration: FIG. 113. APPARATUS FOR DETERMINING CARBON DIOXID IN
KOUMISS.]
=520. Alcohol.=—Half a liter of koumiss, to which 100 cubic centimeters
of water have been added, is distilled until the distillate amounts to
500 cubic centimeters.
If the distillate be turbid 100 cubic centimeters of water are added
and the distillation repeated. The alcohol is determined by the
processes described hereafter.
=521. Lactose.=—The milk sugar may be determined by any of the methods
described, but most conveniently by double dilution and polarization
(=86=).
=522. Fat.=—Evaporate twenty grams of the sample to dryness and extract
with pure ether or petroleum spirit in the manner already described
(=455=).
The analysis is more quickly accomplished by the volumetric method of
Babcock or Gerber (=473-475=).
=523. Proteids.=—The total proteids are most easily estimated by the
official kjeldahl method.[530] The separation of the proteid bodies is
accomplished by the methods described in paragraphs =475-489=.
In addition to the methods already described for separating the soluble
and suspended proteid bodies in milk, and which may be used also for
koumiss, the following should also be mentioned as of especial worth:
_Separation by Filtration through Porous Porcelain._—A purely physical
method, and one which is to be recommended by reason of the absence
of any chemical action upon the different proteid matters, is that
proposed by Lehmann, depending upon the principle that when milk is
forced through porous porcelain, the albumin passes through together
with the milk, sugar and other soluble constituents as a clear
filtrate, while the casein and fat are perfectly retained.[531]
By this method it is quite certain that the albumin and other perfectly
soluble proteids of milk may be obtained in the purest form.
_Separation by Precipitation with Alum._—Probably the best chemical
method of separating the two classes of proteid matters is that
proposed by Schlosmann, which is effected by means of precipitating the
casein with a solution of alum.[532]
The principle of this separation rests upon the fact that a solution
of potash alum, when added to milk diluted with four or five times its
volume of water, will completely separate the casein without affecting
the albumin or globulin. The operation is conducted as follows:
Ten cubic centimeters of the milk are diluted with from three to five
times that quantity of water and warmed to a temperature of about
40°. One cubic centimeter of a concentrated solution of potash alum
is added, the mixture well stirred and the coagula which are formed
allowed to subside. If the coagulation of the casein does not take
place promptly, a small addition of the alum solution is made, usually
not exceeding half a cubic centimeter, until the precipitation is
complete. The temperature during the process should be kept as nearly
as possible 40°. After a few minutes, the mixture is poured upon a
filter and the filtrate, if not perfectly clear, is poured back until
it is secured free of turbidity. In difficult cases the filtration may
be promoted by the addition of some common salt or calcium phosphate,
the latter acting mechanically in holding back the fine particles of
casein. The precipitate is washed with water at a temperature of 40°,
and afterwards with alcohol, not allowing the alcohol wash water to
flow into the filtrate. When the water has been chiefly removed from
the precipitate by washing with alcohol, the fat of the precipitated
casein is removed with ether and the residue used for the determination
of nitrogen in the usual way. The albumin is removed from the filtrate
by a tannin solution in the manner already described (=480=). If it be
desired to separate the albumin and globulin, the methods described in
paragraph =399= may be used.
=524. Mercurial Method.=—A volumetric method for determining the total
proteid matter in milk has lately been proposed by Deniges.[533] It is
based upon the observation that in the precipitation of proteid matter
by mercury salts, a definite quantity of mercury in proportion to the
amount of proteid, is carried down therewith. The precipitation is made
with a mercurial salt of known strength and the excess of the mercurial
salt in the filtrate is determined by titration. For the details of the
manipulation, the paper cited above may be consulted.
=525. Water and Ash.=—From two to five grams of the koumiss are dried
to constant weight in a flat platinum dish over ignited sand, asbestos
or pumice stone, and the dried residue incinerated.
=526. Composition of Koumiss.=—The composition of koumiss varies with
the character of the milk used and the extent of the fermentation. Some
of the data obtained by analysts are given below:[534]