CHAPTER III
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 | 87.50 |
| Total solids | 12.5 |
| —— | |
| 100.0 | |
| —— |
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:
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.
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 | 100.0 | 100.00 |
| Specific Gravity | 1.032 | 1.0315 | 1.033 | 1.033 | 1.038 | 1.034 | 1.033 | ... | ... |
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
Fig. 5. The Creamometer 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.
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.
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.
The simplest method of testing the acidity is to procure a few little books of blue and red litmus
Fig. 7. Lactometer and Test Tube 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.