We have dealt in a previous communication (1) with the effects-of commercial pasteurization on the nutritive value of milk. We present here the results of an enquiry into the nutritional effects of the more drastic heat treatment to which milk is subjected during commercial sterilization. There is no legal definition of either sterilized milk or of the process of sterilization, but in commerce it is customary to apply the term to milk which has been heated to at least 212° F. for varying lengths of time.

1. The biological values and true digestibilities of the proteins of raw and commercially sterilized milks from the same bulk have been compared on rats by the method of Mitchell.

2. For raw milk a biological value of 84·3 and a true digestibility of 96·4 were obtained, while the figures for sterilized milk were 79·1 and 95·3 respectively.

3. Statistical tests showed that the difference in true digestibility was not significant but that the lowering of the biological value by about 6% could be ascribed to the effect of the heat treatment, the odds against chance factors being 625: 1.

1. In experiments in which the biological value of the proteins was measured spray dried skimmed milk was fed separately from or mixed with a basal “nitrogen free” diet.

2. At a 6·9% level of protein (N x 6·38) intake the first method of feeding yielded a biological value of 87·2, the second of 90·3. The difference was not statistically significant (odds of 9: 1 against chance factors).

3. The findings do not account for the low biological value reported by us previously for the proteins of raw liquid milk.

1. The vitamin A and carotene content of raw milk have been compared with that of milk from the same bulk after it had been commercially sterilized.

2. For this purpose the fat was obtained by ether extraction from eleven samples of raw milk and from eleven corresponding samples of sterilized milk. As the sterilized milk had been previously homogenized it was not possible to churn it.

3. The vitamin A and carotene content was estimated by colorimetric (Lovibond tintometer) and by spectrophotometric tests.

3. The vitamin A and carotene content was estimated by colorimetric (Lovibond tintometer) and by spectrophotometric tests.

4. The results showed that the efiect of the heat treatment was negligible, neither method demonstrating a loss in vitamin A, while for carotene only the colorimetric method indicated a loss of about 2 %.

5. The vitamin A and carotene contents of sterilized milk were not decreased after storage for several weeks in a cool dark place.

1. The effect of commercial sterilization on the vitamin B complex, vitamin Bx and vitamin B2 (flavin) of milk has been investigated in experiments on rats using milk from the same bulk before and after heat treatment.

2. No quantitative comparisons were carried out regarding the vitamin B complex, but the tests showed that a considerable destruction of some component or components of it had been caused by the heat treatment and that vitamin B1 was, most probably, one of them.

3. Separate tests showed that about one-third of the vitamin B1 originally present in milk was destroyed by sterilization.

4. Sterilization appeared to have no adverse effect on vitamin B2 (flavin).

Improved methods have been evolved for the separation of the total protein, casein, albumin, globulin, and proteose-peptone substances of milk.

I. The use of acetic acid-sodium acetate solutions for the precipitation of casein is elucidated, and it is shown that the maximum precipitation of casein is rapidly effected from milk samples of varying casein content by the addition to 10 ml. of milk of about 80 ml. of water at 40° C. and 1·0 ml. of 10% acetic acid solution, followed, after 10 min., by 1·0 ml. of N sodium acetate solution.

This maximum precipitation was found to be 1·0–1·4% greater than by Moir's method, and 2·4–3·8% greater than by the method of the Association of Oflicial Agricultural Chemists.

A procedure is suggested for the determination of casein which avoids the tedious transfer and washing of the precipitate, and gives enhanced ease, accuracy and speed of working.

II. The advantages of trichloroacetic acid for the precipitation of proteins in determinations of the total protein and the non-protein nitrogenous substances of milk are discussed.

The trichloroacetic acid methods at present in use are shown not to give complete precipitation, and for this a rapid method employing, at room temperature, a final concentration of 12% acid in the milk-acid mixture is recommended.

III. An accurate method for the precipitation of globulin uncontaminated with either albumin or casein is described.

IV. Methods are given for the precipitation and separation of the albumin and proteose-peptone substances.

Details are given of a semi-micro Kjeldahl method for the determination of the total, casein, albumiij, globulin, proteose-peptone, and non-protein nitrogen of milk. The method is accurate, rapid, and particularly suitable for the determination of the smaller nitrogen fractions.

The various nitrogen fractions are separated by the improved procedures described in the preceding paper.

The N distribution in terms of casein, albumin, globulin, proteose-peptone, and non-protein N is given for a number of samples of normal milk, of milk from cows with mastitis, and of milk persistently low in solids-not-fat content. Improved methods of determination have been used.

The average N distribution in normal milk is as follows: casein N 78·5, albumin N 9·2, globulin N 3·3, proteose-peptone N 4·0 and non-protein N 5·0% of the total N. The casein N figure is the least variable, ranging from 77·3 to 80·7. On this basis, normal milk contains, as an average: casein 2·63, albumin 0·31, globulin 0·11, and proteose-peptone substances 0·13%. The total protein content is 3·18%, and the casein: albumin: proteose-peptone: globulin ratios are 23·8: 2·8: 1·2: 1·0.

The milk from cows with clinical mastitis is low in solids-not-fat content, and characterized by a decreased content of casein and an increased content of albumin, globulin, and proteose-peptone substances, particularly of globulin.

The milk samples of low solids-not-fat content were of normal appearance and reaction, and produced by apparently healthy cows known to have yielded milk of low solids-not-fat content for some time. The samples were examined by strict bacteriological methods for the presence of mastitis streptococci, and formed two distinct groups: (1) samples from cows with subclinical mastitis, having the protein distribution of the mastitis samples described above, the cause of the low solids-not-fat being pathological; and (2) samples from uninfected cows, having the protein distribution of normal milk, the cause of the low solids-not-fat being physiological. The two groups of low solids-not-fat cases are discussed, and the conclusion drawn that subclinical mastitis may account for a large proportion of these widespread and puzzling cases.

Many cases of mild subclinical mastitis cannot be detected by the simple chemical tests which will reveal pronounced infection, and satisfactory bacteriological tests, when available, demand much skill and time. For the detection of milk from cows with subclinical mastitis, and for the differentiation of milk samples with pathological and physiological low solids-not-fat, the solids-not-fat, the figure is suggested, and the advantage of this “casein number” is illustrated.

An enzyme which hydrolyses tributyrin is present in all samples of fresh cows' milk examined. A method has been developed for the determination of the tributyrinase activity of a given milk or fluid.

In milk the enzyme is present in the aqueous rather than the fatty portion. Its optimal range of activity is in the region pH 8·2–8·7. It is rather more thermolabile than phosphatase.

Its concentration in milk varies considerably from cow to cow, and in individual healthy cows there is a fairly regular variation throughout the lactation cycle. It is highest in concentration in colostrum, then the concentration falls to a minimum at about 10 days, rising later to a figure intermediate between this minimum and the colostral value. It shows no sign of increase towards the end of the lactation period.

Its relationship to the “true lipase” of milk is discussed.

Standardization of annatto in aqueous solution has been shown to be unsound, and it has been recommended that standardization be carried out on the basis of the depth of colour produced in cheese itself.

A paper colour standard resembling the German butter standard has been suggested for cheese, the same standard being suitable for the evaluation of annatto solutions for cheese-making.

1. Although chemical analysis, bacteriological analysis and graders' reports failed to distinguish the alcohol-glycerol rennet cheese from the brine rennet cheese, the marked qualities in body and texture during making of the alcohol-glycerol rennet curd, and the sterility or low count of the alcoholglycerol extract warrant further trials under practical conditions.

2. Where brine rennet may be suspected as the cause of bacterial faults, the alcohol-glycerol rennet could well be used in its place.

3. Alcohol-glycerolrennets have superior keeping-qualities to brine rennets.

The abilities of type cultures of lactic acid bacteria to reduce five O/R indicators in plain milk and yeast dextrose milk have been described and the factors influencing their behaviour in this respect discussed. Methylene blue, litmus and safranine appear to be of some diagnostic value for the differentiation of types. There is a marked correlation between rapidity of growth in milk, ability to reduce O/R dyes and the proportion of by-products. This relationship holds not only for the different groups and types, but also for variations in behaviour observed in species kept under laboratory observation. It is suggested that rate of growth is fundamentally related to the amount of by-products and hence presumably to flavour-producing substances such as diacety and acetic acid. Thus in cheese-ripening enforced slowness of growth may be as important as the presence of specific types.