A cover for a 40 gal. cheese vat has been specially designed to allow cheese to be made under conditions which prevent the entry of micro-organisms into the vat from the dairy environment. The technique employed is described. Using milk free from lactobacilli it has been possible to make Cheddar cheese in which no lactobacilli grew during the ripening period. An assessment of the role of these and other micro-organisms in cheese ripening should now be possible.

The technique should also be of help in other cheese investigations where control of the microflora is desired.

1. In commercial practice dairy liquids of fat contents ranging from 3 to 90% are subjected to deodorization by steam distillation treatment.

2. Increase in fat test of cream causes a rise in the vapour/liquid equilibrium coefficient for tainting substances of low butterfat/skim-milk distribution ratio, and a fall for substances of high-distribution ratio, in cream.

3. Increase in fat content of cream reduces the amount of tainting substance in the cream per unit weight of butterfat in the cream, for substances of all butterfat/skim-milk distribution ratios.

4. Separation of milk to cream of a high fat content greatly facilitates the elimination of substances of low butterfat/skim-milk distribution ratio from the cream and butter.

5. Variation in temperature of separation, by its effect on the distribution ratio for tainting substances, could affect the content of these substances in the final cream, and could possibly cause a change in the character of the cream.

6. In the partition of tainting substance between butter and buttermilk in the churning process, fat content of cream affects the proportion of residual taint passing from the treated cream into the butter.

7. It is pointed out that, because of the effect of salt in increasing the vapour/liquid equilibrium coefficient of some steam volatile substances, the threshold concentration for a tainting substance in butter can be expected to vary with variation in the rate of salting of the butter.

1. Continuous growth of lactic acid streptococci in autoclaved milk allows strains, originally insusceptible, to become susceptible to inhibition by raw milk.

2. Variants of the original organisms, the growth of which is inhibited in raw milk but not in autoclaved milk, appear to predominate in strains maintained in autoclaved milk.

3. The time required to allow these changes to occur in autoclaved milk varies with the strains used.

The citric acid content of 440 samples of milk has been determined. The average citric acid content of bulk milk of an estimated 10,000 cows was 0·172%. The average citric acid content of the mixed milk of a Holstein herd of 35 cows was 0·168%, and the average citric acid content of the milk of individual cows was 0·171–0·189%. The coefficients of variation of the citric acid content were: in bulk milk 3·49, in mixed herd milk 7·74 and in the milk of individual cows 10·6–13·5. The citric acid content of the milk of individual cows may differ significantly. The citric acid content of milk in the grazing season was slightly lower and more variable than in the housing season. The stage of lactation had no significant effect on the citric acid content of milk. The time of day of the milking had no influence on the citric acid content of milk. Marked differences may occur in the citric acid content of milk from two successive milkings of an individual cow, and in this study differences up to 0·034% or 23·6% of the lowest value have been found.

1. Data are given on the changes which occurred in the composition of sow's milk during the course of 24 lactations. The mean percentages of the major constituents (192 samples) were: total solids, 20·0%: crude protein, 5·7%: lactose, 4·7%; fat, 8·6% and ash, 0·89%.

2. There were differences in milk composition between sows and between successive lactations, but these were only appreciable for milk fat percentage between sows.

3. A highly significant negative correlation (P < 0·001; r = –0·62) was found to exist between the percentages of protein and lactose in the milk, quite apart from the general trend for protein content to fall with advance in lactation. Possible reasons for this are discussed.

Anoptral contrast, a modification of phase-contrast microscopy, appears to be suitable, particularly perhaps when used in conjunction with fluorescence microscopy, for studying casein particles above 0·1–0·2μ in diameter, and different aggregation and coagulation forms of casein, which may comprise irregular flocks of aggregated particles, thin membranes and micro-fibres, nodulated or smooth.

1. When the concentration of fat-free milk-powder is increased, other factors being kept constant, the elastic and viscous setting-parameters of the curd also progressively increase, being related to the concentration by power-equations. The intercepts, which correspond closely to ‘coagulation times’, likewise increase, but nearly linearly with concentration

2. These effects depend mainly on protein concentration. Quite large quantities of added lactose make little difference to the parameters.

3. The slopes of the logarithmic setting curves vary quite independently of their intercepts, so that ‘coagulation times’ provide a very incomplete indication of the rate of setting of curd.

4. A comparison of setting data obtained in the ordinary way at a series of temperatures with those from experiments in which the rennet is first allowed to act on the milk at near freezing-point, show that, in the latter case, there is considerable delay before the normal equation of setting is established in the curd. It is also clear that high temperatures are detrimental to this process of setting, just as they are when the usual procedure is followed.

5. The general law that the rate of setting is, at any time, proportional to the amount of still unset casein, is established much sooner if the time zero is defined in such a way as to take all the processes involved together. If an ‘induction period’ is subtracted, either by letting the rennet first act in the cold, or by taking the time zero in the normal process at the coagulation, and not the renneting, time, a much longer time elapses before the curves become linear.

We are indebted to many colleagues at the N.I.R.D. for helpful discussions, especially Dr S. J. Rowland and Dr R. Aschaffenburg.

It has been shown that the lactose concentration in separated milk can be determined rapidly with an accuracy of ±1·5% by infra-red absorption methods, using wavelengths in the 10μ region.

The method can also be used to determine lactose in condensed whey and should prove applicable to continuous recording techniques.

The numbers of lactobacilli present in milk for cheese-making and in the cheese at various stages of ripening, have been determined for 38 Cheddar cheeses made during studies on flavour enhancement. These organisms multiplied rapidly even during the first few days of curing. Maximum levels were attained at 3–6 months; at 1 year counts had declined appreciably.

Flavour intensity in the experimental cheese appeared to be correlated with the level of lactobacilli present in (a) milk at the start of cheese-making and (b) at subsequent stages of ripening. These two count levels were usually closely correlated. Factory raw milk had the highest counts and gave the highest degree of flavour, followed by similar milk pasteurized and inoculated with selected strains of lactobacilli.

1. The administration of 0·5–1·5 kg. sodium acetate to cows in which the milk fat percentage had been reduced by diets low in hay and high in concentrates usually brought about an appreciable improvement in fat percentage. The extent of the response varied from slight to complete. The Reichert value of the milk fat fell with diets low in hay, but was raised by administration of acetate.

2. The daily administration of 500 g. sodium acetate to cows receiving diets containing normal levels of hay and concentrates according to their milk yields did not affect the milk fat content.

3. The daily administration of 414 g. sodium propionate did not restore fat percentages lowered by the diets low in hay. Butyrate appeared, in a test with one cow, to possess the restorative properties of acetate.

4. With diets containing either 50 lb. silage as the sole roughage or 60 lb. fodder beet and only 6 lb. hay, fat percentages were the same as with a normal diet containing 16 lb. hay.

1. An experiment to investigate the seasonal increase in the solids-not-fat content of milk, which usually occurs in Britain when cows go from winter feeding to spring grazing, is described.

2. Two similar groups of cows were fed good quality winter rations at the same high level both before and after calving (about 115% of the Woodman standard after calving), until spring grass was available. One group of 7 cows was then fed for 6 weeks almost entirely on cut grass, returning to winter rations for a further 3 weeks. The control group of 6 cows continued to be fed throughout the experiment on winter rations and both groups remained indoors for the whole of the experiment. The estimated nutrient intake of both groups in all periods was kept substantially constant.

3. Changes in milk yield and composition were mainly those associated with advancing stage of lactation, and grass feeding had little specific effect. It is concluded that the increase in milk protein content which is normally observed soon after the start of spring grazing probably results from an improved plane of nutrition and not specifically from an increase in the intake of soluble carbohydrates.

4. Increases in the total nitrogen content of the milk as the experiment proceeded were contributed to uniformly by the casein, total albumin and β-lactoglobulin fractions.

Theoretical and practical aspects of the apparent or effective viscosity of milk have been studied by many workers: see reviews by Scott Blair(1,2,3). Seidler & Elke(4), Cox(5), and others have considered the relations between observation temperature and viscosity measurements; Mohr & Oldenburg(6) and Puri & Gupta(7), for example, have investigated the effects of differences between individual viscometers. Other workers, including Puri & Gupta(7), Kobler(8) and Spöttel & Gneist(9), have examined the viscosity of whole raw milk after different times and temperatures of storage; and Weinlig(10), Trout & Weinstein(11) and Whitnah, Rutz & Fryer(12,13) have studied changes in viscosity values in connexion with various processes such as pasteurization and homogenization.

Butterfat has been shown to contain the normal odd-numbered saturated acids n-nonadecanoic acid (C19), n-heneicosanoic acid (C21), and n-tricosanoic acids (C23).

The presence of the normal even-numbered acids n-octadecanoic acid (C18), n-docosanoic acid (C22), n-tetracosanoic acid (C24) and n-hexacosanoic acid (C26) is conclusively established.

n-Eicosanoic acid (C20) formerly assumed to be present in butterfat has been isolated and identified by its physical and chemical properties.

1. Four species of Medicago, six species of Trifolium and fourteen grass species, all frequently found in British dairy pastures, have been tested for oestrogenic activity by the mouse uterine weight method. Activity was detected in four, four and six species respectively.

2. It is probable that the quantity of oestrogen present in British pasture plants is insufficient to affect milk composition in cows at pasture. Some further experiments with cows are, however, suggested which might elucidate this.

1. Four test organisms were used for the microbiological assay of vitamin B6. The test organisms and the forms of vitamin B6 active for them were: Saccharomyces carlsbergensis 4228 (pyridoxine, pyridoxamine and pyridoxal), Streptococcus faecalis R ATCC8043 and Str. faecium Ø51 (pyridoxamine and pyridoxal), and Lactobacillus casei ATCC7469 (pyridoxal).

2. By simultaneous assay of the test material with the four organisms the amounts of pyridoxine, pyridoxamine and pyridoxal present were determined. The differential assay technique has been improved by the use of Str. faecium, which is more specific for measuring total pyridoxamine plus pyridoxal, instead of Str. faecalis.

3. The optimum conditions for measuring vitamin B6 in milk were studied and the differential assay technique was applied in an investigation of the effect of heat on the vitamin B6 activity of milk.

4. It was found that pyridoxal accounted for about 80% of the vitamin B6 activity of raw milk, the other 20% being due to pyridoxamine.

5. In the manufacture of evaporated milk there was a marked fall in the pyridoxal and a slight increase in the pyridoxamine content. The overall loss was some 60%.

6. On storage of evaporated and of sterilized liquid milk at room temperature there was a further loss of vitamin B6 activity. Most of the residual activity in stored evaporated milk appeared to be due to a substance other than pyridoxal or pyridoxamine, possibly pyridoxine.

1. For chicks and rats pyridoxine, pyridoxal and pyridoxamine were equally active in terms of the free bases when given separately from the diet.

2. Under our experimental conditions pyridoxine mixed with the chick diet was stable, but 20% of pyridoxamine, and a variable amount of pyridoxal was lost.

3. The vitamin B6 activities measured with Saccharomyces carlsbergensis, chicks and rats respectively and expressed as μg. pyridoxine/g. freeze-dried milk were: raw milk 3·4, 3·2 and 4·9; evaporated milk 1·0, 2·1 and 2·7; stored evaporated milk 0·6, 1·4 and 2·0. For the chicks the milks were mixed with the diets; they were given separately to the rats.

4. The microbiological and biological results for raw milk agreed within the limits of experimental error. For the processed milks the differences between biological and microbiological tests were statistically significant.

5. All three methods of assay showed a 45–70% loss of vitamin B6 activity on processing and a further loss of 30% of the remainder after storage for 6 months at room temperature.

We are indebted to Mr J. Rothwell, Department of Dairying, University of Reading, for preparing the evaporated milk and to Dr B. Record, Ministry of Supply, Microbiological Research Establishment, Porton, for freeze-drying the milk. We should like to thank Dr S. K. Kon for his interest in this work.