THE CHEMISTRY OF MILK
_The Composition of Milk._--Like all other organic substances, or those
built up in connection with the life processes of plants and animals,
milk is of complex composition. It is also very liable to change--every
one is acquainted with its tendency to "go bad." This instability is
more or less inherent in all highly organised chemical compounds, and,
indeed, it seems to be necessary that the materials used in growth and
nutrition should be very plastic in a chemical sense, in order, _e.g._,
that the constituents, say of a plant, may easily be transformed into
the substances of the body of the animal which feeds on it.
The perishable nature of milk--the food of young and growing animals--is
therefore essential, so that it may be changed easily into the blood,
bone, muscle, etc., so abundantly required in the early stages of
existence.
Milk is a complete food, and, therefore, naturally it is not a simple
chemical compound, but a mechanical mixture of a number of substances.
The present state of chemical knowledge on the subject does not permit
of its composition being given in detail, but for practical purposes,
such as those of measuring its purity and food value, this is not
necessary.
A proximate analysis, in which, at least, some of the ingredients are
lumped together, is sufficient, and has been adopted everywhere by
analysts. On this basis the average composition of cows' milk may be
stated as follows:
Per cent.
Water 87.50
Fat 3.50
Casein and albumen 3.65
Milk sugar 4.60
Ash 0.75
----
100.00
------
The constituents other than water added together form the "total
solids," and they amount to 12.5 per cent.
Per cent.
Water 87.5
Total solids 12.5
----
100.0
-----
[Illustration: THE CONSTITUENTS OF MILK
In the illustration, a pint of milk is shown in a glass
jar, and the various percentages of water, casein, sugar,
ash, albumen, and fat, which make up its constituent
parts, are shown in separate bottles, the percentage of
each being stated beneath.]
Milk varies a good deal in composition; the different breeds of cows
give varying qualities. The Short-horn gives large quantities of milk of
rather poor analysis, while the Jersey yields smaller proportions of
very rich milk. During the period of lactation (the time which has
elapsed since the cow gave birth to a calf), care in milking, food,
health, etc., all have an effect on the quality of the milk.
The limits of variation may be stated as follows:
Per cent. Per cent.
Water 87.5 to 82.5
Fat 2.5 " 6.0
Casein and albumen 3.0 " 4.5
Milk sugar 3.5 " 6.0
Ash 0.6 " 0.8
These figures are extreme, and it is very seldom indeed that either the
minimum or maximum is reached. Indeed, by the regulation laid down under
Clause 4 of the British Sale of Food and Drugs Act of 1899, when the
percentage of solids not fat falls below 8.5 per cent., and fat under 3
per cent., it is assumed that the milk has been adulterated. This
regulation is a perfectly just one. While genuine milk may, in rare
instances, show figures as low as 7.1 per cent. of solids not fat, or
2.5 per cent. of fat, the right can hardly be claimed of supplying such
an abnormal article to the public as milk of proper quality, and the
dairyman who understands his business, and wishes to deal fairly with
his customers, can, by attention to the conditions enumerated above
which influence the composition of milk, entirely avoid the production
of such a low-grade article.
In the nutrition of both plants and animals large quantities of water
are needed. The solids must be supplied in solution or dissolved in the
assimilative processes, and this cannot take place without water, which
also conveys the dissolved solids to the various parts of the economy,
and in the case of animals removes waste materials. For the most part,
water passes through the body unchanged, but a certain proportion unites
chemically with the food materials and assists in their digestion. It is
therefore not surprising that seven eighths of milk is composed of
water. Blood contains a similar proportion, and this agreement
emphasises the fact that milk is a perfectly balanced food.
The fat of milk, which yields cream and butter, differs in some
important respects from other fats. Like these, it is made up chiefly
of stearin, palmitin, and olein, but, in addition, it contains an
abnormally large proportion of compounds of certain of the volatile
fatty acids. It is these which give to butter its agreeable flavour. By
the methods of Duclaux, the following is the approximate composition of
butter fat:
Per cent.
Stearin, palmitin, olein, and traces of
myristin and butin 91.50
Butyrin 4.20
Capronin 2.50
Caprylin, caprinin, and traces of laurin 1.80
-----
100.00
------
Myristin occurs in nutmegs; butyrin in another combination flavours
pineapples and rum; caprinin is found in cocoanut fat, mutton fat, and
in the offensive odour given off by the goat (from which the name is
derived); caprylin is a by-product of alcoholic fermentation, and also
occurs in cocoa fat; laurin is found in sweet bay; from which it is
evident that there are some curious relationships in flavouring
materials.
Fats are very concentrated foods, furnishing a large amount of energy to
the body. At one time they were classed together with starch, sugar,
and other carbohydrates as heat-producers, but the distinction which
was drawn between the kinds of food which were thought solely to keep up
the temperature of the organism, and those which produced force in work
and other forms of bodily energy, has broken down, and by direct
experiment has been found not to exist. It is usually calculated that
one part of fat is equal in food value to about two and a quarter parts
of any of the other carbohydrates. Milk fat or butter is more digestible
than almost any other fat, and its importance therefore can readily be
realised. All the above constituents of milk fat are composed of
different proportions of carbon, hydrogen, and oxygen, but milk also
contains minute quantities of lecithin, a fat containing phosphorus in
addition. Lecithin is also found in the brain and nerve material of
animals, in the yolk of egg, and in several plants.
The nitrogenous constituents of milk--casein and albumen--are usually
estimated together, and they are reckoned as of equivalent food value.
The name protein is very commonly applied to the total of these bodies
in milk, or other animal and vegetable foods. They are composed of
different proportions of carbon, hydrogen, oxygen, and nitrogen, with
small quantities of sulphur, while casein contains phosphorus in
addition. Albumen exists to the extent of about 0.6 per cent. in milk.
It is very similar in properties to egg albumen. The coagulum which
forms on the surface of milk when boiled is largely composed of albumen.
Casein is combined with, and kept in solution by, lime, soda, and
calcium phosphate, and its amount averages a little over 3 per cent.
The remarkable property possessed by rennet, of curdling or coagulating
casein, is well known; rennet is an extract from the stomach of the
calf, and similar principles are present in the stomachs of man and
other animals, so that the coagulation of milk is the first process in
its digestion. If milk is gulped down in large quantities it is apt to
coagulate in lumps, and digestion is much interfered with, but if it is
taken hot and slowly, it coagulates in small pieces which are readily
attacked by the gastric juice, and milk is then one of the most
assimilable of foods.
Nature provides that the milk for young animals is supplied in finely
divided streams, so that coagulation takes place in the best possible
way.
The proteids are the most important constituents of food; they are
abundant in the blood, and build up the muscles, brain, nerves, and
other bodily structures.
Besides these mentioned, milk contains traces of another proteid of
similar composition called globulin.
The sugar of milk is not found anywhere else. It is a carbohydrate like
cane and grape sugar--that is to say, the hydrogen and oxygen they
contain are in the same relative proportions as in water. Milk sugar is
not so soluble or so sweet as the other sugars. It does not ferment with
ordinary yeast, but certain special yeasts which are made use of in the
preparation of keffir, koumiss, etc., have the power of transforming it
into alcohol. Its most remarkable property, however, is the facility
with which, under the influence of certain bacteria, it is changed into
lactic acid.
Every one is familiar with the souring of milk, but perhaps it is not so
generally known that there are great differences in the results obtained
in accordance with the conditions under which the souring takes place.
The skilled butter-maker, by keeping the milk in a cool and cleanly
dairy, obtains a sour milk of a characteristic and agreeable aroma and
taste, which beneficially affect the flavour of the butter produced. On
the other hand, if milk is kept in hot and dirty surroundings, the
development of acidity is accompanied by different bad tastes and
odours, and it becomes unfit for use as a food. In the first case, the
conditions are favourable to the maximum production of the lactic acid
bacteria, and these occupy the field, and largely prevent the
development of the other bacteria which are present--the survival of the
fittest in the struggle for existence. In the second case, the impure
surroundings swarm with the germs of many kinds of putrefactive
bacteria, and the high temperature assists these to gain the upper hand.
Again, the survival of the fittest, in the particular conditions. Even
in cool and cleanly surroundings injurious taints may develop,
especially if the milk has previously been subjected to a journey by
road or rail, as is the case in the modern creamery system, where the
farmers deliver their milk to a central creamery, where it is made into
butter. In such establishments it is the regular practice to kill the
germs, lactic and others, existing in the milk, by heating it to a high
temperature. This process is called pasteurising, after the great French
chemist and bacteriologist who invented it. Pure lactic cultures are
added to the pasteurised milk, and the souring process is under exact
control, with the result that butter of uniform flavour and quality is
produced. The same method is made use of in making the special sour milk
described in this book, with, of course, modifications in the apparatus
employed, to suit the smaller scale in which the manufacture is
conducted.
The ash is the mineral matter which is left when milk, previously dried,
is burnt in a crucible. It is a complex mixture, and, as we have seen,
it amounts to about 0.7 per cent. of the milk. The process of burning
destroys all the organic matter, and, at the same time, alters somewhat
the state of combination of the inorganic or mineral elements. Attempts
have been made from the analysis of the ash to reconstitute the
composition of the mineral matter as it exists in the milk. The best
known is that of Soldner, and the following is his calculation:
Per cent.
Sodium chloride 10.62
Potassium chloride 9.16
Monopotassium phosphate 12.77
Dipotassium phosphates 9.22
Potassium citrate 5.47
Dimagnesium citrate 3.71
Magnesium citrate 4.05
Dicalcium phosphate 7.42
Tricalcium phosphates 8.90
Calcium citrate 23.55
Calcium oxide, in combination with casein 5.13
------
100.00
------
The presence of citrates will be noted in this analysis. Citric acid,
which gives to lemons their acidity, and is also found in other fruits,
has been proved to exist in milk to the extent of about 0.2 per cent.
When alkaline or earthy citrates are burnt or oxidised in the blood, the
citric acid is destroyed, and corresponding carbonates remain. No doubt
the function of citrates in milk is to furnish to the body the earthy
and alkaline carbonates which are required in certain of its parts.
The mineral constituents of milk have many important functions to
perform in the building up and nutrition of the bodily organism.
Phosphate of lime is the principal constituent of the skeleton, and the
blood must be richly supplied with the alkalies, earths, and acids which
are comprehended in the ash.
Milk contains traces of many other substances, the most important of
which are several enzymes which assist in its digestion.
_General Properties of Milk._--The appearance of milk is known to every
one; it ought to be a pure white opaque liquid, but very generally it is
tinted a cream colour with anatto to give it an added appearance of
richness. The average specific gravity is about 1.031; or, to put it
another way, while a gallon of pure water weighs exactly 10 lbs., a
gallon of milk weighs 10 lbs. 5 oz. It freezes at 31° F. and boils at
about one third of a degree higher than water.
When milk is examined under the microscope, the fat is found to be
distributed through it in a multitude of minute globules varying in size
from 1/16,000th to 1/25,000th part of an inch, and occasionally they are
much smaller and also much larger.
Fig. 1 is a micro-photograph showing the fat globules in whole milk.
Fig. 2 is a micro-photograph of separated milk, and Fig. 3 a
micro-photograph of cream, all under high magnification (450 diams.);
from these figures the comparative number of fat globules present may be
seen.
[Illustration: FIG. 1.--Micro-photograph of a Drop of
Whole Milk, showing distribution of fat globules.
(Magnified 450 diams.)]
[Illustration: FIG. 2.--Micro-photograph of Separated
Milk, showing the almost complete absence of fat globules
as compared with whole milk. (Magnified 450 diams.)]
[Illustration: FIG. 3.--Micro-photograph of Cream, showing
agglomeration of fat globules. (Magnified 450 diams.)]
Fats distributed through a watery liquid in this finely divided
condition form together what is called an emulsion, in which the
particles of fat are kept apart by surface tension. The specific gravity
of milk fat averages 0.93, and compared with water weighing 10 lbs., a
gallon of fat would weigh 9 lbs. 5 oz. It is thus considerably
lighter than the other constituents, and when milk is left at rest, the
fat globules gradually rise to the top and float there, forming cream.
The difference in specific gravity between cream and milk is taken
advantage of in the mechanical separator, now so much used, and which
makes such a thorough separation between the two. Cream is an article of
the most varied composition, according to the ideas of the person who
produces it, but it ought to contain at least 20 per cent. of butter
fat, and may be made with a much larger percentage if necessary. When
cream is agitated in a particular way, as by churning, the surface
tension of the particles is overcome, and they run together into a mass
which forms butter.
The casein of milk is not held in solution in the ordinary sense, but in
a peculiar state of suspension called the colloidal condition,
practically the whole of it remaining behind when milk is filtered
through clay filters.
It is this state of suspension of the casein which makes milk opaque,
but the opacity is considerably increased by the emulsified fat.
The coagulation of the casein in milk by the addition of rennet has
already been referred to. Acids, either mineral or organic, also
precipitate it in the form of flakes. Skimmed milk is now largely used
for the preparation of casein by this method, and the washed and dried
precipitate is used very extensively in the arts for such varied
purposes as the manufacture of billiard balls, paints, cements, etc.
The clear liquid which separates when milk is curdled with rennet is
called whey, and contains the milk sugar and mineral salts. The sugar is
manufactured from it on a limited scale, and is used as an ingredient in
infant foods, and as a convenient medium in certain medical
preparations. In Sweden a kind of cheese is made from whey, but the
great bulk of it everywhere is used for feeding pigs.
The comparative composition of different varieties of milk is given in
the following table:
-------------------------------------------------------------------------
Human Cow Buffalo Goat Sheep Mare Ass Reindeer Whale
-------------------------------------------------------------------------
Water 88.32 87.75 82.57 86.34 81.08 90.38 90.30 67.7 60.47
Fat 3.43 3.40 7.63 4.25 7.67 1.00 1.30 17.1 20.00
Protein 1.55 3.50 4.69 4.40 6.08 1.98 1.80 10.9 12.42
Milk Sugar 6.44 4.60 4.30 4.26 4.26 6.28 6.20 2.8 5.63
Salts 0.26 0.75 0.81 0.75 0.91 0.36 0.40 1.5 1.48
-------------------------------------------------------------------------
Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00
-------------------------------------------------------------------------
Specific 1.032 1.0315 1.033 1.033 1.038 1.034 1.033 ... ...
Gravity
-------------------------------------------------------------------------
[Illustration: FIG. 4 is a photograph of two Petri dishes,
which have been inoculated with ordinary milk (A), and
milk that has been subjected to sterilisation (B). The
whitish bacterial colonies on A are due to enormous
numbers of organisms, while B is quite free from such
growth.
For the production of a reliable lactic food, it is
essential that certain precautions as to the treatment of
the milk, and the maintenance of a suitable temperature
during the growth of the lactic bacteria, should be
observed.
In the first place, milk immediately after extraction from
the cow contains only a few organisms, but these multiply
so rapidly that in a few hours the bacterial content may
amount to many millions per ounce. In preparing a pure
culture of any specific organism, then, care must be taken
to destroy all the bacteria that have accidentally found
their way into the milk, inoculating with the organisms it
is desired to cultivate. This is best accomplished by
heating the milk to the boiling-point of water for about
thirty minutes, by which time almost all the undesirable
bacteria have been killed.]
The milk of the cow differs a good deal from human milk, and where the
former is used for the feeding of children it is usual to add milk sugar
to it, and otherwise alter it to bring its composition more in harmony
with the human article. The high concentration of the milk of the
reindeer and the whale is noteworthy. Perhaps this may be due to the low
temperature conditions in which these animals live, necessitating strong
nutriment to enable their young to make proper progress in growth and
development. On the other hand, the milk of the ass is poor in quality,
and probably on this account it is more readily assimilated by those of
weak digestion, to whom it is sometimes recommended. Goats' milk is
richer than either cow or human milk, and its nourishing properties are
well known. The goat is usually free from tuberculosis and other
diseases which affect the cow, and its milk is therefore a very safe
article to use.
_The Analysis of Milk._--While the analysis of milk can only be made by
a competent chemist, there are a number of simple tests and observations
by which any intelligent person can obtain a fair idea of its quality.
The taste and smell afford some guide, as also the general appearance.
To judge of the latter, place some of the milk in a tumbler or other
clear glass vessel. If the milk is of good quality it will be quite
homogeneous and opaque. Any flocculent matter indicates either disease
in the cow or that the milk is old and bacteria have multiplied in it
and altered its composition. When the milk has stood long enough for the
cream to rise freely, the latter should form a perfectly homogeneous and
strongly defined layer on the top. The quantity of cream may be measured
in a creamometer, which consists of a small glass cylinder graduated at
the top (Fig. 5). It is filled with milk to the top graduation line, and
when the cream has risen, the percentage quantity of the latter which
has separated can be taken off.
[Illustration: FIG. 5. The Creamometer]
The colour should be like that of porcelain, but, as already stated, it
is a common thing for the dairyman to add a small quantity of anatto or
an aniline dye of a similar shade, to give the milk a rich creamy tint.
If the milk is of a reddish colour this may be caused by blood from the
udder, although certain foods, such as beets, mangels, and carrots
sometimes give a similar tint. The milk given by cows immediately after
calving is called "colostrum" or "biestings," and is of a yellow or
yellow-brown colour. It is much thicker than ordinary milk, and
coagulates in boiling.
In dirty byres in which care is not taken in milking, quite considerable
quantities of hairs, pieces of manure, and other filth may get into the
milk. Usually the milk is strained by the dairyman, but sometimes this
is omitted or carelessly done. To test for dirt, a ribbed glass funnel
is useful. Get a piece of the finest muslin about twice the diameter of
the funnel, fold over twice, so that it becomes one quarter of its
original size; open one of the sections and place in the funnel; pass
the milk into this. It will run through quickly and some water may be
run into the funnel to clear away the last traces of milk. The filter
cloth can then be opened out and any dirt retained will become visible.
The apparatus is shown in Fig. 6.
[Illustration: TESTING-GLASS FOR EXTRANEOUS MATTER IN
MILK.
FIG. 6.--A piece of muslin is folded as shown and a
measured quantity of milk is passed through the funnel;
from the sediment left in the muslin, the percentage of
extraneous matter may be arrived at.]
If a glass funnel is not available, a very small jelly bag can be made
of fine gauze and used in the same way. The washing water should be used
in small quantities and directed to concentrating the dirt in the apex
of the bag. After washing, the latter can be turned outside in, to
permit of readier examination of the dirt. The bag should be well
washed in cold water, then boiled and dried, and is then ready for
future use.
The acidity of milk is a very useful guide to its age. Milk has the
curious property of being "amphoteric," _i.e._, it is both slightly acid
and slightly alkaline when fresh. As its age increases, however, so does
its acidity, and at a rate varying with the temperature and moisture
contents of the atmosphere in which it is placed. Old and acid milk is
heavily contaminated with bacteria, a proportion of which are likely to
be injurious to health.
[Illustration: FIG. 7. Lactometer and Test Tube]
The simplest method of testing the acidity is to procure a few little
books of blue and red litmus test papers, and these can be had from any
philosophical instrument maker or laboratory furnisher. The strips of
test paper are torn out and dipped in the milk. When the milk is quite
fresh it will, owing to its amphoteric condition, change the red litmus
paper slightly blue, and the blue litmus paper slightly red. Old milk
changes blue litmus paper to a bright red because of its decided
acidity.
The above tests do not indicate if the milk is poor or rich, but this
can be determined by the lactometer, an instrument for ascertaining in a
simple way the specific gravity. The lactometer is shown in Fig. 7.
It is graduated usually from 25° to 36°, corresponding to specific
gravities 1.029 to 1.038. It is graduated to degrees and half degrees.
Sometimes a thermometer is combined with the instrument. The specific
gravity rises as the temperature is lowered and decreases with increase
of temperature, so that it is important to make the test at the figure
at which the lactometer was graduated, which is usually 60° F. Failing
this, an allowance has to be made for higher or lower temperatures. The
milk to be tested is well mixed, and placed in a deep vessel, and the
lactometer placed in it, holding it at first at an angle. It stands
upright and remains deeper or higher according to the specific gravity.
The reading is taken on the stem at the level of the milk. As the latter
is drawn up a little round the stem, about a half degree should be added
on to get the true figure. Thus, if the apparent reading is 31, the true
reading may be taken as 31.5. This is the average figure for good milk,
corresponding to a specific gravity of 1.0315; anything above this is
all to the good. Lower readings mean inferior quality, the latter being
proportionate to the lowness of the readings. The tests are most
conveniently made in a glass cylinder (Fig. 7), which may be purchased
with the lactometer. As there are many inaccurate instruments in the
market, it is necessary to go to a reputable maker, because an
unreliable lactometer is worse than useless.
The following table gives, in a condensed form, the allowances to be
made when the temperature is above or below the standard (60° F.):
------------+----------------------------------------------------------
Temperature.| Reading of Lactometer.
------------+------------------------------------------------------------
Degs. F. |
40 | 23.5 24.5 25.5 26.4 27.3 28.2 29.1 30.0 31.0 31.9 32.8 33.7
45 | 23.8 24.8 25.9 26.8 27.8 28.6 29.3 30.4 31.3 32.3 33.2 34.2
50 | 24.1 25.1 26.1 27.0 28.0 29.0 29.9 30.9 31.8 32.8 33.7 34.7
55 | 24.5 25.5 26.5 27.5 28.5 29.5 30.4 31.4 32.4 33.4 34.3 35.3
------------+------------------------------------------------------------
60 | 25.0 26.0 27.0 28.0 29.0 30.0 31.0 32.0 33.0 34.0 35.0 36.0
------------+------------------------------------------------------------
65 | 25.5 26.6 27.6 28.7 29.6 30.7 31.7 32.8 33.8 34.8 35.8 ...
70 | 26.1 27.2 28.2 29.3 30.2 31.3 32.4 33.4 34.5 35.5 36.5 ...
75 | 26.8 27.8 28.8 29.9 30.8 32.1 33.1 34.2 35.2 36.3 ... ...
80 | 27.4 28.4 29.5 30.7 31.6 32.8 33.9 35.9 36.1 ... ... ...
------------+------------------------------------------------------------
Thus if the thermometer indicates 40° F., and the lactometer 29.1°, the
true reading at the standard temperature of 60° F. is 31°, corresponding
to a specific gravity of 1.031. Intermediate figures can readily be
averaged. Care should be taken to wash the lactometer with cold water
under the tap, as otherwise the milk will dry on it and render it
inaccurate.