1. The influence of the age of cows on the milk yield and on the milk solids is traced. The milk yield reaches a maximum at about the sixth lactation and then declines. The concentration of fat and solids-not-fat shows a gradual decline from the first lactation onwards.

2. During lactations in which the yield is below the normal productivity of a cow, the quality of the milk also tends to be subnormal.

The results of the present experiments and of those reported in the previous paper(1) may be summarised as follows:

1. When minimal quantities of protein are fed in the production rations of milking cows, a deficiency of either lysine or tryptophane will lead to a marked reduction in milk yield. There is, however, some evidence of the storage of reserve N, which can be utilised when the food protein is inadequate.

2. The feeding of a lysine- or tryptophane-deficient ration causes a marked increase in urinary N, indicating a poor utilisation of food protein. On the other hand, the feeding of a ration containing adequate quantities of these essential amino acids reduces the urinary N, indicating efficient protein utilisation.

3. The utilisation of body tissue in an attempt to maintain normal milk production on a deficient protein ration is shown by the high creatine excretion. The fact that the S: N ratio of the excess sulphur and nitrogen excreted during the deficient protein periods approximates that of body tissue (circa 1: 15·7) confirms this conclusion.

1. The incidence of twins has been one twin birth in every forty-eight Red Poll births, and one twin birth in every thirty-three Friesian births.

2. So far as is known no identical twins have been dropped.

3. Thirteen of the fourteen freemartins reared have been barren.

4. The birth weight of twin calves has been only slightly lower than that of single calves. Bull calves have been approximately 6 lb. heavier than heifer calves.

5. The mortality amongst twins is approximately equal to that amongst single calves.

6. Twinning appears to be inheritable and to run in strains.

7. The gestation period of a cow bearing twins has been from 8 to 10 days shorter than that of a cow bearing a single calf.

8. The percentage of twins dropped increased as the age of the dam at calving increases.

9. Twinning is not influenced by the season of the year.

10. The twins and dams of twins have been consistently heavier producers of milk and butterfat than the other herd members.

11. There is some evidence to suggest a close genetic relationship between the following three characters: (i) multiple births, (ii) high milk and butterfat production, (iii) long life and fertility.

1. Of 21,569 samples of mixed cow's milk taken under fair conditions of cleanliness and examined by means of the coliform test, 48·3 per cent, gave negative results with 1/10, 1/100 and 1/1000 c.c. amounts; 21–4 per cent, gave positive results with only 1/10 c.c.; 14·0 per cent, gave positive results with only 1/10 and 1/100 c.c.; and 16·3 per cent, gave positive results with 1/10, 1/100 and 1/1000 c.c. It is probable that these results are better than those generally obtained.

2. The proportion of coliform-positive samples was much higher in summer and early autumn than during the winter and spring. This seasonal variation was largely due to atmospheric temperature, there being a well-marked direct correlation between the proportion of coliform-positive samples and the mean of the minimum and maximum atmospheric temperatures.

3. Of 21,857 samples examined, 10,458 were coliform negative and had an average bacterial content of 25,294 per c.c; 11,399 were coliform positive and had an average bacterial content of 160,577 per c.c. The coliform-positive samples contained on an average 6·3 times as many bacteria as the coliform negative.

1. A modification of the method for estimation of the amino-acid content of foods is described.

A series of analysis of the more important cattle foods is given.

In a previous communication (Allen (1)) it was shown that Cheddar cheese made from milk of a very low bacterial count (Grade A (T.T.)) exhibited a retarded rate of ripening, as evidenced by proteolysis and development of flavour and texture, when compared with similar cheese made from ordinary market milk, and the view was advanced that the miscellaneous flora of market milk produces sufficient proteolysis of the casein in the early stages of manufacture to provide the starter organisms with necessary nitrogen compounds for rapid growth.

1. A comparison has been made of the accuracy of the ordinary plate and Burri smear-culture methods for the determination of the bacterial content of milk samples.

2. In determinations made by the smear-culture technique the percentage error has been shown to be markedly higher than in those made by plating.

3. Evidence is presented in support of the view that the inaccuracy of the Burri method (as ordinarily employed) when compared with the plate technique is partly due to the errors involved in loop measurements and partly to the inferiority of the conditions for growth on slopes to those in plate cultures.

1. The solids content of fat-free milk shows little variation throughout the milking process, i.e. first and last drawn milk are of similar concentration.

2. Curves are presented showing the normal variation in the solids content of fat-free milk throughout a lactation period. These curves are influenced by the age of the cow and by pregnancy.

1. Some cases of sudden failure in the growth of cheese starter cultures are described.

2. It is shown that the cause lies in the method of pasteurisation of the milk in which the cultures are grown.

3. Aeration of the milk immediately before the inoculation of the culture is shown with certain cultures to have an inhibitory efEect on the growth of the organisms. Normal starter cultures are not affected by aeration of the milk; they may, however, suddenly develop this sensitivity to aeration and may just as suddenly revert to the normal state.

4. The mere absorption of atmospheric oxygen while milk is being cooled after pasteurisation in a wide-mouthed cylindrical vessel may constitute an aeration sufficient to inhibit sensitive cultures of lactic streptococci. Milk pasteurised and cooled in conical flasks plugged with cotton-wool does not reabsorb sufficient oxygen to lead to inhibition, provided that the flasks are not violently agitated.

5. The possible mechanism of the inhibition of the sensitive streptococci is discussed. Evidence is brought forward which tends to show that the action is connected with the oxidation-reduction reactions by means of which the organisms normally obtain their growth energy.

6. The practical significance of the phenomenon in the process of cheese manufacture is emphasised.

It is well known that the quantity of salt added to Cheddar cheese curd has an important influence on the quality and rate of ripening of the resultant cheese. The character of the cheese body is, indeed, dependent to a large extent on the rate of salting. An investigation of the salt content of cheeses of tender body, normal body, and harsh body showed that the percentage concentration of salt in the moisture in cheese of normal body in general did not exceed 5·5 per cent. (1).

Seventy boxes of salted Victorian butter were held at 12° F. for three months. Bacteriological and chemical examinations were carried out before and after storage in order to observe the changes which occurred and the factors which controlled deterioration.

The bacteriological examination showed that on the average only slight changes in bacterial numbers occurred during the actual cold storage period. None of the bacterial, yeast or mould counts revealed any positive association with keeping quality.

The catalase test gave negative results. It is suggested that enzyme activity is not an important factor in controlling deterioration of the butters.

The chemical examinations indicated that the variations in curd or salt content did not affect keeping quality. Fat hydrolysis, as measured by ether soluble acidity of the fat, and fat oxidation as measured by peroxide value, did not appear to be important factors in deterioration. Considerable variations were observed in the acidity of butters made from cream supposed to be “neutralised” to the same acidity. Acidity of the butter as measured by titration of water-soluble material was closely related to the pH of the butter. Butter acidity and copper content appeared to be the most important factors controlling keeping quality. The mechanism by which acidity causes deterioration was not revealed by the investigation.

1. The process of ripening in experimental cheeses was followed by periodic estimations of the amounts of various types of protein degradation product formed.

2. No connection between the type of nitrogen partition and the commercial quality of the ripened cheese could be traced.

3. The proteolytic powers of the various pure cultures of streptococci used as starters in the cheese vats seemed to have little influence on the rate of proteolysis in the cheeses during the ripening period. The main trend of the protein breakdown was governed by the rate and total extent of acid formation in the milk and curd during the manufacturing process. This suggests that the rennet enzyme is the principal proteolytic agent, and that the most significant function of the streptococci is the formation of acid which in turn influences the rate of action of the rennet enzyme.

4. No specific connection between the numbers of streptococci present in the cheese milk at the time of renneting and the rate of ripening of the cheese could be traced.

5. Some evidence was found tending to suggest that minor differences in the type of protein degradation occurring during the ripening period influenced the changes of bacterial flora in the cheese. Lactobacilli appeared to develop earlier where the proportion of simpler protein derivatives was high.

1. Data, taken over a period of eight months, are presented, concerning the quantitative changes which take place in the microflora of sweet-cream salted cold-stored butter of good keeping quality.

2. Tables showing the influence of varying periods of cold storage on the development of acidity, total bacterial count, yeasts, moulds, gelatin liquefying bacteria and types of micro-organisms in forty-nine churnings of butter cold stored at 15° F. are given.

3. There was a slight increase in the titratable acidity.

4. There was a noticeable increase of the yeast count, in many cases without apparently injuring the keeping quality.

5. The numbers of Oidium lactis present tended to decrease.

6. The following types of bacteria persisted in the sweet-cream salted butter in decreasing numbers: (a) various types of micrococci; (b) lactic acid bacteria of non-coagulating and acid coagulating types, including “slow” varieties of Str. lactis; (c) acid with coagulation and gas; (d) rennet digesting; (e) alkaline-forming; (f) alkaline digesting.

A description is given of the characters of certain streptococci which in small numbers are sometimes found in the milk of individual cows. Although the organisms possessed high haemolytic activity, they showed no evidence of pathogenic power, either for the cow's udder or for ordinary small laboratory animals. Not only are they different from the streptococci of bovine mastitis, but they are also to be distinguished from streptococci of the Str. pyogenes group which are occasionally found in milk. One of their principal characters is the formation on solid media of minute colonies, consisting of relatively minute cocci, which grow in broth as very short chains.

The fifty-eight strains studied could be divided into two groups on biochemical grounds: (a) those fermenting glucose and lactose with production of small amounts of acid, (b) those which ferment mannite and salicin as well. The former are referred to as “low-acid” strains and the latter as Str. infrequens, as they bear a resemblance to the organism given this name in Holman's classification. Fermentation of sorbite and trehalose occurred with streptococci of both groups.

The two groups could be distinguished from Str. pyogenes and the ordinary mastitis Streptococcus by direct agglutination. For distinguishing the two groups from each other, agglutinin absorption methods were required. It was then found that the low-acid Streptococcus possessed two antigens of which one only was represented in Str. infrequens.

The heating of milk causes denaturation of the lactalbumin and lactoglobulin to an extent depending on both the temperature and the time of heating.

A series of data has been given for the amounts of albumin and globulin denatured on heating samples of milk at various temperatures from 63 to 80° C, for periods of from 2½ to 60 min.

An average of 10·4 per cent, of the total soluble protein was denatured after 30 min. heating at as low a temperature as 63° C.

Smooth curves were obtained for the progress of denaturation with time at each temperature, and, over the range of 63–75° C, the relative increase in velocity of denaturation for each rise in temperature of 1° C. was found to be constant, the temperature coefficient of the reaction being 1·5.

II. When added as an oxidant to cheese, copper, to the extent of 36 p.p.m., produced an abnormal flavour and markedly inhibited the ripening process. The remaining oxidants (KNO3, KCIO3 and KCIO4) held the cheese at a higher potential and decreased the amount of proteolysis, especially after five months.

Potassium cyanide had no effect on the proteolysis or the oxidation-reduction potential.

It is concluded, therefore, that normally the flora of cheese holds the oxidation-reduction potential at such a point that proteolysis is diminished if the potential is raised to the oxidative side.

III. Variations in the amounts of pepsin and rennin appeared to have little effect on flavour, texture, body or ripening. There was apparently no correlation between clotting time and extent of proteolysis. The addition of lactobacilli with the starter appeared to accelerate the protein degradation in the early stages of ripening.

1. Using the method described by Sanders for protein and fat precipitation from cow's milk, a fairly satisfactory method of determining the phosphorus partition in cow's milk has been devised.

2. It is found that the gravimetric procedure of the American Association of Official Agricultural Chemists ((10), p. 3), which was not, in any case, specifically recommended for the analysis of milk, cannot be applied with safety to the determination of inorganic phosphate in filtrates prepared by Sanders' (1932) method, and that the conclusion that there is no appreciable amount of organically combined phosphorus in these filtrates is therefore invalidated.

3. The ester P (organically combined, acid-soluble P) in milk obtained by the usual milking routine varies from 1/7 to 1/4 of the inorganic P of the milk (from 7 to 21 mg. P per 100 c.c. milk).

4. An active phosphatase is present in cow's milky, and brings about changes in the ester P content of the milk on standing, possibly within and certainly without the mammary gland. Like phosphatases of all mammalian tissues, its optimum pH is in the neighbourhood of 9·0. In freshly drawn milk it is working, therefore, considerably on the acid side of its optimum.

5. The phosphoric ester P of the milk appears to vary with the breed of cow, being low in the miiy of Canadian Holstein-Friesian cows and higher in that of Jersey cows.

1. The effect of increasing concentrations of milk solids on the degree of ionisation of electrolytes cannot be measured by determinations of electrical conductivity, owing to the influence of viscosity on ionic mobility.

2. An approximately linear relationship exists between concentration of milk solids and the freezing-point depression. Above a certain concentration (equivalent to 34 g. milk solids per 100 g. of water), however, the solutions are saturated with respect to lactose, and any increase in freezing-point depression beyond this point is limited to that due to the other osmotically active constituents of milk.

3. There is no evidence of the existence of “bound” water in solution of milk solids up to concentrations equivalent to 65 g. milk solids per 100 g. water.

4. An approximately linear relationship exists between concentration of milk solids and hydrogen-ion concentration, increase in the concentration of milk solids resulting in a markedly increased acidity.

5. The applications of these results to the heat coagulation of evaporated and dried milks are briefly discussed.

A method is described for the quantitative determination of phosphatase in raw milk. Using this method, it has been found that phosphatase is sufficiently thermolabile to be destroyed completely by pasteurisation if this process is properly carried out. The absence of phosphatase from a sample of milk or cream indicates with a fair degree of certainty that the milk or cream has been heated sufficiently to destroy such pathogenic organisms as were originally present, though it is of course no guarantee that the product is free from these organisms at the time of testing.

A simple, qualitative test-tube method is described which may be used for differentiating between raw and heated milk, or between raw and heated cream, and which with slight modifications may also be used for distinguishing between butters made from raw or from heated cream.