An examination of over 5000 milk samples has shown that penicillin is frequently present. Its concentration, however, is usually below the level which is believed to be necessary to cause significant interference in cheese-making, but a significant proportion of the samples contained more than a third of this quantity. This suggests that conditions under which difficulty would arise are not remote.

In order that the results recorded in Part II of this series(1) may have meaning, it is necessary to know the effect of different low concentrations of penicillin in cheese-making. Preliminary experiments suggest that this is mainly the result of their effect on the cultures used as starters.

1. A statistical analysis has been made of lactation records from 500 dam-daughter pairs of the Ayrshire breed. Measurements were taken of milk yield, fat, S.N.F., crude protein, casein and lactose.

2. A correction was made for the age of the cow. This contributed a substantial fraction of the total variation for milk yield, S.N.F. and lactose contents. Month of calving had a trivial effect on the lactation average for the characters when compared with the individual variation between cows.

3. After correction for age, the variation between lactations was highest for fat content (standard deviation 0·35%) and slightly less for S.N.F. (0·26%).

4. The heritability was estimated by the dam-daughter correlation within herds. Values in the neighbourhood of 0·5 were obtained for S.N.F., crude protein, casein and ‘ash’ (determined by difference), about 0·35 for fat and lactose, 0·25 for milk yield and 0·13 for non-casein nitrogen.

5. The phenotypic correlations between yield and lactose were positive for both dam and daughter groups. The correlations between yield and the other constituents were more negative in the daughter group than in the dam group. The correlation between yield and S.N.F. was, in fact, positive in the latter. The correlation between fat and S.N.F. averaged 0·36, being derived mainly from the correlation between fat and casein (0·43), though that with lactose was also positive (0·13).

6. The variation in S.N.F. was mostly determined by that in casein (r=0·85). There was a positive correlation between lactose and casein (0·24), and a negative one between lactose and non-casein nitrogen (–0·41). Lactose was negatively correlated with ‘ash’ (–0·31). ‘Ash’ was positively correlated with non-casein nitrogen (0·23)

7. There were positive and significant genetic correlations between casein and both fat and lactose, and that between the last two was almost significant.

8. No significant genetic correlations were found between yield and the chemical constituents, although figures of practical importance may exist which did not reach significance in an experiment of this size.

9. The practical significance of these results is discussed. It is concluded that selection for yield alone is not likely to cause any great decline in S.N.F. content of the milk, and that this could probably be held in check by paying some attention to the fat content of the milk.

Further studies of immersion cleaning have shown that no deposition of calcium soap and inorganic calcium salts occurs if the immersion solution contains ethylene diamine tetraacetic acid. Compared with results for the previous year there was a marked improvement in the bacteriological state of liners as a result of the absence of calcium deposits. It has also been shown that the addition of tetramethylthiuram disulphide to the rubber mix from which liners are made gives useful bactericidal properties. The intensity of the bactericidal action was little reduced after liners had been in farm use for 4 months.

Antibiotic treatment eliminated organisms from 61% of staphylococcal and from 80% of streptococcal udder infections. In 84% of quarters from which organisms were eliminated the cell counts of the milk were reduced, about half of them to a normal and about one-third to a late-lactation type of cell count in which the majority of the cells are epithelial cells. The remainder became reinfected before there was time for the cell counts to return to normal. It was found that it may take as long as 5 weeks for the cell counts to return to normal after elimination of mastitis organisms, but in quarters in which the cell count eventually fell there was a decrease in the percentage of polymorphs in the milk to less than 30% one week after treatment.

The addition of penicillin to cheese milk had the effect of delaying acid production by starter in the cheese curd; any effect on cheese quality could be traced to the delay in acid production and to a high final pH in the cheese. There was no indication of any direct effect of penicillin on the ripening process.

A concentration of 0·10 unit/ml. of penicillin in the cheese milk was a borderline amount with the particular starters which were used. Cheese quality was sometimes adversely affected. A smaller concentration (0·05 unit/ml.) delayed the cheese-making process slightly but had no adverse effect in final cheese quality. Higher concentrations regularly resulted in poor quality cheese.

Penicillinase added to cheese milk neutralized any penicillin present but with a short time of contact of about 30 min., a large excess of penicillinase had to be used.

Variation in the temperature of measurement, pH, and concentration of calcium, citrate and phosphate ions have been consistently found to affect the reflectance of separated milk in such a way that the entire curve of reflectance versus wave-length in the visible spectrum is raised or lowered. At temperatures between 10 and 50°C. the reflectance rises reversibly with temperature. The change is not instantaneous, and if the temperature is changed abruptly the reflectance change to the new value is exponential, with a time constant of about 40 min.

Reflectance is independent of pH in the range 5·8–6·6, but for greater values it drops sharply.

Added calcium ions increase the reflectance of separated milk, and added citrate and phosphate ions decrease it, independently of the simultaneous changes in pH.

It is suggested that the reflectance changes which have been found to occur are all caused by changes in the sizes of the casein particles in milk which are brought about by the factors considered. The results given by other authors on viscosity and bound water content, and on direct electron-micrographic observation of casein, are cited to support this contention.

It is further suggested that the change in casein particle size with temperature may be due to a change in the calcium-ion distribution between solid and liquid phases.

The viscosity and reflectance results for separated milk heated to 70°C. and above are apparently anomalous, but they can be explained on the hypothesis that denatured soluble proteins aggregate into particles having a high ration of length to breadth.

During an investigation of the surface microflora of Stilton cheese during ripening, Brindley(1) isolated lactobacilli from the surface scrapings of normal and slipcoat cheese. Some of these isolates were sent to the National Institute for Research in Dairying to form part of a collection of lactobacilli which was being built up for classification purposes. During the work on classifying lactobacilli these organisms from Stilton cheese were examined and all but one were serologically grouped and identified by physiological tests. They were also tested against a number of type sera which were available, and it was found that, although isolated fromcheese of different origins, most of the strains belonged to the same serological type.

One of the most important stages governing the accuracy of lactose determinations in milk is the preparation of the protein-free serum. Factors influencing the lactose content of the serum are: (1) the complete precipitation of protein, (2) the formation of lactose-protein complexes, and (3) the degree of hydration of the protein coagulum.

Experiments are described which show how these factors are influenced by different clearing agents operating at different pH values. On the basis of the results obtained a reagent containing zinc acetate, phosphotungstic acid and acetic acid is suggested; it reduces to a minimum any errors introduced by the above factors.

The results of examining the fat globule size in milk samples from thirty-one cows which were in-patients in a Veterinary Hospital, have been recorded. Eleven of these cows did not develop a body temperature over 102·4° F., and samples collected on the second day after admission and 5 days later were examined and compared. Although the rise in yield was significant, the differences in fat percentage and fat globule size were not. The other twenty cows developed raised body temperatures, and the association between body temperature and milk composition was considered in the following temperature ranges: 104° F. or over, from 102·5–103·9° F., and 102·4° F. or below. The results showed that a rise in body temperature was accompanied by a fall in yield and also by an increase in the fat percentage and in the average fat globule diameter.

The carotene and vitamin A contents of the butterfat from the milk yielded by the cow in early lactation may be higher than normal for a period of up to 10 or 12 days after parturition.

Seasonal changes in carotene content of butterfat from small herds of cows were governed almost entirely by the availability of carotene in the pasture and not by lactational influences.

Seasonal changes in vitamin A content of the herd butterfats were relatively small during most or all of the lactational period. In certain years an increase in the vitamin A content during the autumn months was probably due to seasonal conditions and not to a lactational effect.

Iodine values of milk fats of grazing cows show wide variations which are chiefly determined by differences in oleic acid content. Among the factors promoting the variations are: individuality of the cow, stage of lactation, breed differences, diurnal and incidental (irregular) day-to-day effects, nature of the food and plane of nutrition.

Results of experiments with feeding an unsaturated vegetable oil indicate that considerable proportions of the unsaturated long chain acids of milk fat were derived from the ingested oil. It is deduced that herbage lipids may contribute materially to the production of milk fat of grazing cows.

From comparisons of the extents to which dietary long chain unsaturated acids are incorporated into milk fats of grazing and semi-fasting cows, it would appear that the rise in iodine value following partial inanition does not result from increased uptake of fatty acids withdrawn from the more highly unsaturated glycerides of the body reserves.

A comparison is made of the stripping characteristics of the types of plant studied in previous papers of the series. The effects of alterations to the type of plant, the number of vessels used or a change in processing conditions on the volatile flavour components are illustrated by calculations for solutes having equilibrium coefficients m of 2, 10 and 50. For design purposes a hypothetical taint of m=10 is used, the final taint concentration being set at 10% (of initial) for high-taint pastures and 25% for low-taint areas. Using the above hypothetical conditions, calculated steam-demand figures are shown to agree with known typical steam-demand values. The use of a flash cooling vessel is examined and shown to effect a useful steam saving.

The effect of steam velocity on the mechanical break up of the liquid in a counterflow spray column is shown; an increase in steam flow increased the liquid side disturbance and thus reduced the liquid side resistance giving a greater rate of taint transfer, the characteristic thus more nearly following a line of constant saturation than a line of constant number of transfer units.

A number of flow circuits for deodorizing plant are described. The general conditions limiting the continued reduction in steam flow are discussed.

1. Reactivation of alkaline milk phosphatase is affected by the presence of ions, Mg2+, Zn2+ and Mn2+ being activating and Cu2+, Ni2+ and Co2+ being inhibitory.

2. Cu2+ will inhibit reactivation induced by Mg2+, but has no effect upon Zn2+-induced reactivation.

3. Using washed cream, either alone or in admixture with boiled whey treated with cationic resin, reactivation occurs in the presence of added Mg2+, but not when Zn2+ is added.

4. Experiments with EDTA. confirm that metallic ions play a part both in alkaline phosphatase activity and in reactivation.

5. It is suggested that either Mg2+ or Zn2+ or both play a part in reactivation.

A lysogenic culture, prepared in the laboratory from a strain of Streptococcus lactis, was used as a cheese starter in commercial factories. It was attacked in turn by two other unrelated phage races. The lysogenic condition, which involved slight morphological and physiological changes, persisted in the subsequent forms resistant to one or both the new phage races. Acquired resistance to any one of the three phages did not protect the culture from the other two phages.

In nature such interactions between phage races and lactic acid bacteria must be constantly taking place, giving rise to similarly related strains.

Two of the three phage races produced spreading haloes around their plaques due to a lysin released during phage action. The lysin may also interfere with the survival of secondary growth after attack by these phage races. Production of this type of lysin is thus a property of the phage race and not of the bacterial strain.

A reaction closely resembling the nascent phage phenonomenon was studied. When lysing the homologous strains of Str. lactis, two phage races were found to be capable of simultaneously inducing lysis of a heterologous strain and of other phage-resistant homologous strains. Seitz filtered potent lysates of the homologous strains also caused lysis. Subjecting the lysates to a temperature of 60°C. for 5 min. destroyed the lytic activity on heterologous strains but did not inactivate the phage.

These two phage races produced haloes around their plaques while a third phage, unable to induce lysis of heterologous cultures, showed no haloes. Addition of I3-to the medium suppressed the appearance of haloes, providing further evidence that they are due to the production of bacterial lysins.

Evidence was obtained to show that several other possible mechanisms were not responsible for the observed lysis, and it was concluded that a heat-labile lysin or lysins capable of destroying the heterologous strain was produced during lysis of the homologous strain. The relationship of this to the nascent phage and other similar phenomena observed elsewhere is discussed.

1. The nutritive value of milk sterilized according to the Stork procedure has been studied in laboratory experiments by the method of Mitchell, the rat-repletion method and growth trials with rats and baby pigs.

2. Determinations of biological value of the milk proteins with three groups of six rats by the Mitchell method yielded mean values of 87·0, 88·0 and 85·1 for raw, pasteurized and sterilized milk respectively. The corresponding digestibilities amounted to 96·8, 96·3 and 94·4. The differences were not statistically significant.

3. The relative nutritive value of the three types of milk for the repletion of protein-depleted rats was studied with three groups of eight rats. The average weight gain on feeding daily 40 ml. of either raw, pasteurized or sterilized milk during 5 days was 27·7, 26·3 and 22·9 g. respectively. Again these differences were not statistically significant.

4. No differences in growth rate were obtained if three groups of twelve male and twelve female young rats were fed on raw, pasteurized or sterilized milk supplemented with trace minerals for a period of 6 weeks. The growth rate of rats on exclusive milk diets was, however, far below normal.

5. In growth experiments with baby pigs, fed equal amounts of pasteurized or sterilized milk supplemented with trace minerals, small but statistically non-significant differences in weight gain were found, in favour of the pasteurized milk. The average weight gain on sterilized milk in four separate growth trials amounts to 97, 95, 98 and 98% of that on pasteurized milk. The average 3% less gain on sterilized milk was ascribed partly to more difficult adaptation and less intake of milk solids.

6. These experiments failed to demonstrate a significant decrease in nutritive value of milk sterilized according to the Stork procedure.

The problem of bacteriophage in cheesemaking has been reviewed recently by Whitehead(1) and Mattick(2). Results since reported by other workers and ourselves call for some modification of and addition to the existing concepts of the problem.