An investigation has been made into the accuracy and speed of response of temperature-measuring instruments suitable for use in the control of H.T.S.T. pasteurization. Thermocouples used in conjunction with a ‘d.c. amplifier’ recorder system are shown to be markedly superior to both vapour-pressure and mercury-in-steel thermometers. The accuracy of temperature measurement is about ±0.3°F. as against ±1°., while the lag constant is about 0.2 sec. as against about 3 sec. Response to small, rapid and transient changes of temperature is also shown to be greatly improved. The operation of control contacts by the thermocouple recorder was found to be reliable to well within the limit of accuracy of calibration, but the other types showed variations of the order of ±1°F., in addition to the uncertainties of calibration.

The work described in this paper was undertaken at the request of the National Institute for Research in Dairying as part of the research programme of the National Physical Laboratory, and is published by permission of the Director of the Laboratory. The author wishes to acknowledge the help he has received from Dr A. T. R. Mattick and Miss E. R. Hiscox of the National Institute for Research in Dairying and from his colleagues Mr C. Wigley and Miss V. M. Leaver. Dr Mattick and Miss Hiscox supplied the experimental results given in the introduction, for the drafting of which they are largely responsible. Mr Wigley derived the equations used in computing the theoretical curves of Text-fig. 6 and Miss Leaver assisted with the observational work.

1. During four periods equally spaced throughout the year raw milks coming from surrounding farms into the Buckingham receiving station were examined by (a) the 10 min. resazurin test, (b) the 1 hr. resazurin test, (c) the resazurin test in which the time to reach disk 1 was recorded, (d) the modified methylene-blue test, and (e) a keeping quality test based on taste and alcohol precipitation. Altogether, 2588 samples were examined.

2. Table 2, in which the correlation coefficients are worked out between the results of the dye and the K.Q. tests, shows that there is little to choose between the methylene-blue test and the time to reach disk 1 in the resazurin test; but since an average of only 15 min. was saved by carrying out the resazurin test, there seems little advantage to be gained by the use of this test. Both of these tests are superior to the 10 min. and the 1 hr. resazurin tests.

3. A comparison of Tables 5–7, in which the average K.Q. of the milks corresponding to different disk numbers or reduction times is recorded, shows that the methylene-blue test affords the best indication of the K.Q.

4. A study of Table 3, setting out the relation between the 10 min. resazurin and the methylene-blue reduction tests, shows that 99% of milks giving a 0–½ reading in the resazurin test reduce methylene blue within 30 min. and 80 % of them within 10 min.; but that, of milks reducing methylene blue within 30 min., only 53% give a disk reading of 0–½ in the 10 min. resazurin test. If it is agreed that no raw milk on arrival at the creamery which reduces methylene blue within 30 min. and which has, according to Table 7, an average K.Q. of only about 5 hr. at the time of testing, is fit for human consumption, then it is clear that the present rejection standard in the 10 min. resazurin test passes nearly 50% of unsatisfactory milks.

5. The data recorded in this paper all go to show that the best indirect index of the K.Q. of raw milk is afforded by the methylene-blue test as carried out by the method officially laid down by the Ministry of Health.

6. On the other hand, if time is an overruling consideration, then a slightly less accurate estimate of the K.Q. of raw milk is afforded by the 1 hr. resazurin test. It must be pointed out, however, that the figures given in Table 6 suggest that below disk 2 there is some irregularity in the relation between the disk numbers and the K.Q. If the 1 hr. resazurin test should come into use for the routine grading of milk, a revision of the present tints at the lower end of the scale might prove desirable.

1. The diacetyl production of betacocci isolated from starters, cheese and rye-grass was investigated at pR values ranging from 5.5 to 44. The betacocci produced diacetyl and subsequently partially or wholly destroyed it.

2. There was a considerable variation in the amount of diacetyl produced by the different strains of betacocci. Capacity to produce diacetyl could not be correlated with any other property of the organisms.

3. The rate of growth of the organisms was progressively slower the lower the pH, but at least as much diacetyl was eventually formed at low pK values as at high pH. values. The results showed that there was an alteration in the metabolism of the organisms, the diacetyl-producing power per unit cell increasing with decreasing pH.

4. The results found enable an explanation of the rapid production of diacetyl in mixed starters to be offered.

The amount of CO2 formed during the fermentation of sugars by the heterofermentative lactic acid bacteria depends on several factors:

(1) The sugar fermented. Gas is produced more readily from glucose than from lactose, even in the case of active lactose fermenters.

(2) The reaction and the buffering capacity of the medium. The optimum initial reaction of the medium is approximately pH 7 and the greater the buffering capacity the more active is the gas production.

(3) Factors which occur in plant juices and yeast. They may promote gas formation without increasing cell proliferation and apparently have a specific activating effect on the mechanism of the fermentation.

(4) The concentration of sugar, the optimum being about 5%.

The formation of CO2 by Bacillus licheniformis is influenced in a similar way, but in this case the optimum initial reaction of the medium is in the region of pH 8–9 and the factors of (3) have little effect.

1. A series of experiments with rats is described in which the growth-promoting properties of butter fat have been compared with those of the solid and liquid fractions of butter fat, with margarine and with certain vegetable oils.

2. The separation of butter fat into fractions containing a preponderance of glycerides of saturated or unsaturated acids was effected by means of crystallization from acetone.

3. The following results were obtained by incorporating fats in liquid skim milk:

(a) No differences were detected between the growth-promoting properties of butter and margarine or of butter fat and deodorized arachis oil; in the latter comparison the arachis oil was more economically used than the butter fat.

(b) No differences were found between butter fat and maize oil alone or containing the solid or liquid fractions of butter fat in the approximate proportions in which they occur in the original fat. The less saturated oils, i.e. maize oil and maize oil + the liquid butter fraction, were more economically used than butter fat and its solid fraction mixed with maize oil.

(c) No differences were observed between the growth-promoting values of butter fat, arachis oil, cottonseed oil or soya-bean oil. The butter fat was rather less well utilized than the less saturated oils, but the differences were not significant.

(d) Poorer growth was observed with the more saturated fraction of butter fat than with the original fat or the liquid fraction.

4. When butter fat, margarine fat or arachis oil was incorporated in a dry basal diet, no differences were observed in the growth-promoting value of these fats, but the arachis oil was more economically used than the other fats. This finding was not con- firmed in a second experiment of longer duration in which poorer growth was observed with the arachis-oil diet, though the economy of gains was the same with all diets.

5. It is concluded from these experiments that it is unlikely that butter fat possesses superior nutritive properties to those of other fats, and that the more saturated fraction of butter fat is certainly not superior in growth-promoting value to that of the more unsaturated fraction or to more unsaturated vegetable oils.

Further work on the standardization of annatto for cheese colouring supports the author‘ earlier conclusions (l), viz.

(1) The colouring power of annatto extracts in cheese can be estimated by their tinctorial power in dilute alkaline solution.

(2) A simple tintometer of the ‘Limitester’ type is suitable for this work, a standard orange disk, or Lovibond yellow and orange glasses being used as the standard.

1. Previous work on bacteriophage infection of starters as a cause of slow acid development in cheese manufacture is reviewed. Air-borne phage seems to be the form of infection which is most insidious and most difficult to exclude. Hypotheses are developed to explain the sequence of events observed in commercial practice. The spread of phages as dust or droplet infections seems to explain the gradual increase of trouble with a given starter culture within a single factory and also throughout an entire dairying district.

2. When phage is established in the surroundings of a dairy factory a cycle of infection can develop. Besides the air-borne droplet infection there may be daily reinfection of cheese milk from incompletely sterilized factory and farm equipment which comes into contact with the milk. A description is given of methods which have been used to determine in any given case whether slow acid development is due to phage infection of the starter itself or to infection of the milk from factory or farm utensils.

3. The problem of preventing phage infection of the starter culture is fully discussed. The extent of the air-borne infection to be guarded against is indicated. The equipment which has so far been developed in New Zealand for the aseptic handling of starter cultures is described in detail. The most promising lines of development are indicated.

1. Frequent examination of the mother and bulk starters from several cheese-making creameries during the 1942 season has shown bacteriophage to be prevalent in some factories and apparently absent from others. Outbreaks of phage attack could not be correlated with heat treatment of the cheese milk, and were geographically widespread over the country.

2. The phage-organism relationships of these starter strains and their phage races were studied, and on this basis it was possible to arrive at the relationship and in some cases the identity of some of the strains circulating in commerce. It was found that for the isolation of some phaging strains, milk was a more sensitive medium than broth. Commercial starters appear to be composed of more than one distinct strain, and no starter can be judged by the reactions of one of its component strains.

The nascent phage phenomenon was found not to be common in the selected starter strains. The reaction itself may, however, possibly be used to demonstrate fundamental relationships between strains.

The heat resistance, in milk, of bacteriophages attacking strains of lactic streptococci isolated from cheese starters has been determined. Active phages did not usually survive a temperature of 75°C. for 7½ min. but many phages were not destroyed by heating for 50–60 min. at 65–67°C. and most survived at 70°C. for 10–15 min. The practical implications of these findings in the ‘sterilization’ of milk for starter making have been discussed. Growth in yeast dextrose broth was found to be superior to methylene blue reduction for the demonstration of residual phages after heating.

1. Cheshire, Cheddar and Stilton cheeses were made on one occasion from winter, and on another from summer, milk.

2. The distribution of vitamin A and carotenoids, vitamin B1, riboflavin and vitamin C between milk, whey and curd was measured during cheese-making.

3. The fate of these vitamins during ripening was determined.

4. Some 80% of vitamin A and carotene, 15% of vitamin B1 and 25% of riboflavin originally present in milk were recovered in the green cheese and persisted throughout ripening.

5. Vitamin C disappeared progressively during cheese-making and was absent from the cheeses.

6. Vitamin B1 and especially riboflavin were recovered in cheese in larger proportions than would be expected from the partition of water between curd and whey, indicating that they were partly present in milk in combination with the coagulable solids.

(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.

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. 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.