some heat-treated milks and its absence in others could be understood.

Without attempting to review completely the work done to demonstrate the presence or absence of the vitamins in preserved milks, attention may be called to some of the more important investigations.

DRIED MILK.

Vitamins A and B.-In 1913, before it had become an established fact that vitamins are necessary constituents of food for normal growth, Osborne and Mendel (3) showed that a food mixture consisting of milk powder, starch, and lard contained all that was essential for the normal existence of rats through at least two generations. This experiment, when viewed from the standpoint of our present knowledge of the vitamin requirements of rats, may be regarded as the first demonstration of the presence of vitamins A and B in powdered milk.

Although vitamin B is probably never entirely absent from the dried milk, the results of various investigations indicate that it is present in varying amounts. Thus McCollum and Davis (4) demonstrated that 2 per cent of milk powder added to a ration complete except for vitamin B supplied enough of this factor for almost normal growth of rats; Osborne and Mendell (5) found that when dried milk was used as a source of this vitamin at least 24 per cent of a diet containing adequate amounts of protein, salts, butterfat, and starch was necessary for inducing rapid growth; and Sherman (6) has found that 0.8 gram of dried skimmed milk per day, or approximately 7.5 per cent of the ration, furnishes the amount of B required for normal growth.

This apparent variation in the amount of vitamin B in dried milk is undoubtedly due to the fact that this factor is not a constant quantity in milk, rather than to destruction of the vitamin by the process of drying. However, there is no definite proof of this assertion, for up to this time the literature contains no reference to comparative experiments in which the same milk was fed in the fresh condition and, at the same time, in the dried form.

Sherman (7) and his colleagues showed that when the diet contains corresponding proportions of fluid or solid milk from different sources there is equality of growth. To arrive at this result they used three groups of experimental animals, giving one group Pasteurized milk, another group milk powder as reconstructed milk, and still another group the same milk powder fed in the dry form. The milk was given in addition to bread, the proportion of milk solids to bread solids being the same in all three cases. As all three groups grew at the same rate throughout a period in which they more than doubled the weights with which they entered the experiment, it seemed safe to these investigators to conclude that the growth-promoting power of milk was not injured by the heat of drying. Johnson (8) has arrived at the same conclusion by feeding groups of growing rats, in addition to a basal ration, which was complete except for vitamin B, a reconstructed milk made of a skimmed milk powder (spray process), a raw milk, and a Pasteurized milk. On each kind of milk his rats made equally good gains where the milk was given

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in the proportions of 2 parts of milk to 1 part of ration, thus demonstrating that the heat of Pasteurization and drying did not injure vitamin B. Johnson (9) has also shown that the antineuritic vitamin of skimmed milk powder is not destroyed or reduced in its activity by the drying process. He found it necessary to feed pigeons about 6 to 7 grams daily of skimmed milk powder in order to protect them from polyneuritis. This amount would equal approximately 75 cubic centimeters of liquid milk, an amount somewhat less than Gibson and Concepcion (10) give as the minimum protective amount of fresh milk.

The literature contains very few references to studies of the vitamin A content of milk powders. Sherman (11) has shown that skimmed milk powder contains a significant amount of vitamin A, probably about one-half as much as whole milk powder. He has presented experimental evidence to show that young rats have grown steadily, though at less than one-half the maximum rate, for a period of three months, trebling their weights and remaining free from xerophthalmia, on a diet in which skimmed milk powder was the sole source of vitamins. Such results can only be obtained on diets containing significant amounts of vitamins. He has also been able to cure rats which have been brought to a typical condition of declining body weight and the characteristic eye disease due to deficiency of vitamin A in their food by feeding skimmed milk powder.

While these investigations on the nutritive value of dried milk were being carried on in the United States, similar investigations were being carried on in England, notably by the medical research committee. The report by Winfield (12) of his experiments on animals with dried milk is of interest in this connection. The manner in which he carried out his experiments differed somewhat from the experiments reported above. The diets which consisted of whole milk powder made both by the spray process and the drum process were offered either in the dry form or as a thick paste with water supplied separately ad libitum. The feeding experiments were begun as soon as the rats could be separated from their mothers, which was earlier than the normal time of weaning. In every case, except a few in which the rats died from accidental causes, the animals remained in good health upon the milk as their sole food. Some of the rats were kept on the diet for a period of 16 months, approximately half of their lifetime, and their general health from all appearances remained perfect to the end of the experiment. Although perfect maintenance was apparently secured on this diet, the rate of growth fell off from the normal when the animals had reached from one-half to two-thirds of their full adult weight. However, it is noteworthy that the addition of fresh milk alone to the diet produced no acceleration of growth, so that the failure in nutrition may be attached to the milk per se and not to the dried preparation. Mattill and Conklin (13) have shown that there is the same retardation of growth between the fiftieth and one hundredth day, approximately the period of adolescence, on both dry milk and fresh milk when fed as the sole food. That this result was not due to a deficiency of vitamin A was proved by the fact that when 10 per cent butterfat was added to a whole milk powder no better growth was obtained. The possibility of a deficiency of vitamin B was also

overruled by the fact that satisfactory growth was obtained when starch and lard, which are devoid of vitamin B, were added to the milk powder. Winfield emphasizes the fact that dried milk used as the sole food maintains an animal in good health and permits normal growth for periods which long outlast those corresponding to infancy and early childhood in man.

COMPARATIVE TESTS OF SPRAY AND DRUM PROCESS DRIED MILKS.

Experiments have been recently carried on in this laboratory to determine which of two processes most commonly used for drying milk has the least destructive action on the vitamins of the milk. In the pressure-spray process the milk is first condensed in vacuum at a temperature of 50° to 55° C. and is then sprayed under pressure into a chamber where it comes in contact with a current of hot air at a temperature of 80° to 85° C. and falls to the floor of the drying chamber as a powder. In the drum process a thin film of the milk is allowed to flow directly onto a steam-heated drum where it is dried in a few seconds and immediately scraped from the drum. In this process the exposure of the milk to the ill effects of high temperature and oxygen of the air of the air is not so prolonged as in the spray process and as a result the vitamins which are labile to these agencies are better protected.

The spray-process dried milk used in the following experiments was made from a full-cream milk and the drum-process dried milk from milk from which approximately one-half the fat had been removed. These milks were reconstituted by the addition of sufficient water to give to the resulting milk the freezing point of normal milk and were fed to albino rats to supply either vitamins A or B in otherwise adequate diets in a daily dosage of 10 cubic centimeters per rat. This amount was chosen because it had been found to be the minimum dose that would produce normal growth had fresh milk been used (14). Eight groups of rats each containing 12 rats, divided into two lots, were started on the feeding experiment at intervals throughout the year so that any differences in the vitamin content of the milk due to seasonal changes in the food of the cow could be explained as such and not wrongly attributed to the process used in drying. The results of this work showed that there was a change in the vitamin content of the two samples of milk which corresponded to changes found in fresh milk due to seasonal changes in the feed of the cow. No pronounced differences in the vitamin content of the milk due to the method of drying were found, although the drum-process dried milk gave better growth when this milk furnished vitamin A than when the spray-process dried milk furnished this factor. As part of the fat was removed from the former milk before drying, it must be concluded that the drum process of drying is less destructive to vitamin A than the spray process. Vitamin B did not appear to be affected by either process of drying. Vitamin C.-The situation is quite different in regard to the presence or absence of vitamin C in dried milk from that which is found with respect to the other two vitamins. In order that milk may retain its antiscorbutic properties after drying, it must be rich in this factor before drying, and it must be dried quickly and at once

protected from the action of oxygen. It is not the degree of heat to which the milk is subjected that is all important but rather the time of heating and of exposure to oxygen as well as the initial amount of vitamin in the milk which determine the antiscorbutic potency of dried milk. If the milk is exposed for a considerable period of time to heated air in the course of drying, as in the spray process, it appears to lose much if not all of its antiscorbutic vitamin, but if the drying takes place almost instantaneously on a highly heated drum the action of the air is so slight that the powder retains a considerable part of the vitamin C of the fresh milk. Thus, Hart (15) and his associates found that milk powders varied in their antiscorbutic properties not only because fresh milk varies in its vitamin content, but also because one process of drying may be more destructive to the vitamin than another. In fact, they found that the spray process of manufacture was much more destructive to vitamin C than the drum process. Hess (16) has also shown that milk dried by the Just-Hatmaker (drum) process will protect guinea pigs against scurvy when reconstructed to resemble fresh milk and fed at a level of 80 cubic centimeters daily. Whether this amount would approximate that of fresh milk which is necessary to protect a guinea pig against scurvy can not be stated, as different investigators have given as protecting doses amounts ranging anywhere from 30 to 150 cubic centimeters of fresh milk daily. Furthermore, in order to determine definitely the effect of the drying process it would be necessary to feed the same milk both before and after drying. No experiments of this kind have been reported. Hess has also given evidence that milk thus dried may be stored for a period of at least six months and still cure babies of scurvy (17).

Contrary to these results are those of Barnes and Hume (18). who found that a milk dried by the drum process failed to protect guinea pigs and monkeys from scurvy when fed in an amount equivalent to an amount of fresh milk which protected these experimental animals. These results may have been due to a low content of the milk in vitamin C before drying, or to improper storage conditions, because from the evidence reported in a preceding paragraph it has been demonstrated that the drum process of drying does not entirely destroy the vitamin C.

From this brief survey of the vitamin value of dried milk it may be concluded, so far as can be determined by experiments on animals (1) that milk powders, as a class, contain practically the same amount of vitamin B as the fresh milk from which they were made, whether manufactured by the spray or drum process; (2) that vitamin A is preserved better in drum-processed milk than in sprayprocessed milk; and (3) that vitamin C will be retained in the dried milk only when the process of manufacture is such as to prevent oxidation and long periods of heating.

POWDERED MILK AS INFANT'S FOOD.

Important as experiments on animals are for obtaining absolute proof that a food substance has retained its initial amount of vitamins, conclusions can not be drawn from such experiments as to the value of the same dried milks in infant feeding. When the

rat is used as an experimental animal it must be remembered that its span of life is only approximately 3 years and that when the young rat is weaned and ready to be placed on artificial food its age of 35 to 40 days corresponds to that of a child 8 to 10 years old. Again, when using guinea pigs for the study of scorbutic diets it is well to bear in mind that this animal is much more susceptible to scurvy than a child, often succumbing to the disease in 2 to 3 weeks, whereas an infant may not show the first symptoms for several months. Therefore because of the economic importance of powdered milk, it has been found necessary to determine if this form of milk could satisfactorily replace fresh milk in the dietary of infants and children. This work has been accomplished by actual feeding tests under as controlled conditions as are possible where babies are concerned. There are many difficulties to be met in carrying out tests of this kind, and owing to the comparatively short periods of time over which such tests can extend the results have not been entirely satisfactory. However, the reports of the various agencies carrying on these tests show, on the whole, that powdered milk may be used as a substitute for natural milk. Jordon (19) concludes from his study of 319 babies fed either on modifications of grade A milk, modifications prepared from whole-milk powder, or modifications prepared from milk reconstructed from unsalted butter and skimmed-milk powder that the powdered milk is an adequate food for babies over short periods of time, although there seems to be a tendency for the babies not to do quite so well on a powdered milk for a long interval of time as on natural milk. Hess (17) reports that in his experience dried milk does not lead to scurvy and that it may even contain enough of the antiscorbutic factor to cure scurvy. Winfield (20) has also shown from results obtained from infant feeding that dried milks which have been exposed for a short time to a temperature above the boiling point are not wholly deprived of their vitamins, and that there is no evidence in the large number of records obtained by him that the use of dried milk is associated with any greater tendency to rickets and scurvy than is seen in infants fed upon fresh cow's milk. When wholecream dried milk was fed he did not observe any cases of xerosis and keratomalacia, a condition which demands careful watching among artificially fed infants. Winfield also reports that the growth curves of children fed exclusively on dried milk from birth closely resemble the average growth curve of breast-fed children, and although at first the rate of growth may be somewhat slower it catches up with the normal and may exceed it. He concludes that cow's milk during the process of drying loses none of the characters which are necessary for the support of normal growth in infants.

CONDENSED MILK.

Much less work has been done on the vitamin value of condensed milk than on that of powdered milk, and there also appears to be less uniformity in the results obtained. Reasons for this lack of uniformity in results are found not only in the methods used in the