Samples of milk, skim-milk and raw cream from one factory and of pasteurized cream, buttermilk and butter from two factories were taken twice monthly for varying periods for estimation of total phospholipids and (in butter only) of lecithin, cephalin and sphingomyelin.

Seasonal variations in the phospholipid contents of milk, skim-milk and raw cream over 13 months probably were due to the greater proportion of small fat globules in autumn (late lactation) milk compared with spring (early lactation) milk.

There were regularly recurring seasonal variations in total phospholipid content of the butters over 3 years. Maximum results were found during autumn and minimum results in winter. Lecithin, cephalin and sphingomyelin contents followed the same trends as the total phospholipid contents.

Seasonal variations in phospholipid contents of buttermilks and butters were due mainly to variations in the amount of phospholipid per unit of fat in the cream. Differences in butter-making technique had little effect on the proportion of phospholipids from the cream retained in the butter.

Average results for total phospholipid content of the products examined were: milk, 0·038% (0·031–0·050%); skim-milk, 0·018% (0·014–0·023%); buttermilk, 0·156% (0·103–0·191%); butter, 0·133% (0·099–0·181%). The average result for percentage of total phospholipid in the fat of the raw creams examined was 0·44% (0·38–0·51%); and in the fat of the pasteurized creams 0·41% (0·35–0·49%). Average weighted results for total phospholipid and for lecithin, cephalin and sphingomyelin contents of butter were 0·139, 0·041, 0·051 and 0·047% respectively.

1. The composition of the milk of six low-plane and six high-plane ewes has been studied. Definite lactational trends have been observed for all the major constituents.

2. For all the ewes the initial colostrum was characterized by very high protein and fat contents and by low lactose content.

3. Analyses of the mineral constituents—calcium, magnesium, sodium, potassium, phosphorus, and chloride—are reported covering the whole lactation of twelve sheep. The colostrum of the low-plane animals was higher in total mineral content than that of the high-plane animals.

The author wishes to thank Mr L. J. Lambourne for providing the milk samples and Mr J. E. Allen for the spectographic analyses.

The gross energy content of the milk of the cow, sow, ewe, goat and of human milk has been determined by bomb calorimeter, and compared with results obtained using conversion factors and chemical analysis.

Correction for non-protein nitrogen content led to values agreeing within 0·5–1·0% for the milk of all species. For human colostrum other factors may be involved, and a closer estimate of gross energy value of human milk was obtained by treating all nitrogenous constituents as being protein.

Organisms identified as Pediococcus strains were found to constitute about a quarter of the normal lactic-acid flora of ripening New Zealand Cheddar cheese. The development and the frequency distribution of these pediococci, in relation to the starter streptococci and lactobacilli were briefly studied.

The purpose of any pasteurizing or sterilizing plant is to bring about the heat inactivation of bacteria or bacterial spores. It is generally considered for convenience that the bactericidal effect takes place almost entirely in a holding period, during which the milk can be considered to be held at a fixed temperature for a minimum time. In practice, since the milk cannot be instantaneously heated and cooled, there is a progressively increasing lethal effect throughout the period when the milk is approaching its holding temperature, and a decreasing effect in the cooling period. There is, therefore, a considerable lethal effect above that due to the nominal holding time. This has been shown in a paper by Ball(1) in which methods of calculation developed for the canning industry are applied to the problem of high-temperature, short-time pasteurization of milk. The higher the final milk temperature, the greater will be the proportion of lethal effect due to the come-up time, until, at temperatures of the order of 135° C., the lethal effect during holding may be relatively slight.

An extraction-titration method is described for the determination of free fatty acids in small samples of fresh milk.

Recoveries of fatty acids added to milk ranged from 63% for butyric acid to 97% for stearic acid. For a composite sample approximating milk fat in fatty acid composition the recovery was 83%.

Interference from citric and lactic acids was not significant for reasonably fresh samples.

The method gives a higher recovery of the lower molecular weight fatty acids than is possible by earlier methods.

For titrating very dilute solutions of fatty acids α-naphtholphthalein was shown to be a more accurate indicator than phenolphthalein; it also provides a better defined endpoint.

For a survey of the numbers of lactobacilli present in milk used for cheese-making in creameries, counts were made on an acetate-agar medium (1) selective for lactobacilli. It was observed that the numbers of lactobacilli in the milk in the cheese vat sometimes greatly increased after the addition of the starter to the milk. When these starters were plated on acetate agar growth of lactobacilli, sometimes in large numbers, suggested contamination of the starters with these organisms. The starters in use at sixteen different creameries were then examined on acetate agar, and seven were found to contain lactobacilli. Representative colonies of these organisms were picked from each starter and these strains identified physiologically using the methods of Briggs(2) and Wheater(3,4), and serologically using the group sera of Sharpe(5) and Sharpe & Wheater(6). Table 1 shows the numbers and species of lactobacilli present in the starters. Each starter was sampled several times, usually at monthly intervals.

A method for the determination of the fat content of milk from measurements of the turbidities of diluted milk suspensions in the near infra-red region has been shown to provide greater accuracy than the alkali-addition method in the visible region. Variations in results due to breeds of cows and lactation times have been studied, and excluding early lactation samples, the maximum deviation within a given breed was found to correspond to ±0·2% in the fat concentration.

1. Four batches of soft cheese were made using rennet coagulation from both raw and pasteurized separated milk standardized to contain 1·5% fat and 7% added sodium chloride.

2. The yield of pasteurized milk cheese was about 3·33% higher than that of raw milk cheese.

3. Pasteurization of the milk improved the flavour and the texture of the fresh cheese, but had no marked effects on the body and general appearance.

4. The shelf keeping property after 4–5 days under the conditions of these experiments was better for the cheese made from raw milk. Reasons for this apparent anomaly are discussed.

5. Chemical analyses showed a greater moisture retention by the pasteurized milk cheese, leading to a lower content of fat and total nitrogen but increased sodium chloride in solution. Some retention of heat coagulable protein in the pasteurized milk cheese was suggested by a higher total nitrogen in dry-matter figure and by a lower nitrogen content of the pasteurized milk whey.

6. Pasteurization of the milk greatly reduced the total bacterial population of the fresh cheese and also the acidity of the whey produced.

The lactobacillus flora of two pairs of duplicate cheeses, each pair being made under the same conditions and from the same milk, was examined during ripening. A different distribution of serological types of lactobacilli was found not only within a pair of cheeses but in the top and bottom of the same cheese. A third pair of cheeses, to one of which a known Lactobacillus had been added, was found to have a more regular distribution of lactobacilli owing to greater numbers being present in the cheese milk.

A comparison of three different media, yeast dextrose agar, tomato dextrose agar and acetate agar (which is selective for lactobacilli) showed that the acetate medium was satisfactory for the isolation and counting of lactobacilli from Cheddar cheese during ripening. Lactobacilli were isolated on this medium in the early stages of ripening when the large numbers of starter streptococci growing on the other media were completely suppressed. Lactobacilli isolated were identified biochemically and serologically. Serological typing was found to be a useful method of identifying large numbers of isolates and of assessing the incidence and distribution of particular types in the cheese.

We wish to thank Dr A. T. R. Mattick for his interest and helpful advice in this work; Dr L. A. Mabbitt for preparation of the cheese samples; Miss H. R. Chapman for making the experimental cheese; and Mrs P. Jarrett for technical assistance.

1. This paper is Part I of a series dealing with the relation between the chemical composition of milk and the stability of the caseinate complex to ethanol (Part II), rennet (Part III) and heat (Part IV).

2. A description is given of the milk samples, which were taken from herd bulk milk, from individual cows in different stages of lactation and also from cows with subclinical mastitis.

3. The methods used to measure the stability of the caseinate complex to the three coagulating agents are described, together with the methods used to make a detailed chemical analysis of the milk samples.

4. The chemical composition of the samples is given and also the relation between composition and stage of lactation, and the interrelations of the concentrations of certain milk constituents.

The authors thank Miss M. H. King and Miss R. E. M. Stevenson for technical assistance, Mr N. H. Strachan and Mr R. C. Voss (West of Scotland Agricultural College) for the sodium and potassium determinations and Dr P. S. Blackburn for the differential cell counts and bacteriological examination of the milks.

Lactobacilli occur in large numbers in cheese (1,2,3,4) and may be responsible for or influence the formation of flavour. It is therefore desirable to find the source of the lactobacilli and the route by which they enter the cheese, so that the cheese flora may be controlled

1. The variation in the stability of milk protein to ethanol and the relationship between milk composition and ethanol stability were examined.

2. Samples of herd bulk milk were very similar in stability to ethanol; the range of aqueous ethanol solutions required to coagulate the caseinate complex in an equal volume of milk was only 80–84% (v/v) ethanol. Samples from individual cows showed a wide variation in stability; coagulation was caused by ethanol solutions ranging in strength from 66 to 90% (v/v) ethanol.

3. Colostrum was very unstable to ethanol but stability rapidly increased during the post-colostrum period to higher levels in mid-lactation. Late lactation and subclinical mastitis milk showed no definite bias towards stability or instability.

4. The strength of ethanol required to coagulate the caseinate complex in an equal volume of milk was inversely related to the concentration of ionized calcium in the milk. The correlation coefficient was –0·76 (significant at 0·001 level) indicating that approximately 60% of the variation in stability was accounted for by the variation in the concentration of ionized calcium.

5. The relationships between the concentrations of other milk constituents and stability to ethanol could be attributed to the interrelations of the concentrations of these constituents and the concentration of ionized calcium.

1. The influence of a number of factors on the utilization of carotene by rats has been investigated. A high dose level equivalent to that derived from pasture by cows has been employed and liver storage of vitamin A has been used as a measure of the efficiency of carotene utilization.

2. Utilization was influenced by level of carotene intake, by concentration of carotene in the oil used as vehicle, by the degree of unsaturation of the vehicle and by the presence of non-digestible wax.

3. Carotene was better utilized from the rye grass and ‘low cyanide’ white clover than from ‘high cyanide’ clover, but added cyanide had little or no effect on the uptake of carotene from cyanide-free pasture or from a solution in oil.

4. In so far as the results of the investigation can be applied to cows grazing typical New Zealand dairy pasture, it seems that factors which may throw some light on the poor utilization of carotene at certain times of the year are (a) small day-to-day variation in the carotene content of the pasture, (b) variations in the ‘ether extract’ fraction of the pasture, and (c) variations in the degree of hydrogenation of the fat in the rumen.

5. Insulin and adrenalin appear to have no influence on the levels of vitamin A alcohol in the blood plasma of rats.

The results of this investigation form a section of a thesis submitted by one of us (N.A.W.) in partial fulfilment of the requirements for the degree of Ph.D. of the University of New Zealand. The authors are indebted to the Department of Scientific and Industrial Research for a Grant towards this investigation and to Miss Fay Frecklington for technical assistance; one of us (S. Y. T.) is indebted to the Royal Society, the Nuffield Foundation, and the Board of the Dairy Research Institute (N.Z.) for a travelling fellowship during the tenure of which the work reported here was carried out.

1. Certain starter cultures have been found to be inhibited in the presence of milk peroxidase.

2. A high correlation has been established between the inactivation of milk peroxidase and of the inhibitory action by use of critical values of any of the following: temperature, pH, concentration of either sodium azide or hydrogen peroxide.

3. The inhibitory action can be overcome by the addition of certain reducing substances such as cysteine and sodium hydrosulphite.

4. The inhibitory action is exhibited by the corresponding separated milk and whey.

5. It is suggested that this inhibition may be due to the formation of a specific inhibitory oxidation product having a quinonoid structure.

6. The addition of 1–2% of pasteurized milk or whey to autoclaved milk caused total inhibition, whereas such additions to milk heated to 190° F. showed no such effect. This is attributed to the presence of SH groups in the latter milk capable of reducing the oxidation product.

7. It is suggested that the inhibition due to peroxidase is identical with that ascribed to lactenin 2, and that the inhibition described in our previous paper as due to ‘agglutinin’ is identical with that ascribed to lactenin 1.

1. Microbiological methods were used to measure biotin, nicotinic acid, pantothenic acid, riboflavin, ‘vitamin B6’ and vitamin B12 in milk from Guernsey, Friesian and Shorthorn cows.

2. A study was made of day-to-day variations and breed differences in the B-vitamin content of such milk. Fluctuations in each vitamin were observed from one day to the next in milk from individual cows, these variations being especially marked with biotin and vitamin B12. In addition, the variations in the mean vitamin levels of milk from cows of the same breed were generally greater than any breed difference.

3. Samples of milk from ten Friesian and ten Shorthorn cows were taken at intervals throughout one complete lactation and analysed for each of the vitamins mentioned above. The results of this study of lactational trends are presented in detail.

1. Thirty-six 9–11-week-old Large White pigs in nine blocks of four litter-mates were individually fed on diets containing either white fish-meal or various amounts of dried skim-milk as the protein supplement. The pigs were fed according to live weight and a scale up to a maximum of 6½ 1b. of meal/pig/day.

2. There were no significant differences in rate of growth or efficiency of food utilization between pigs given the diets containing as protein supplements either 10% white fish-meal reduced to 7% at 130 1b. live weight, 10% dried skim-milk reduced to 7%, or 15% dried skim-milk reduced to 10½%.

There was, however, a trend towards an improvement in both rate of growth and efficiency of food utilization with increasing levels of skim-milk in the ration, and the differences between the high skim-milk diet (20 reduced to 14%) and the low (10 reduced to 7%), and also between the former and the fish-meal diet, reached statistical significance.

3. The difficulties of drawing any definite conclusions from the results about the relative value of the protein in white fish-meal and dried skim-milk were discussed in relation to (a) the probability that the white fish-meal diet used contained over-generous supplies of protein, and (b) the higher energy value of dried skim-milk as compared with white fish-meal.

4. The commercial grading of pigs on all four treatments was very good. The carcass measurement data indicated a tendency for the skim-milk diets, particularly those containing either 10 or 15%, to result in some increase in both back fat and belly thickness. The possibility that this was associated with the higher energy/protein ratios in the skim-milk as compared with the white fish-meal diets was discussed.

1. The variation in the time required for the coagulation of milk by rennet and the relationship between milk composition and renneting time were examined.

2. The renneting times of herd bulk milks varied over the relatively narrow range of 3·2–5·4 min., whereas the renneting times of milks from individual cows varied from 1·4 to 12·9 min. and two did not coagulate.

3. In general, milk quick to coagulate gave a firm curd (early lactation milk) and milk slow to coagulate gave a soft curd (late lactation and subclinical mastitis milk).

4. The renneting time of milk was related to the stage of lactation of the cow. In early lactation, renneting times were short and as lactation advanced there was a progressive increase in renneting time; the increase became more pronounced towards the end of lactation.

5. The property of milk most closely related to renneting time was acidity. As pH increased from 6·4 to 7·2, there was a curvilinear increase in renneting time from 1·5 to 13 min. Titratable acidity was related to renneting time in the opposite way to pH.

6. Although milk containing a relatively high concentration of ionized calcium coagulated quickly with rennet, the renneting time of milks containing average, or less than average, amounts of ionized calcium was not related to their content of this constituent.

7. Weak relationships between the concentrations of other milk constituents and renneting time could be attributed to the interrelations of the concentrations of these constituents and pH.

1. The variation in the stability of milk protein to heat and the relationship between milk composition and heat stability were examined.

2. The coagulation times of the majority of the milk samples decreased by a factor of about 3 with an increase in temperature of 10°C. over the range 130–150°C. Because of the general proportionality of the coagulation times at 130, 140 and 150°C., the coagulation time at 130°C. only were used as a measure of the stability of the samples to heat.

3. The coagulation times of herd bulk milks ranged from 17·2 to 59·0 min. at 130°C., whereas the range for samples from individual cows was 0·6–86·2 min.

4. Samples of colostrum were very unstable to heat, and milk from cows in late lactation tended to have the longest coagulation times, but otherwise there was little relation between the heat stability of milk and the stage of lactation of the cow.

5. Although colostrum samples were comparatively rich in ionized calcium, their marked instability to heat appeared to be caused solely by their high content of lactalbumin plus lactoglobulin.

6. The stability to heat of the calcium caseinate-calcium phosphate complex in all samples, other than colostrum, could not be closely related either to the concentration and composition of the complex or to the composition and salt-balance of the aqueous phase.

7. When the calcium phosphate content of the caseinate complex was relatively low, the heat stability of the complex tended to be inversely related to the concentration of ionized calcium in the milk, but in general coagulation time was not related to the concentration of ionized calcium.