materials to the food as fenugreek, anise, and caraway seed is to be recommended. According to Temesvary (57a) beer increases the amount of milk fat. Morgen, Beger, and Fingerling (58) have also investigated the influence of fat and other substances on milk production when fed in connection with a scanty basal meal. They have observed an increase in the yield of milk and an increase in the percentage of fat amounting to 0.14 per cent when such quantities of fat were added to the food. The addition of large quantities of fat to the food caused a further increase in the yield of milk, but was found to vary in its effect on milk-fat production, sometimes causing an increase, sometimes a decrease. Caspari (59) has shown that iodized fats appear in the milk even though the food be poor in fats and rich in carbohydrates. He therefore concludes that some of the fat of milk comes from the fat of the food. Later (60) he showed that when iodocasein and iodoalbumin are fed to an animal no traces of iodized fats appear in the milk. On the other hand there are those who hold that the addition of fat to food does not increase the quantity of fat in milk and that there is no direct migration at least of the fat of the food to the milk. Such a conclusion was arrived at by Einecke (61) from his experiments with goats. With liberal comprehensive rations the yield of milk and fat depends, according to this observer, on the individuality of the animal. The milk from cows grazing off the poor, dried-up grass on the plateau of Sétif, in Algeria, has been compared by Malméjac with that of cows fed on rich forage with the following results:

Except for the proteids, the differences in composition are obviously in favor of the milk produced on the richer diet. Woll (63) observed that as a food for milch cows silage increases the yield of milk and butter 3 per cent over that produced with maize fodder when the area of land required to produce the two foods is taken into account. Some studies have also been made of the effects of certain definite nitrogen and phosphorus compounds on the production of milk and milk fat. Morgen, Beger, and Fingerling (55) have investigated the effect of adding lecithin to food. This substance seemed to increase the yield of milk and also the live weight of the animal. It was found, however, to be favorable to the production of milk fat only

when it was fed in conjunction with other foods deficient in fat. Pfeiffer, Einecke, and Schneider (64) have shown that asparagin when substituted for proteids, along with cane sugar, caused no diminution in the yield of milk, in fact in some instances it seemed to cause an increase, but the amount and percentage of fat in milk was diminished. The feeding of this compound also acted unfavorably on the increase in live weight, and caused a reduction in the percentage of proteids and dry matter in the milk. Morgen, Beger, and Westhauser (65) have reached the conclusion that amino compounds can not take the place of proteids in milk production, but that they exert a greater effect than carbohydrates.

It has been observed that the actual yield of milk diminishes in the later period of lactation. According to Trunz (66), however, the specific gravity of the milk, and most of the solids, including the proteids, are relatively increased, while the proportion of albumin to casein remains remarkably constant throughout the entire period of lactation. This same investigator (67) has also made an exhaustive study of the mineral constituents of cow's milk and their variation during the period of lactation with the result that he has found considerable variation in the ash contents during the lactation. period and that the total quantity of ash varies from time to time throughout the period, being as a rule less during the spring and summer months than during the autumn and winter months.

Hardy (68) claims to have shown that the milk of a given cow varies in composition at the different stages of milking. Thus taking the milk in quantities of one-half liter at a time the milk of one cow gave the following successive numbers for fat: 2.2, 2.9, 3.5, 3.75, 3.8, 3.9, and 4.65 per cent.

The solids rose from 10.52 to 12.70 and the ash from 0.74 to 0.75 per cent. The composition of the milk serum was found to remain the same throughout the milking. On the other hand Ackermann (69) claims that the conclusion that the fat increases in milk regularly during the process of milking, as this is ordinarily carried out, is incorrect. He has found, however, that by milking the teats singly or in pairs the fat did show an increase up to a maximum at the end of the milking and that on drawing the milk from the second pair of teats the quantity of fat was slightly more at the commencement than that given by the first pair and rose at the end of the milking to a higher maximum. The increase is probably due to a mechanical or physiological stimulus.

The effect of work and fatigue on the quantity and quality of the milk has also been studied by several observers. Hill's (70) results would seem to show that there is a slight falling off in the quantity of milk produced as a result of fatigue, 122.5 pounds against 131.4

24907-Bull. 41-08-21

pounds after rest. The total solids and the fat were found to be slightly higher during the period of fatigue than after rest. Dornic (71) also has shown that the yield of milk is diminished slightly as the result of work. The dry matter and the amount of acid were slightly increased. It was further observed by this investigator that work exerts a harmful influence on the quality of the milk, especially on its keeping qualities. For example it was found in the case of a certain cow that ordinarily her milk curdled when the acidity reached 70°-75°, whereas the milk of the same cow when fatigued by work, curdled when the acidity reached 45°. According to Moerman (72) also, work lessens the amount of milk secreted and raises the proportion of solids. The differences, however, in the quantity and quality of the milk in all of these investigations were only slight, indeed in some instances the results obtained were not very definite.

PART II-CHANGES IN THE COMPOSITION OF MILK.

The

On account of the milk sugar and proteids which milk contains, it is an exceedingly unstable liquid. When first drawn from the cow, it has a characteristic odor and a sweet taste. Even in the perfectly fresh state, it reacts acid to phenolphthalein. The acidity of fresh milk is due primarily to carbonic acid and acid phosphates and also in part to dicalcium caseinogenate. According to Thörner (1) the acidity of fresh milk varies between 12 and 16 degrees. According to Richmond (2) it has an acidity of 20 degrees. On standing exposed to the air for some time it gradually loses its sweet taste. sugar of milk is gradually transformed into lactic acid through the action of bacteria. The milk becomes sour to the taste and ultimately clots or curdles as the result of the precipitation of the caseinogen by the combined action of acids and soluble calcium salts. Stokes (3) gives figures and tests to show that milk having an average acidity of 44 degrees, corresponding to 0.396 per cent of lactic acid, tastes sour. According to Richmond (2) milk tastes sour when the acidity reaches 45 degrees, corresponding to 0.405 per cent lactic acid, and when it has an acidity of 85 degrees, equivalent to 0.765 per cent of lactic acid, it curdles at ordinary temperatures.

Under certain conditions, milk may also develop rancid and cheesy odors which render it quite disagreeable.

The principal changes occurring in milk are those produced by— (1) The action of heat and acids.

(2) The action of milk enzymes.

(3) The action of the digestive enzymes.

(4) Bacteria and various other micro-organisms.

PART II, (1) CHANGES IN MILK PRODUCED BY THE ACTION OF HEAT

AND ACIDS.

When milk is heated a film or skin forms on the surface, which, according to Jamison and Hertz (4), is due to the drying and coagulation of a part of the proteids which the milk contains. They have shown that such a skin may be formed on the surface of any albuminous' solution containing fat or paraffin. Rettger (5) also has arrived at the conclusion that its formation is dependent on the presence of proteid. This proteid is caseinogen. Surface evaporation and the presence of fat facilitate its formation although neither is absolutely essential. According to Harris (6), also, the scum of boiling milk consists very largely of caseinogen. It is also well known that certain changes occur in the odor and taste of milk as the result of boiling. These changes seem to be due to the partial decomposition of certain of the proteids with the liberation of a volatile sulphide, probably hydrogen sulphide. That such is the case has been proven by Rettger (7), and also by Franz Utz (8). According to the former, when milk is heated to 85° C., a volatile substance, probably hydrogen sulphide, is liberated. The amount of this, though small, suffices to blacken lead acetate paper and to decolorize dilute solutions of potassium permanganate. He found that alkalis and alkali phosphates accelerate the formation of the sulphide, whereas acids and acid phosphates retard this change. According to this author this change is believed to indicate proteid decomposition, and may partly account for what some observers describe as the injurious effect of heating milk. These observations have been confirmed by Utz (8), who was able to recognize the hydrogen sulphide resulting from the boiling of milk by lead acetate paper and also by Ganassini's reagent.

When milk is boiled there seems also to be a partial fixation of the calcium salts which it ordinarily contains. These are probably partially precipitated in the form of tricalcium phosphate. This would account for the fact that the coagulation of milk by rennin takes place more slowly in boiled milk than in unboiled milk. (See p. 332.) In this connection Wassermann and Schütze (9) have pointed out that cooked milk is not coagulated by lactoserum. According to P. T. Müller (10) the fact that cooked milk can not be coagulated by lactoserum is in some way associated with a diminution in the quantity of soluble calcium salts contained in the milk, this diminution having been caused by the action of heat. On the other hand, both Moro and Müller (10) have observed that certain milks do not show any diminution in coagulability by lactoserum after boiling. According to Müller (10) this is to be attributed to the large amount of soluble calcium salts present in the milk of certain particular

localities, and in this connection he has observed that the coagulability by lactoserum may be restored to boiled milk by the addition of soluble calcium salts.

Hammarsten observed that milk curdles when it is heated to 130° to 150° C. (see p. 344). Cazeneuve and Haddon (11) observed that milk which had been coagulated at 130° C. became very acid. According to these observers it then contained formic acid. They also reached the conclusion that the discoloration and coagulation of milk by heat is due to the oxidation of lactose in the presence of the alkaline salts of the milk, one product of the oxidation consisting of formic acid, which, like other acids, precipitates the caseinogen. The latter undergoes no further change except that it is discolored by the products of the decomposition of lactose.

Bruno Bardach (12) has also studied the coagulation of milk by heat. He found that about twelve hours' heating at 100° C. was required in order to coagulate perfectly fresh milk, whereas at 150° C. it coagulates in three minutes, and at 130° C. in one hour. He found only the merest traces of formic acid to be formed at 130° C. He concludes from his study of the subject that the coagulation of milk by heat is a complex process; that it is brought about by the action at the high temperature of the small quantities of acid which are formed from the lactose, and which ordinarily are powerless to coagulate the original unchanged casein (caseinogen), and that it is only after the casein (caseinogen) has been changed by the action of heat that such small amounts of acid can cause its coagulation.

The part played by calcium salts in the acid coagulation of milk has been studied by Loevenhart (13). According to this author the very small quantities of acid required to effect the coagulation of milk at temperatures at or below boiling accomplish this change by rendering the calcium salts normally present in milk available for the coagulation of the caseinogen. Therefore the temperature at which a given specimen of slightly sour milk will coagulate on heating depends partly upon the degree of acidity and also upon the nature and amount of the calcium salts present in the milk.

Von Soxhlet (14) has also recently investigated the coagulation which occurs on boiling faintly acid milk. He observed that at the commencement of the souring of milk boiling causes a .coagulum to form. This occurs when only one-eighth of the amount of acid necessary to produce coagulation at ordinary temperatures is present. It depends, according to this author, on the formation of an insoluble compound of caseinogen with soluble calcium salts, the acid first produced forming monocalcium phosphate from the dicalcium phosphate present in the fresh milk.

The fact that milk occasionally curdles in the pasteurizing apparatus during pasteurization makes the accumulation of data bearing