1. Storage tests on spray-dried full-cream milk powders prepared from milk preheated at 160,170,180, 190 and 200° F. for approximately 20 sec. and dried by the Krause process have been carried out at 47, 37 and 15° C. or room temperature. The storage tests were carried out independently at two different research stations. At the Hannah Institute the powders were packed in plain tin-plate containers as received from the factory. At Cambridge grease-free plain tin-plate and lacquered tin-plate containers were used. Deterioration was followed at both stations by a tasting panel and by determination of the amount of oxygen absorbed by the powder and of the accumulation of peroxide in the fat.

2. When fresh the powder pre-heated at 180° F. had the best flavour, followed in order of preference by those pre-heated at 190 and 200° F., which had a definite but quite pleasant ‘boiled’ or ‘cooked’ flavour, and by those pre-heated at 160 and 170° F. which had an incipient tallowy flavour, but were nevertheless still acceptable. The 200 and 190° F. powders gave a strong reaction for volatile sulphur, the 180° F. powder a much weaker but quite definite reaction, and the 170 and 160° F. powders a negative reaction. The copper content of the 180° F. powder which, with the 170° F. sample, was the highest of the group, may have been partly responsible for the weakness of the reaction for volatile sulphur given by this sample. The solubility of the 180, 190 and 200° F. powders was not adversely affected by the high pre-heating temperatures, and the moisture contents of all the powders were sufficiently low to prevent any obvious loss of solubility, which remained very good indeed throughout the storage tests.

(1) Increasing the pre-heating temperature of the milk from 165 to 190° F. before spray-drying greatly improved the resistance of the powder to the development of tallowy ‘off’-flavours on storage. From the results of accelerated tests at 47 and 37° C. a storage life (in air) of the order of 2 years could safely be predicted for the powders from milk pre-heated at the higher temperature. Confirmation has now been obtained by storage of the powders for 2½ years at 15° C. and at room temperature.

(2) Improvement in the bacteriological quality of the milk supply, and more careful control during its passage through the plant, produced some improvement in the keeping properties of powders prepared from milk pre-heated at the lower temperature, but had relatively little effect on the keeping properties of the powder when the higher preheating temperature was used.

(3) The shapes of the deterioration curves for the various powders showed that the effect of the higher pre-heating temperature and of improvement in the milk supply was to extend the induction period which precedes rapid deterioration. The protective effect of both factors was greater when measured at the ‘slightly tallowy’ or ‘fairly good’ stage than when comparison was made at a more advanced stage of decomposition.

The beneficial effects of high-temperature pre-heating of the milk before spray drying constitute the main features which emerged from previous experimental work (1). The bacteriological advantages became obvious during the experimental period, and high-temperature pre-heating at 190°F. was introduced as standard practice following the experimental runs. Further observations have been made during two complete drying seasons, in ordinary commercial operation wherein the milk was collected once daily without special selection and pre-heated to 190°F. without preliminary clarification. This work has afforded opportunity to discover any possible practical difficulties, and also to examine a much wider range of samples than was possible during the experimental period.

A knowledge of the density of air-free milk solids is required for calculation of the quantity of oxygen originally enclosed in sealed cans containing milk powder, and for conversion of percentages of oxygen obtained by analysis of the headspace gas to absolute units. Determination of this figure is a matter of some difficulty owing to the presence of entrapped ‘air’ in the spray-dried product. In the liquid displacement method there is also a danger of error due to incomplete ‘wetting’ of the particles, or to solution of one or other constituent of the powder, while in the gas-displacement method error may be introduced by solution or adsorption of the gas by the powder. Lea, Moran & Smith(1) obtained values of 1·29 by a pyknometer method with ethylene glycol, and 1·31–1·32 by a nitrogen displacement method for full-cream solids containing 27% of fat. Muers & Anderson(2), on the other hand, reported values of 1·26–1·28 by a liquid-displacement method using propylalcohol. Further determinations have therefore been made by various modifications of all these methods.

1. A sample of roller-dried skim-milk powder, found to possess the very low biological value of 55.2, supplemented slightly but definitely the proteins of national wheatmeal bread when it supplied 23% of the total nitrogen. No such effect was observed with another sample of milk with a biological value of 71.1 when it supplied 17 % of the total nitrogen, or with a further sample of milk with a biological value of 81.3 when added to bread at a 6 % level supplying about 15 % of the total nitrogen. On the first two occasions the biological value for two different samples of national wheatmeal bread was 48.9 and 55.0 respectively for young rats; for the same rats when mature the values were 55.8 and 64.6. No differences attributable to age were found with milk. Baking did not affect the biological value of the proteins of the milk added to the flour.

2. The biological value of a sample of spray.dried skim milk declined from a value of 88.5, observed 18 months after manufacture, to 71.1 some 36 months later. The true digestibility remained unchanged.

3. Commercial samples of spray.dried and roller.dried skim milk of good quality, and of over.neutralized and overheated roller.dried skim milk, yielded biological values of 78.6, 80.9, 78.2 and 76.2 respectively and true digestibilities of 90.5, 90.1, 89.4 and 88.8 respectively.

1. Of a number of substances tested for antioxidant activity in laboratory-made spray-dried milk powder, ascorbic acid and ethyl gallate proved most promising. Both of these substances materially increased the resistance of the powder to the development of tallowiness without producing any foreign flavour in the milk.

2. The actitivy of both substances has been confirmed in factory-made spray-dried powder, ethyl gallate being much the more powerful of the two. Ethyl gallate at a concentration of 0·07% increased the storage life of powder in the accelerated tests 2½–3-fold.

3. Ethyl gallate remained unchanged during storage of the powder, but the concentration of ascorbic acid decreased, rapidly at first and then more slowly.

Bacteriological standards are used in the assessment of the quality of dried milk powder. From a knowledge of total numbers of bacteria and the relative numbers of the various types deductions are made as to the quality of the raw milk supply, the cleanliness of the plant and the over-all efficiency of the process. Figures have been published by several workers showing plate counts of roller- and spray-dried milk powder, but the technique of reconstituting and of plating varied. Sterile water appears to have been the general reconstituting fluid, but sometimes lithium hydroxide (N/10 solution) was used, especially when the powder was difficult to dissolve. The temperature of the reconstituting fluid is not always mentioned, but it appears to have varied from room temperature to 60–65° C. The usual incubation temperature for the plates was 37° C, though 21–22 and 55–56° C. were also used for special purposes. In Standard Methods for the Examination of Dairy Products, eighth edition (1), the technique laid down mentions water, carefully warmed to 43–49° C, as the reconstituting fluid, and either 37 or 32° C. (for 48 hr.) as the incubation temperature, the choice being left to theenforcement officials having jurisdiction. American workers have for some years strongly advocated the use of 32 rather than 37° C. as the incubation temperature for routine plate counts of samples of raw milk, the advantage being that slight inaccuracies in the temperature of the incubator have less influence on the plate count which is, moreover, closer to that obtained by direct counts. The data offered in this paper show that the advantage of a similar incubation temperature (30° C. was used in these experiments) is equally obvious in the plating of dried milk powder.

1. Eighteen samples of colostrum obtained from Large White sows during parturition were examined for total solids, fat, vitamin A, vitamin B^ riboflavin and vitamin C.

2. The mean values and ranges for the total solids (fourteen samples) and fat (fifteen samples) were 23.9% (20.6–27.2%) and 3.4% (1.7–5.8%) respectively.

3. The mean vitamin A content of eighteen samples was 247 i.u./100 ml. with a range of 103.462 i.u./lOO ml., or 89 i.u./g. fat with a range of 42–137 i.u./g. fat. No β-.carotene was detected in any of the samples.

4. The mean vitamin B1 content of seventeen samples was 145 /µg./100 ml. with a range of 81.260 µg./100 ml.

5. The mean riboflavin content of sixteen samples was 45.6 µg./100 ml. with a range of 21–78 µg./100 ml.

6. The mean ascorbic acid content of seventeen samples was 23.8 mg./100ml. with a range of 12–36 mg./100 ml.

1. A technique (B) for the reconstitution of both roller- and spray-dried milks is described which yields a liquid which is more satisfactory to handle, and gives clearer plates, higher counts and better replicates than the technique (A) at present recognized. The most important improvement in the new technique involves reconstitution, with a short holding period, at 50° C.

2. For spray-dried full-cream milks the mean increases in the counts obtained by technique B as compared with technique A were: for 3 days at 37°C, slightly more than a three-fold increase; for 5 days at 30°C, slightly more than a two-fold increase; and for 3 days at 55° C. an 86% increase. The corresponding increases for spray-dried separated milk were for 3 days at 37°C, a two-fold increase, for 5 days at 30°C. a three- fold increase, and for 3 days at 55°C, a 23% increase. For roller-dried full-cream milk the increases were markedly smaller, i.e. 44, 41 and 29 %.

3. After 6 months' storage spray-dried full-cream milks showed a slightly greater difference between counts by technique B than by technique A.

4. A mean increase in plate count of about 10% was obtained by incubating plates for 5 instead of 3 days at 37° C. Although this increase was statistically significant, it is not considered that 5 days' incubation offers any practical advantage over 3 days' incubation.

5. At 30°C. incubation for 5 days is essential. Thus the mean count after 5 days at 30° C. showed, for spray-dried powders, at least a two-fold increase over that at 3 days, though for roller-dried powders the increase was only 20 %. The percentage increase for spray-dried milks varied for individual samples and for samples from different plants.

Some years ago the annual wastage of cows in dairy herds in Britain was estimated by Wright(1) to be approximately 22%, and of this about one-quarter was due to sterility. Recent figures(2) from Shropshire, Staffordshire and Warwickshire suggest that this position has not changed in the past few years. Thus in forty-eight herds containing 1925 cows there were 620 disposals in the 12 months ending September 1940. 23% of the disposals were due to sterility. The annual losses from bovine sterility in Britain have been calculated to be equivalent to £7,000,000 in milk alone. This fact is cited by Day(3) in a concise article on the control of sterility in which he discusses various factors which contribute to the problem. Some of these factors, such as contagious abortion and trichomonas infection, are pathological, but many others are functional or physiological. The present review is concerned only with the latter. Mackintosh (4), in an article on breeding and rearing to maintain the milk supply, summarizes for the practical dairy farmer many of the methods by which the incidence of functional sterility can be reduced. Both Day and Mackintosh emphasize the importance of pregnancy diagnosis, of detecting the short winter oestrus and of testing the fertility of the bull. They also mention that hormone therapy can frequently be used to great advantage for inducing oestrus. Knowledge of these factors and how best to deal with them has greatly increased during the past few years as a result of the work carried out in many research stations throughout the world.