There can now be little doubt that glucose is the principal precursor of lactose in the mammary gland. Evidence for this comes form in vivo and in vitro studies, using both analytical and isotopic techniques (1), though considerable species differences have been noted.

The proportion of fat present in cream as small globules (approximately 1 μ and under) is estimated from the proportion of the total fat which remains in the serum when a sample of the cream, diluted to 10% fat, is centrifuged at 60° C. for 30 min. at 3000 r.p.m.

The fat present as large globules (7 or 8μ and over) is estimated by finding the proportion of the fat which rises 1 cm. in 60 min. when the cream, diluted to 10% fat, is held at 60° C. in a modification of the Andreasen particle-size apparatus in which the jar is replaced by a conical flask and the tip of the sampling pipette is held 1 cm. from the flat bottom of the flask.

The thanks of the author are due to Dr F. H. McDowall for helpful advice and criticism and to Miss R. P. Johnson for assistance with the experimental work.

1. Criticism is directed towards the method of assessing cheese-starter activities, whereby pasteurized milk (vat milk) is inoculated with starter and its activity judged by the acidity developed after 6 hr. at 30° C.

2. It has been found that certain starters are slow when subjected to this laboratory test, owing to the fact that they are agglutinated in such milk and consequently carried up into the cream layer, thus producing a lower acidity in the milk.

3. Whilst such starters show higher activity in homogenized or autoclaved milk, it is suggested that starter activity tests should be carried out in the presence of agar in order to prevent cream rising and thus simulate conditions in the cheese vat. Agar is preferred to rennet for this purpose.

4. It is emphasized that such apparently slow starters are not slow when used in the cheese vat.

5. Evidence is given that keeping-quality tests on pasteurized and homogenized milks involving storage of the milk are also affected by the same factors, which may result in homogenized milk having a poorer keeping quality than pasteurized milk.

1. The distribution of carotenoids and vitamin A in milk-fat globules has been further investigated. The results obtained support previous observations that the carotenoid content of the fat obtained by solvent extraction from the smaller globules in milk is greater than in the fat obtained from the larger globules. It has also been shown that the vitamin A alcohol content of the extracted fat varies in the same way as the carotenoid content, but that the reverse appears to be the case for vitamin A ester.

2. It is suggested that these differences in concentration are not due to a surface layer of carotenoids and vitamin A alcohol on the fat globule but to the presence in mammary secretions, as in blood, of protein-bound forms of these substances. In support of this it has been shown that when fat is obtained by centrifuging rather than by solvent extraction, its carotenoid and vitamin A content appears to be almost independent of globule size. Furthermore, some vitamin A and carotenoids remained in the virtually fat-free serum obtained from normal milk, colostrum and pre-partum secretion and was precipitated with the various serum proteins, particularly the globulin fraction. The increased carotenoid and vitamin A alcohol content of mastitis milk may also be explained by the presence of abnormally large amounts of these protein-bound forms.

The author is grateful to the Department of Scientific and Industrial Research for a grant towards the cost of the investigation; also to the farm staff of The Dairy Research Institute (N.Z.) for assistance with the collection of milk and colostrum samples.

The effect of pH on the extent of splitting and clumping of globules in hot cream agitated by treatment in a Vacreator, by vigorous mechanical stirring or by simple aeration has been investigated. The size distribution of globules was determined in cream before and after treatment either by the microscopic method or by a rapid method involving separation of small and large globules by centrifuging and gravity creaming respectively.

It was found that at pH 8 splitting of globules was at a maximum and clumping at a minimum. As the pH of the cream to be treated was lowered towards pH 6, the amount of splitting decreased and clumping increased. The ‘clumping’ of globules in cream during single-stage homogenization was also increased as the pH was lowered.

Practical applications of the observations are suggested.

The thanks of the author are due to Mr J. A. Singleton, superintendent and Mr B. Le Heron, buttermaker of The Dairy Research Institute (N.Z.) experimental factory, for arranging and carrying out the Vacreator treatment of cream; to Dr F. H. McDowall for helpful advice and criticism, and to Miss R. P. Johnson for assistance with microscopic and analytical work.

1. The vapour/liquid equilibrium coefficient for diacetyl in 35–40% cream has been found to be of the order of 26–28, compared with 39·3 for water. In a 5% aqueous solution of lactose, equivalent to that present in milk and in cream serum, the coefficient was 30. The depression of the equilibrium coefficient by lactose in solution was shown to be independent of the concentration of diacetyl present in the liquid at time of partition, over the range 1–65 p.p.m.

2. The slower rate of removal of diacetyl from cream, per unit weight of steam condensed as compared with its removal from water, was observed to be due mainly to this depressing effect of lactose on the equilibrium coefficient; but there was some evidence that delay in re-establishment of the solubility partition equilibrium as between butterfat and cream serum also contributed to the lowering of the equilibrium coefficient for diacetyl in cream as measured in the laboratory continuous vaporization equilibrium still.

3. Sodium chloride in the solution caused a rise in the equilibrium coefficient for diacetyl from the value of 39·3 for water to about 69 for a 10% salt solution and 82 for a 20% salt solution.

4. Lactose had no effect on the equilibrium coefficient for acetoin, but salt caused a rise from 1·29 for water to approximately 1·75 for a 10% salt solution.

5. The possible effect of lactose on the effectiveness of steam in cream deodorization equipment is referred to.

The processing of liquid milk, particularly sterilization ‘in-bottle’ or ‘in-can’, may cause considerable destruction of its content of vitamin B12. This loss can be greatly reduced by thorough deaeration of the milk before heating.

The author would like to thank Dr S. K. Kon for helpful advice and criticism.

1. The effect of the level of clover intake on the vitamin A potency of milk fat has been studied, using Jersey cows from the herd of the Dairy Research Institute (N.Z.).

2. In preliminary experiments in which composite bi-weekly butterfat samples were obtained from the whole herd of thirty-eight cows no relationship could be established between level of clover intake and potency of fat produced. This was assumed to be due to the rapidity with which the level of clover intake varied, since under normal grazing management the cows were moved to a fresh paddock (which was often of widely different botanical composition from that previously grazed) after every second milking.

3. In later experiments in which three pairs of monozygous twins from the herd were used, a clear-cut relationship was established between clover intake and potency of fat produced and between clover intake and iodine value, oleic acid percentage and tocopherol content. Diets high in clover tended to depress total potency, iodine value, oleic acid percentage and tocopherol content of the fat, while diets low in clover had the reverse effect. The clover used in these experiments was mainly an improved strain of white clover containing approximately 0·03% cyanide. Repetitive experiments using improved strains of red clover produced negative results.

4. The mechanism by which diets high in clover depress fat potency, iodine value, oleic acid percentage and tocopherol content is not clear. It is suggested, however, that it may be due to (i) the low tocopherol content of clover relative to grass, (ii) the high HCN content of certain species, and (iii) the possibly lower oleic acid content. The possibility is also considered that it may be due to some factor (or group of factors) associated with the stage of development of the pasture as a whole at a time when clover happens to be the dominant pasture species.

5. Drenching cows with thiocyanate, the probable detoxication product of HCN in the ruminant, could not be shown to have any effect on the levels of vitamin A alcohol, ester, carotene, or xanthophyll in the blood and milk fat.

Some of 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 M.Agr.Sc. of the University of New Zealand. The authors are indebted to the Department of Scientific and Industrial Research for a grant towards the investigation and to Mr M. R. Patchell and farm staff of The Dairy Research Institute (N.Z.) for assistance in collection of milk and blood samples. The technical assistance of Miss Fay Frecklington is gratefully acknowledged.

1. Pasteurized, sterilized, U.H.T. treated, U.H.T. treated-sterilized, evaporated, sweetened condensed, roller- and spray-dried milks were prepared simultaneously from one bulk of raw whole milk.

2. The preparation of any of these products entailed little or no apparent loss of riboflavin, pantothenic acid, nicotinic acid, vitamin B6 or biotin.

3. Thiamine was destroyed in amounts ranging from less than 10% to 40–50%, increasing with the severity of the heat treatment.

4. Prolonged high temperature used in the sterilization of evaporated milk and of in-bottle sterilized milk caused nearly complete loss of vitamin B12.

5. The new process of sterilization by the U.H.T. treatment alone when it was not followed by the, so far necessary, in-bottle sterilization caused little more loss of the vitamins measured than did H.T.S.T. pasteurization.

1. Despite the presence of appreciable quantities of steam volatile fatty acids as glycerides in butterfat, no short-chain fatty acids with odd carbon number have been detected by gas-liquid chromatography of the acids.

2. Mild oxidation of butterfat for short periods of time produces steam volatile fatty acids with odd carbon number. A large proportion of these acids was formic, with smaller proportions of propionic, valeric and nonanoic acids.

3. The relative ease with which short-chain fatty acids are produced on oxidation of fats, points to the danger of contaminating naturally occurring fatty acids with extraneous acids.

4. A total analysis of the fatty acids of New Zealand butterfat by gas-liquid chromatography compared well with similar analyses carried out by previous workers by means of esterfractionation analysis.

The size distribution of fat globules was determined microscopically in cream samples taken from different parts of the Vacreator during treatment of cream.

A considerable increase in numbers of small globules (splitting) was found in samples which had passed the equilibrium valve. Smaller increases took place in the pasteurizing chamber and in the extractor pump. Increase in numbers of large globules (clumping) occurred in the cooler section.

Splitting of globules could be reduced: in the pasteurizer by reducing velocity of incoming steam; at the equilibrium valve by reducing the vacuum drop across the valve or by introducing part of the steam at the infuser instead of into the pasteurizer.

It is concluded that splitting of globules in the Vacreator is due to a homogenizing effect where steam and cream travel at a high velocity. Flash-boiling causes clumping but not splitting of globules. These conclusions should apply to any equipment for pasteurization or deodorization of cream with direct steam.

The thanks of the author are due to Mr J. Singleton (Superintendent) and Mr B. Le Heron (Buttermaker) of the Institute factory for arranging and carrying out Vacreator treatments of cream, to Dr F. H. McDowall for helpful advice and criticism and to Mrs R. P. Haverkamp for assistance in measuring fat globules.

1. The levels of vitamin A alcohol and ester, carotene and xanthophyll in the blood plasma of six cows, grazed out-of-doors on pasture under typical New Zealand conditions, have been followed throughout one lactation. The same constituents have also been followed in the milk fats of these animals and in commercial factory butters over the same season.

2. The seasonal trend in the vitamin A and carotenoid content of New Zealand milk fat is a reflexion of the changing level of these substances in the blood plasma.

3. The decreased level of carotene, xanthophyll and vitamin A ester in the blood plasma over the summer months is consistent with a decreased absorption or utilization of carotenoids over this period.

4. Throughout the year the vitamin A in the milk fat is predominantly in the ester form with no increase in vitamin A alcohol indicative of any increased utilization of hepatic vitamin A reserves.

The author is indebted to the Department of Scientific and Industrial Research for a grant towards this investigation; to Mr M. R. Patchell and the farm staff of The Dairy Research Institute (N.Z.) for assistance in the collection of milk and blood samples; to Dr F. H. McDowall, Mr A. K. R. McDowell and the staff of the Chemistry Laboratory, The Dairy Research Institute (N.Z.), for the preparation of the fat samples and for the supply of factory butters. The assistance of Miss Fay Frecklington and Miss Jane Monroe with part of the experimental work is also acknowledged.

Milk samples were collected, between 17 April and 30 May 1956, from fifty-six Ayrshire and forty Friesian cows, which had been calved for between 45 and 165 days, when they were running out at grass day and night. The results showed that differences in the fat percentages of their milk were accompanied by corresponding variations in the average diameter of the globules in both breeds and in the numbers of globules per cu.mm. in the Friesian cows. These changes were not significant for either variant because there were some individuals in which changes in fat percentage appeared to be accounted for by either an increase in fat globule diameter or an increase in the number per cu.mm. with correspondingly low figures for the other variant.

1. During the period September 1954—August 1956 monthly samples of milk were obtained from individual cows in herds selected because of known history of low solids-not-fat. Records were kept of general feeding and management. Milk was analysed for total solids, butter fat and s.n.f. (by difference). Crude protein and lactose were also estimated at three-monthly intervals.

2. Mean lactation averages were calculated for s.n.f. These showed a normal distribution around a mean of approximately 8·5%.

3. Throughout the period a high proportion of samples failed to reach 8·5%. The seasonal effect is at its worst in March and April when 70% of the samples were below 8·5%.

4. Effect of advancing lactation (uncorrected for the effect of season) agreed broadly with previously published work but marked differences in the shape of the lactation curve were found between different herds. Variations in s.n.f. are largely a reflexion of variations in crude protein.

5. Age distribution of the cows agreed favourably with published figures for the national herd, and a mean regression of s.n.f. with age gave 0·11% per year.

6. Supervision of the feeding of the herds in the investigation gave no measurable improvement in s.n.f. and no significant response was obtained from short-term trials where extra s.e. was fed.

7. There was no undue incidence of disease of the udder, either clinical or subclinical.

8. Little information could be obtained on breeding, but it is concluded that the major cause of the problem is the genetically poor potentialities of the cows as judged from the average composition of the milk they produce.

The relationships between nineteen strains of Str. cremoris and nineteen phage races were investigated. It was found that in addition to clear-cut lytic reactions there were other actions between some phage races and bacterial strains whereby in some cases a phage could show adaptation to a strain and become changed in host range, and in other cases could inhibit growth of a strain without any phage multiplication.

There is some evidence to indicate that bacterial strains and phage races fall into family groups within which most of the phages act in one way or another on most of the strains. The suggestion is made that the strains and races in a group may have originated from a parent bacterial strain and parent phage race.

The selection of strains of Str. cremoris for use as cheese starters is discussed in the light of the findings reported in the present paper. The diffculty of isolating a large number of strains unrelated in every respect is indicated.

1. The effect of varying levels of vitamin A ester in the blood plasma of cows on the levels of vitamin A ester in their milk fat has been investigated.

2. The milk fat levels reflect plasma levels, but it appears that, irrespective of the immediate carotene or vitamin A intake of the animals or of their liver reserves, some of the milk fat vitamin A ester is derived from vitamin A of other than immediate dietary origin.

3. It is suggested that the bulk of the plasma carotene contributes little to the level of carotene in the milk fat. The latter derives mainly from the carotene of immediate dietary origin which, during transport from the intestines to the storage organs, is associated with dietary fat and vitamin A ester in the chylomicrons.

The author is grateful to the Department of Scientific and Industrial Research for a grant towards this investigation; also to Mr M. R. Patchell and the farm staffs of The Dairy Research Institute (N.Z.) and Massey Agricultural College for assistance with the feeding and dosing of the animals and the collection of milk samples. The assistance of Miss Fay Frecklington with some of the experimental work and of Mr A. C. Glenday with statistical examination of the results is also acknowledged.

1. Plasma levels of vitamin A and carotenoids prior to and following parturition have been followed in groups of cows and heifers maintained under typical New Zealand farming conditions.

2. Despite the high carotene intake prior to calving, the cows, and to a lesser extent the heifers, showed decreases in plasma vitamin A and carotenoid level at or near to parturition, the decreases in the levels for the cows being of the same order as those reported in Great Britain and the United States.

3. Although the concentration of vitamin A and carotenoids was higher in the colostrum of heifers than of cows, the total daily secretion of these components by the cows was much greater over the first few days of lactation.

4. Evidence is advanced to support the suggestion that the decrease in plasma components is related to the extra drain of colostrum synthesis rather than to any hormonal changes associated with parturition.

We are indebted to the following: the Department of Scientific and Industrial Research for financial assistance towards the cost of this investigation; Mr R. M. Patchell and the farm staff of The Dairy Research Institute for the collection of milk samples; Miss Fay Frecklington for help with much of the analytical work; one of us (S.Y.T.) 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. The trends of starter bacteria population during the manufacture and maturing of Cheddar cheese have been investigated. Fourteen strains consisting of Str. lactis, Str. diacetilactis and Str. cremoris have been studied.

2. Physiological and morphological characteristics of the fourteen strains were examined.

3. Population studies during manufacture showed that the three species of starter organism attain quite different maximum populations and at different stages of the cheesemaking process. Str. lactis (4500 millions/g.) and Str. diacetilactis (2900 millions/g.) reached their maximum half way through Cheddaring, whereas Str. cremoris (960 millions/g.) reached its maximum at milling.

4. Str. lactis was shown to maintain its maximum population until the cheese left the press, whereas Str. diacetilactis and Str. cremoris steadily declined after reaching their maximum.

5. Counts of the different starter cultures used showed considerable variation between and within strains, particularly in the case of Str. cremoris.

6. No relationship was shown to exist between acidity development and starter populations developed.

7. The study of cell distribution in the cheese curd has shown that whereas Str. lactis and Str. diacetilactis gave an even scattering of diplococci, Str. cremoris showed colony formation.

8. Str. lactis strains maintained a high but declining population during maturing of the cheese, the starter beginning to die out gradually after 8 weeks. Str. diacetilactis showed similar trends but at lower populations levels. Str. cremoris, on the other hand, gave much lower populations, the starter dying out rapidly after 2 weeks, and virtually disappearing at 8 weeks.

9. Persistence of starter bacteria in the cheese appears to be correlated with salt tolerance and chain length.

10. Starter bacteria isolated from the cheese made with Str. lactis have shown a low proportion of slow acid-producing types in contrast to a high proportion found with Str. cremoris.

11. The non-starter bacteria in the cheese were found to consist on the average of 30% micrococci and 70% lactobacilli. Negligible numbers of other genera were isolated.