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. An experiment with ten dairy cows has shown that the daily feeding of 30 or 60 g. of shark-liver oil containing 80,000 i.u. of vitamin A/g. is without effect on milk yield, fat yield, the fat and solids-not-fat percentage of the milk and on the health and wellbeing of the cow.

2. The concentration of aneurin, riboflavin and vitamin C in the milk was not affected by the feeding of shark-liver oil.

3. The total carotenoids of the milk fat decreased when shark-liver oil was fed. The vitamin A content of the fat increased 1100% when 30 g. were given and 1850% when 60 g. were fed. The highest individual value reached was 390 i.u./g. fat.

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.

The collection of samples of milk from individual quarters has usually been done either by hand milking or by sampling by hand at the commencement of milking, before the application of the teat-cups for milking by machine. The former method is laborious, and may not be entirely accurate, as it depends a great deal on constant care throughout the milking to ensure that each quarter is milked into the correct container. If the quarters are milked separately by hand, the yield of milk from the individual quarters may be affected, and if the cow is not accustomed to being milked by hand, hand milking for quarter sampling may disturb her, and affect the yield, and so the composition of the milk. Quarter sampling by hand before the commencement of machine milking does not provide a representative sample of the yield of milk from the quarter, and it does not permit a measurement of quantity of milk produced by each quarter. In connexion with some work on the composition of milk from quarters showing a positive brom-thymol blue test (Hume(i), McDowall(2, 3)) and on the influence of such milk on the quality of cheese and butter made from it (McDowall(4)); the collection of full quarter samples of milk on a considerable scale was required. For this purpose a special milking-machine claw, a milking-machine bucket, and a strip bucket were constructed. These are illustrated in Fig. 1, I–IV.

1. The sources of error and general significance of the presumptive coliform test have been studied, with special reference to manufactured dairy products.

2. Eosine methylene blue agar has been found reasonably satisfactory for isolation of cultures but unsatisfactory as a direct plating medium.

3. A wide survey of the coliform flora of raw milk and milk products has been made over a period of 11 years. The data were collected from 24,952 samples and include the incidence of presumptive positive tests at each stage of manufacturing processes, the occurrence of ‘false-positive’ tests, the detailed distribution of coliform types in individual dairy products, and some preliminary observations on ‘heat-resistant’ coliform strains.

4. Material examined included raw milk, pasteurized milk, ice cream, milk powder, condensed milk, pasteurized cream, clotted cream, butter, soft cheese, Cheddar cheese, processed cheese, swabs of dairy plant, churn rinses, and the atmosphere of dairy factories.

5. Detailed confirmatory tests were performed on 2490 presumptive positives, from which 2508 coliform cultures were isolated and classified.

6. A general ecological survey of the frequency of individual coliform types in dairying operations as a whole has been attempted.

7. Particular factors which may affect the distribution of coliform types in specific products or situations have been shown to include heat resistance, resistance to drying, chemical composition of the substrate, the effect of storage, and conditions prevailing on plant surfaces. It is suggested that the coliform flora may be partly the result of adaptation to conditions associated with each individual product.

8. The value of the presumptive coliform test has been discussed in respect of public health requirements, as a plant-control method, and in relation to economic aspects. The test is believed to be of great utility in the plant-control laboratory but of little value for public health purposes.

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.

A cheap, easily prepared and relatively uniform agar medium, containing beef extract, lactose, peptone, yeast extract and phosphate, is described. It gives excellent results with lactic streptococci and a good growth with many other organisms.

The role of phosphate and agar in the medium and the effect of filtration is discussed. It is suggested that the value of phosphate is due partly to its buffering effect and partly to its combination with some constituent of agar to form a substance which has a growth-promoting action on the lactic acid bacteria. This active substance is largely removed from the medium if filtration through paper pulp is practised.

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.

1. The germicidal effect of spraying hypochlorite solutions of varying strength on three different phage races of streptococci from cheese starter has been described. It has been shown that fine mists containing 0·003–0·02 p.p.m. av./cl., usually ensure satisfactory destruction of air-borne phage.

2. Below a relative humidity of about 50% hypochlorite solutions cause a slight or negligible destruction of bacteriophage particles.

3. A gelatine medium has been found useful for collecting phage particles from the air, since it could be liquefied without danger of destruction of the phage after exposure and a quantitative estimation obtained of the phage particles collected.

4. When the relative humidity was low the recovery of phage particles from the air was increased.

5. Some experiments using mists of resorcinol and propylene glycol are also reported.

As moderate dilution causes little change in the surface tension of milk, it is shown to be advantageous to use σ-dilution curves in place of the σ-values of the undiluted fluid as a characteristic of the surface properties of milk. The complications arising from the presence of the milk fat are described, and it is suggested that the influence of the fat is of a physical rather than of a chemical nature. A study of the role of the various milk proteins shows the casein to be of great importance, whilst the heat-coagulable proteins have little influence. The serum obtained after removal of the casein and heat-coagulable proteins contains a residual fraction of protein-like material which is markedly surface active though constituting only about 3% of the total milk proteins. The surface-active material (σ-proteose) has been concentrated and isolated, and its properties are described in some detail.

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. The biological value of proteins at an 8% level of protein intake measured on rats by the method of Mitchell (7, 10) was 52 for white bread of 73% extraction, 76 for Cheddar cheese, 75 for a mixture of both foods when each supplied 50% of the protein, and 67 when the two sources of protein were given on alternate days. The true digestibilities were, respectively, 99, 100, 98 and 99.

2. Similar experiments with dried skim milk and dried cooked potatoes yielded biological values of 71 for potato, 89 for milk, 86 for the mixture and 81 for the substances fed separately on alternate days. The true digestibilities were, respectively, 82, 90, 82 and 87.

3. An attempt to carry out experiments of the same type with dried whey and dried potato failed because of deaths of some rats receiving the whey diet, probably because of its high content of lactose. A biological value of 76 was obtained for the potato proteins and of 70 for a mixture of whey and potato in which whey supplied 25% of the protein. The true digestibilities were, respectively, 82 and 68.

4. The biological value and the true digestibility of the proteins of a sample of edible soya-bean flour were, respectively, 73 and 90.

5. It is concluded that there is a marked supplementary relationship between the proteins of milk and potato and those of bread and cheese when the members of each pair supply equal amounts of protein and when they are given together. No supplementary relationship is exhibited when the sources of protein are given separately on alternate days.

The ferric thiocyanate method of estimating peroxide in fats has been modified to give greater sensitivity and greater stability of the reagent. As the 96% acetone used by Chapman & McFarlane (3)and Lips et al.(4) only dissolves about 0.5% butterfat, the sensitivity of the test was increased by using, as solvent, the mixture of benzene and methanol used by Bolland et al. (6) in the determination of rubber peroxides, which dissolves at least 10% butterfat. Instead of using a prepared reagent which gradually deteriorates, the ferrous salt and thiocyanate are added in aqueous solution during the test, and, moreover, ferrous salts appear to be more stable in this solvent than in acetone. The modified method is sufficiently sensitive to measure the peroxide value of the fat of freshly drawn milk and has been applied to the study of the first stages of oxidation in milk, butter and whole-milk powder.

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. Data on milk consumption, and other relevant information, were obtained from 150, 152, 124 and 135 lower working, working, lower professional and professional class families respectively in the city of Leeds during May-June 1939.

2. An account is given of the factors affecting the consumption of liquid milk at school and at home, and of tinned milk. An estimate is made of the amount of milk consumed by children over and under 5 years of age and by adults, and of the nutritional adequacy of the milk consumption of those three groups.

3. An analysis is made of the use of milk in the home and the importance of the three factors: (a) the number of families making milk dishes, (b) the frequency with which the dishes are made, and (c) the amount used in each dish per person per time, in causing differences in the average liquid milk consumption of the four social classes.

4. An estimate is made of the extent to which families were obtaining a proper amount of milk, and the number of the families who were not spending sufficient on food to purchase an adequate amount of milk.

5. It is concluded that in investigations into the consumption of milk, the various social classes should be studied separately, otherwise fundamental differences between them fail to be disclosed.

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.

The general experience of the retail milk trade is that considerable variations exist in the demand for milk and cream at different times of the year, and that these need careful consideration to preserve the proper balance between the supply of, and demand for milk. Different social classes of consumers are also known to exhibit different habits of milk and cream consumption. This paper examines the fluctuations of demand for milk and cream over a 1-year period of four social classes of consumers.