Milk samples may frequently contain coliform organisms which give very slow reduction of methylene blue at 37° C.

This slow reduction seems to be due to two factors present in raw milk, both of which are destroyed by heating milk to 70° C. for 1 hr. These factors are (a) a specific bactericidal substance, and (b) a growth-inhibitory factor.

All the cultures examined appeared from differential tests to be intermediate types of coliform organisms. All were citrate negative and failed to produce acid or gas when grown in MacConkey's broth at 44° C. This tends to confirm the findings of Taylor (4) that the use of certain tests for the differentiation of coliform bacteria is not justified.

A second outbreak of phage infection with complete cessation of acid production in experimental vats is described. Although occasional positive indications were obtained from the milk supply itself, the original source of the phage was not definitely demonstrated.

The results obtained in testing for phage appeared to be influenced by the size of milk sample examined, and by refrigerated storage for 7–16 hr. before testing.

Trouble with slow working vats in two factories which had continued over a period of 6 weeks was shown to be due to lysis of the dominant strains of the mixed culture of starter organisms by phage.

Attention is called to the need for substitution of a starter of entirely different bacterial strains when such trouble is encountered.

If milk containing the inhibitory substance produced by growth of ‘non-acid’ streptococci is used for the propagation of starter, it can cause delayed coagulation, simulating a starter failure caused by bacteriophage. ‘Non-acid’ milk in the starter culture milk affects cultures differently according to the particular strains of streptococci present; some strains are considerably less inhibited than others.

The belief that over-ripening of starters is responsible for their weakening and for slow acid development during Cheddar cheese-making has been widely held by cheese-makers for many years. Statements that over-ripening is harmful have been made by investigators in several countries(1,2,3,4), but experimental evidence in support of this view is scanty. On the other hand, Whitehead(5) in New Zealand has shown that prolonged over-ripening of single-strain starters under rather extreme conditions merely resulted in a temporary checking of their activity; between wide limits of size of inoculum and period of incubation no permanent damage was done.

The effect of temperature on the growth in milk of several strains of Str. cremoris and their appropriate phages has been investigated. The phage races show a wider diversity of reaction to temperature conditions than do the homologous organisms. They frequently have different optimum growth temperatures quite distinct from the optimum growth temperature of the substrate organisms. Some races fail to multiply at 37° C.

The implication of the results in cheese-making practice is discussed.

Abomasal juice containing rennin was obtained from living calves by means of an abomasal fistula as described by Fomin, in order to ascertain whether the process could be used as an alternative method of manufacturing rennet, should the present curtailed sources fail completely.

The operation for fistula was performed on the calves at the age of about a fortnight by the technique of Phillipson & Innes. The animals received, during the three months for which the experiment lasted, an exclusive diet of whole milk, supplemented by minerals and vitamin D. Although only two calves were used, we feel that the results are nevertheless representative.

Abomasal juice was obtained by allowing them to drink diluted whey and removing it through the fistula after about half an hour. The mean yield of rennin for each ‘perfusion’ from the first calf was 3120 units with a standard deviation of ± 1330. The corresponding figures for the second calf were 5680 and ± 2560 respectively. (One unit of rennin clots 10 ml. of substrate in 100 sec. The substrate is made by dissolving 12 g. spray-dried skim-milk powder in 100 ml. N/50 calcium chloride solution.)

The following summary of the effect of increasing quantities of oxygen in producing ‘off’ flavour in gas-packed full-cream milk powder during storage for long periods has been based on observations made in the present series of experiments, supplemented to some extent by results from later work on the gas storage of spray-dried powder which is not here reported in detail.

(a) Full-cream powders stored in the presence of up to 0.01 ml. of oxygen per g. of powder kept very well at both normal and high temperatures. This figure corresponds to 1 % of oxygen (after completion of desorption) in the free-space gas of a can of spraydried powder packed to a bulk density of 0.55 g./ml., or to 0.5 % of oxygen in a can of roller powder packed to a bulk density of 0.35 g./ml. Tallowiness was never definitely detected under such conditions, and there seems to be little or no advantage to be gained, at least so far as palatability is concerned, by improving on this figure. An atmosphere containing not more than 0.01 ml. of oxygen per g. of powder can therefore be considered an ideal pack for milk powder.