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.

1. Two Ayrshire cows in mid-lactation were fed for over 40 days on production rations containing from 16 to 20 g. of added borax.

2. The daily excretion of borate during the experimental feeding period was 13–16 g. in the urine, 5–7 g. in the faeces, and 0·3–0·6 g. in the milk, compared with figures of 1·5, 1·0 and 0·1 g. for the control period. There was no detectable retention of borate in the body, and the levels of borate excretion rapidly returned to normal after cessation of the experimental feeding.

3. No adverse effects were observed as a result of the borate feeding. The live weights of the animals remained normal, and there was no diuretic effect. The milk yield was also well maintained. The borate content of the milk increased from the normal level of 0·7 to over 3·0 p.p.m.

4. Attention is drawn to the risk of using farmyard manure which has been made from the excreta of animals fed on boronated rations.

In previous papers by the authors(l,2) it was shown that the vitamin A content of the colostrum of cows varied within wide limits and that the wide range was not due to any differences in dietary, breed or date of calving in animals kept under the same conditions of management. It was also established that calves born of cows whose colostrum possessed a vitamin A content of less than 250 blue units (Moore's) per 100 ml. were more liable to infections such as white scour, navel-ill and joint-ill than were calves from cows whose colostrum had a vitamin A content of more than 250 blue units per 100 ml.

1. A bacteriological study was made of the commercial operation over a period of 4½ years of two Kestner evaporating and spray-drying plants, handling a fairly good and a very poor milk supply respectively. Processes investigated comprised handling of raw milk, pasteurization, climbing film evaporation, handling of evaporated milk, and spray drying. Data were also obtained showing the influence of variations during processing on the moisture content and solubility of powders. Separated milk, full-cream milk, and whey were handled.

2. The general bacterial flora of spray-dried powders and the influence of individual manufacturing processes, packing and storage were studied and possible control methods suggested. Data showing the extent of both hourly and seasonal variations were also obtained.

3. A method of bacteriological analysis is described which gave reasonably good agreement between different workers.

4. Examination of 671 powder samples showed a wide variation in individual plate counts from 200 to 19,500,000 per g. The flora on standard milk agar at 37° C. was of a specialized type and comprised comparatively few species. Thermoduric streptococci of the ‘enterococcus’ and ‘viridans’ groups predominated, Str. durans and Str. thermophilus being the commonest species; in addition, five species of micrococci, probably non-thermoduric, also occurred commonly. Aerobic spore-forming bacilli and three Achromobacter species were found regularly, but their numbers were not of practical importance. Spore-forming anaerobic bacilli were present in 14% of powder samples, probably in very small numbers, and were most frequent during the winter months.

5. Coliform organisms were rarely found in 1 ml. of reconstituted milk, but were isolated from 25% of powder samples when 20 ml. of reconstituted milk was examined; coliforms occurred most frequently during the winter months. False positives due to anaerobes were common, especially in stored powders, and confirmatory tests of presumptive positives were essential. Examination of 198 coliform strains from powders and 164 strains from raw milk showed that coli types occurred less frequently in the powders, whilst aerogenes-cloacae types, particularly cloacae, were more frequent. 48·5% of the powder strains were heat resistant, as compared with 2·2% of the raw-milk strains. It was shown that the coliform flora of powder was partly due to plant contamination by heat-resistant strains, although some non-heat-resistant strains could survive spray drying. It seemed possible that coli types were less resistant to drying than aerogenes-cloacae.

A group of species, comprising Bacillus subtilis and related organisms, may be differentiated from other members of the genus Bacillus by (a) morphology and (b) the production of acid and acetylmethylcarbinol from glucose. The group may be divided into three species:

(1) B. subtilis Cohn emend. Prazmowski, the synonyms of which include B. aterrimus, B. globigii, B. mesentericus, B. niger, B. panis, and B. vulgatus.

(2) B. pumilus Gottheil, which resembles strains of (1) that produce a smooth glossy growth. It differs in failing to secrete diastase and in other features, but its separation as a distinct species is still an open question. It is sometimes identified as B. mesentericus.

(3) B. licheniformis (Weigmann) Gibson, the synonyms of which include B. subtilis (as used by Ford), B. globigii and B. mesentericus ruber. This species may be differentiated from (1) and (2) by its cultural characters, slow proteolytic action, vigorous CO2 production and ability to grow anaerobically.

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.

Data are given which show that the solids-not-fat content of the milk produced in a certain dairying area in the early months of 1942 and 1943 was unusually low, a high proportion of the individual herds yielding milk containing less than the presumptive standard of 8·5% solids-not-fat. The freezing-point depression of this milk was normal. The solids-not-fat content showed substantial improvement in May. It is suggested that the low level of solids-not-fat was due to a low plane of nutrition of the cow.

1. Estimations of solids-not-fat by evaporation and by hydrometry were made in two laboratories on samples of the same raw and pasteurized milks.

2. General agreement was found between the results by stated evaporation techniques and those by Richmond's formula (S.N.F. = 0·25G + 0·2F + 0·14) when the latter was applied to milks which had been stored cold (i.e. Recknagel contraction complete as required by Richmond).

3. General agreement at a lower level of S.N.F. (0·2% approx.) was found between Richmond's formula and the density formula (S.N.F. = 0·25D + 0·21F + 0·66) when both were applied to milks heated as required by B.S.S. no. 734 (1937) (i.e. Recknagel effect absent).

4. Adoption of the B.S.S. technique and formula placed a very much greater proportion of samples, below the presumptive legal standard of 8·5% S.N.F.

5. The need is indicated for the standardization of evaporation technique before attempting a permanent revision of the density formula.

6. Seasonal variations between ‘calculated’ and ‘found’ results have been observed.

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 results of a series of determinations of the freezing-point depressions of the milk of the individual cows of a Shorthorn herd over a period of 16 months are reported.

The relative constancy of the freezing-point was confirmed, all but two values deviating by less than 4·5% from the mean Δ values.

Significant seasonal trends were observed, especially during the spring when the supply of ample amounts of young grass may cause the production of milk with somewhat smaller freezing-point depressions.

Systematic trends in the differences between the Δ of morning and evening milk were found, the evening values being generally higher, except for the winter period, during which the cows were housed at night, when the morning Δ values became the higher. The differences appear to depend on the changes in the temperature of environment to which the cows are exposed. These observations provide an explanation for the discrepancies reported in the literature.

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.

That bacteriophage is the cause of much loss to the cheese-making industry of New Zealand has been shown by Whitehead(1). In New Zealand in recent years, cheese has been made almost entirely from flash-pasteurized milk with the aid of starters prepared from single-strain cultures of lactic acid streptococci (Str. cremoris). The use of such starters has been accompanied by the occasional failure of the bulk starter to clot, or, if clotted, by failure to produce the required amount of acid in the cheese vat—the latter condition being described by Whitehead as ‘pack up’. Whitehead has traced the cause of sudden failure to air-borne infection of the bulk starter by specific bacteriophage (2), the most probable source of infection being the spray from whey separators. When adequate protection from phage contamination was provided slowness in cheese making disappeared.

1. Following the experiences gained from an earlier experiment, cheese were made at varying controlled pitching points and rates of scald, the experiment being done both with sweet milk and comparatively acid milk.

2. One of the two cheese made from each vat was submitted to physical tests at regular intervals during the ripening process and the other cheese, when ripe, were judged by a panel of experts who also, together with non-experts and routine analysts, gave judgements of firmness and ‘spring’.

3. Tables were drawn up in which the rank orders of the cheese for each objective physical test and subjective judgement are compared with one another.

4. With the possible exception of vapour pressure and ‘spring’, the effect of pitching consistency was much more marked than that of the differences in rate of scald although the latter were considerable. Many physical properties were well correlated with pitching consistency.

5. The cheese makers are shown to have a greater self-consistency as individuals in picking out replicates and as a group in their decisions as to the rank order, by comparison with the other categories.

The results of freezing-point tests on some 4000 samples of milk as delivered by individual producers to creameries in England and Wales are analysed. Prior to the war about six out of every 100 samples were found to contain added water, but, as cases of heavy adulteration were comparatively rare, the total effect on the bulked milk supplies was small. After 2½ years of war the position had remained virtually unchanged.

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.

In studies previously reported (1) it was found that milk of normal appearance from cows having a latent infection with Streptococcus agalactiae yielded less cheese than that from cows with normal udders. Milk from cows free from Str. agalactiae infection, yet classed as ‘abnormal’ on the basis of biochemical tests, behaved similarly. The lower yield appeared to result from the lower casein and solids-not-fat contents of these ‘mastitis’ milks. With a single exception, the quality of the cheese made from these milks was not inferior to that of cheese made from normal milks. These findings appeared to warrant the extension of the studies to ordinary cheese-factory milks.

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.)

A method is described for the measurement of the firmness of the rennet curd from 100 ml. samples of milk coagulated under conditions simulating those of the cheesemaking process. Tests with a series of homogeneous curds ranging in firmness from very soft to very hard showed that the curds behaved as elastic gels of varying modulus, and the deformation produced under a suitable compressive load was accordingly adopted as a simple objective measure of firmness.

The method was used to observe the variation in the curd firmness of samples of milk from individual cows and separate quarters, with some observations on the relationship between curd firmness and milk composition, and the effect on curd firmness of variations in the following factors: acidity, addition of calcium chloride, amount of rennet, time of coagulation, and the admixture of soft-curd milk.

The results of these experiments indicate that the method provides a satisfactory measure of the rennet curd firmness of small samples of milk. The utility and applications of the method are discussed, and the suggestion is made that a standardized reproducible test on the same principle should be evolved to serve the same function with regard to milk for cheese making as the standardized measurement of curd tension serves with regard to the digestibility and suitability of milk for infant feeding.

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.