The ratio of phage-resistant to phage-susceptible chains in five strains of Streptococcus cremoris and two of Str. lactis was investigated. It was shown that this ratio varied between strains and was decreased when the cultures were subcultured with extremely small inocula ( <50 chains/ml) and increased with relatively large inocula (1%). It is suggested that resistant cells arise by mutations occurring in the absence of phage.

The rates of secretion of milk, fat and solids-not-fat have been measured over milking intervals of 2–24 h in a series of experiments using thirty cows at various stages of lactation. To ensure that the results were not biased by a carryover of residual milk and fat the residual milk was removed after intravenous injections of ‘Pitocin’. The biases that can be caused by previous interval effects and lactation trends were eliminated by the balanced Latin square design used.

Milk and solids-not-fat secretion appeared to be linear with duration of milking interval up to 16 h, but with longer intervals there was, in some experiments, a decline in the rate of secretion. There was considerable variation between cows in the decline in secretion rate with increasing milking interval and part of this variation was associated with differences in milk yield. In all experiments fat secretion appeared to be almost linear with time over a 24 h period.

After milking intervals of 20 and 24 h the rate of secretion of milk and solids-not-fat, but not of fat, was depressed for at least 16 h.

For milk sterilized under partial vacuum in closed bottles the oxidation-reduction potential measured under nitrogen was markedly lower (—280 to —300 mV) than that of milk heated in bottles open to the air, measurements being made in air (+10 to —30 mV).

The Eh (under nitrogen) of commercial sterilized milk and of ultra-high-temperature short-time heated milk was in the range —200 to —300 mV and of the same order as that obtained in the laboratory for deaerated milk sterilized in closed bottles.

A description is given of various methods for the ultrafiltration and dialysis of milk and of the composition of the sera obtained. Ultrafiltrate prepared by the procedure recommended is reasonably representative of the aqueous phase of milk, but its content of lactose and citric acid, and consequently also of calcium, is determined to a slight degree by the sieving phenomenon known to occur often in ultrafiltration. The composition of diffusate obtained from milk at 20°C is not thought to be controlled to any significant extent by a Donnan effect and is regarded as identical with that of the aqueous phase of milk. The lactose content of diffusate suggests that about 2% of the water in milk is bound to protein, and allowance should be made for this when calculating the concentrations of the soluble constituents in milk from the composition of diffusate. Diffusate prepared from milk at 3°C contains slightly more total calcium, ionized calcium and phosphorus than diffusate prepared at 20°C. These differences are attributed to a change in the partition of calcium and phosphorus between the disperse and aqueous phases at the lower temperature, an explanation that is supported by the reversibility of the change. The composition of diffusate prepared by the procedure recommended indicates that about 5% of the sodium and about 6% of the potassium and citric acid in milk are in the disperse phase.

The numbers of lactobacilli, including pediococci and some leuconostocs, in raw and heat treated milks at twenty-six cheese factories in different parts of the country were determined over a period of 12 months, by plating on acetate agar. Counts in the raw milks varied from < 1 to > 100000/ml, but were usually within the range 100 to 10000/ml. Widely different times and temperatures of heat treatment were in use at the different creameries, some of which destroyed most of the lactobacilli present, whilst others had less effect. Post heat treatment re-infection of the milk in the vat with lactobacilli usually occurred.

The effect of the temperature of laboratory sterilization of homogenized milk on the titratable acidity, the colour of the milk, the yellowing of the filtrate from the Aschaffenburg turbidity test, the acid-ferricyanide-reducing substances and on pH has been determined. The greater changes in these properties in milk sterilized under reduced oxygen tension compared with milk sterilized in open bottles was shown to be due, in part at least, to the slower rate of cooling in the evacuated bottles. Samples of commercial sterilized milk have also been examined and their properties have been compared with those of the sterilized milk produced in the laboratory.

The use of starter cultures prepared 3 days, 2 days and 1 day before inoculation at the 1% level into cold cream in the making of sweet cream starter butter was investigated. With Streptococcus diacetilactis starter there was little effect of age of starter on diacetyl production in the cream and on the quality of the fresh butter, but when 3-day-old culture was used in the cream there was a slight reduction in the butter quality after 4 and 8 months' storage. With the mixed commercial cultures Camb and F.D. increase in age of the culture led to reductions in diacetyl production in the cream. The difference was attributed to the effect of holding for the longer time under acid conditions on the equilibrium between streptococci and leuconostocs in the mixed culture. The age of the Camb culture had no effect on the grade of the butter, but holding the F.D. culture for 3 and 2 days instead of for 1 day caused a fall in grade of the butter after storage for 4 and 8 months at 14 °F.

A preliminary examination of strains of leuconostocs collected from many different laboratories has resulted in a division into six groups. Some of these groups are heterogeneous. The name Leuconostoc cremoris is used in preference to Leuconostoc citrovorum for the non-sucrose fermenting species. Reasons are given. Other arguments lead to the conclusion that Streptococcus kefir (Evans) should be regarded as a synonym of Leuconostoc mesenteroides, and the name Leuconostoc lactis is used in preference to Streptococcus kefir (Abd-el-Malek & Gibson).

A distinct fish-oil or cod-liver-oil flavour appeared after storage in commercial butterfat containing, as an antioxidant, nordihydroguaiaretic acid (NDGA) and citric acid dissolved in propylene glycol. The fishy condition was reproducible in the laboratory by the combined effect of NDGA with either citric or lactic acids. The acids alone in butterfat gave oily and rather less clearly defined fishy flavours after storage.

A positive 80% alcohol test was not obtained in sterilized milks containing fewer than 105 bacteria/ml with Bacillus subtilis, B. licheniformis and B. cereus, and usually did not occur until the stationary phase of growth had been maintained for some days. A positive alcohol test with less than 105 bacteria/ml was associated with the lower maximum population density of B. brevis and B. circulans 152.

The alcohol test after 24 h at 37°C was found unreliable as an indicator of the keeping quality of sterilized milk at 22°C. The possibility of using the alcohol test after 3 days at 37°C or a bacterial count after the same incubation period is discussed briefly.