Though the mineral constituents normally represent less than 1 per cent, of cow's milk their importance in many dairying problems is well realised. For example, the calcium-magnesium-phosphate-citrate balance is a most important factor in connection with coagulation by heat and in the alcohol test, even minute alterations in the balance producing a very marked effect.

Curves are produced to show the proportions between morning and evening yield of milk, yield of fat, and fat per cent, in the case of individual cows milked alternatively at 6 a.m. and 9 p.m. and again at 6·30 a.m. and 3·30 p.m. In each case the intervals between milkings were 15 and 9 hours.

It is seen that when the 15-hour interval is between night and morning milkings as specified, a bigger proportion of milk is produced at the morning milking than is produced at night when the same interval is between morning and night milkings. The curves and Tables I and II show that this is quite clearly the case with the six cows in the trial.

It is apparent that, as far as winter conditions are concerned, night itself, or factors operating at night tend to high milk production of low fat content.

The influence of a number of factors on the autoxidation of butterfat has been studied. Metallic catalysts have been found to exert a great effect in hastening the reaction, their intensity being in the following decreasing order: sodium vanadate, copper lactate, iron lactate, nickel sulphate. Zinc lactate has a slight retarding action.

Ultra-violet light, hydrogen peroxide and fat peroxides have a strong prooxidative effect and to a less extent lactic acid. Curd exerts an antioxygenic effect, while humidity, glycerol, triolein, lactose, iodine, potassium iodide and the effect of pasteurisation have little or no influence.

The author wishes to thank the Auckland University College Council and Sir James Gunson for a Sir James Gunson Scholarship during the tenure of which this research was undertaken, Dr F. H. McDowall for his helpful interest in the work, and the Dairy Research Institute (N.Z.) for permission to work in their laboratories.

In Part I(1) the influence of a number of factors on the autoxidation of butterfat was investigated. It was considered of interest, however, to study the oxidation itself more fully to gain some idea of the changes taking place, and also to study certain reactions for detecting such changes.

In tallowy butter as the result of oxidation, the following differences in properties can be observed: (1) the amount of unsaturated compounds is decreased; (2) the peroxide content is increased; (3) the amount of free acid is increased; (4) the colour is bleached. Secondary changes occur simultaneously and result in the formation of aldehydes, ketones, etc.

1. Clean milk has been “flash” pasteurised at 165°F. Portions of the same raw milk have been treated with small amounts respectively of starter culture and lactic acid, so as to raise the acidity, before being similarly pasteurised.

2. Comparison of the whey obtained by rennetting the three raw portions has shown that those from the acidified milk contained more soluble protein than the original.

3. The differences between the raw and pasteurised milk in their coagulation time with rennin have been found to be proportionately much greater in the acidified portions.

4. These differences in coagulation time have been shown to be correlated with decreases in the amount of soluble protein found in. the whey separated from the coagulum.

5. These experimental results, together with those of other investigators, have been made the basis of a suggestion that the decreased coagulability of heated milk with rennin is partly due to flocculation of soluble milk protein upon the casein micelles.

The range of the copper and iron content of 80 samples of fresh, pasteurised and sterilised milk are given. Examples of investigations on the point of entry of copper into milk during processing are given. As a minimum limit, 1·5 p.p.m. of copper in milk can be expected to produce “oiliness” in milk.

The copper content of dried milk is usually normal but the iron content is increased above that expected from the iron content of fluid milk.

Cheese made in copper vessels has a high copper content; and the iron content of cheese is variable.

Butter contains on an average about 0·5 p.p.m. of copper and 1 p.p.m. of iron. Factory made butter invariably contains more iron and copper.

The more a milk product is processed, the greater is the contamination with heavy metal.

An example of determining the amount of blood in milk by a determination of the iron content is given.

1. Two colorimetric methods for determination of copper have been described, viz. the xanthate and the sodium diethyl-dithio-carbamate. Both methods have given satisfactory results but the carbamate method has proved to be more sensitive and less liable to error from possible contamination than the xanthate method.

2. The colorimetric determination of iron by the modification of the ferric thiocyanate method has been proved to give a very high degree of accuracy and excellent results have been obtained.

3. A method of wet ashing has been described whereby organic matter is oxidised by the use of concentrated sulphuric acid and sodium nitrate and copper and iron extracted from the fat.

4. A number of results are given showing the amount of copper and iron contamination that the cream is subjected to during the course of butter manufacture, and the distribution of copper and iron in the cream, butter and buttermilk.

If research work in cheese ripening is to be conducted on an entirely satisfactory basis, a complete control of both temperature and humidity of the ripening room must be available.

Though temperatures are relatively easily controlled, the factor of shrinkage or loss of moisture from the cheese is a more difficult problem. Further, should such loss of moisture be an additional and variable factor due to differences in humidity of the ripening room, research work in cheese ripening is seriously handicapped. It is realised that different cheese, whether large or small, high or low in moisture, new or mature, lose moisture at different rates even though kept under exactly the same conditions of storage. Also, the identical cheese cease to remain the same due to various rates of ripening, if studied as to their loss of moisture at different temperatures.

The results of a routine examination of samples of commercial sterilised milk extending over a period of 15 months are given and reviewed.

Three distinct types of spoilage have been found, and each of these has been shown to be induced by a definite group of bacteria.

Representative members of each group have been isolated and examined. The majority were found to be of the type B. mesentericus, B. cereus and B. mycoides. Two types have not been identified. Of these, one has been found to be the cause of the “coconut” or “carbolic” taint, and is more fully described in Part II of this paper.

The characters are given of the organism causing the “coconut” or “carbolic” taint of commercial sterilised milk. Although certain similarities with B. novus (Plectridium novum) Huss are shown to exist, the organism remains unidentified and the question of its identity has been reserved for a subsequent paper.

In laboratory culture the organism exists in a stable vegetative form which has resisted all efforts to induce it to sporulate. Under commercial conditions, on the other hand, it occurs in a spore form in the bottles of tainted sterilised milk.

Propagated by ordinary laboratory methods, the spore form is rapidly replaced by a vegetative race. Five serial transplants are sufficient to bring about this transformation.

Sporulation can be induced and maintained indefinitely if sporing cultures are heated at 100°C. for 30 minutes at the time of inoculation. The function of the heating process appears to be the destruction of a growth product inhibitory to the germination of the spores. This substance is associated with the growth of the vegetative form and its presence in unheated cultures is inhibitory to the germination of the spores.

Spore formation takes place at temperatures below 22°C., the spore cycle occupying 3 days at 22°C., 6 days at 15°C., and 15 days at 5°C. At higher temperatures, e.g. 30 and 37°C., it is inhibited and the normal process is replaced by an abortive effort at spore formation. The optimum temperature for the propagation of the spores is 22°C.

In the foregoing parts it is indicated that great difficulty may be experienced in the control of the spoilage of commercial sterilised milk which results from the development of spore-bearing bacteria.

A detailed study of the actual sources of contamination is now being made, but in the meantime it may be pointed out that many of these bacteria are typical members of the soil flora. They may therefore enter the milk from the dust either at the farm or the factory. The greater the care with which the milk is handled from the time of milking until the end of the heating process, and the greater the efficiency of the cleaning and sterilisation of the plant and the empty bottles, the less will be the danger of spoilage arising from the use of infected milk. During the course of the investigations recorded it has been necessary to carry out numerous control experiments. The milk used has been Grade A (T.T.) milk and the utensils with which it came into contact and the empty bottles into which it was poured were clean and sterile. There has never been the least difficulty in obtaining a sterile, fresh-smelling and sweet-tasting product after heating at 100°C. (212°F.) for 30 minutes and storing at 22°C. (72°F.).

Sources of error in the adjustment of media to pH 3·5 have been discussed.

Titration curves for whey and beer wort with citric, lactic and tartaric acids have been constructed. It is shown that citric acid is the most convenient acid to use for adjusting media for yeast and mould counts. The data obtainable from these curves cannot be relied upon to give a medium of pH 3·5 ± 0·1, owing to the variations in the composition of whey and wort, but the required accuracy may be obtained by adding to the medium a bufEer mixture of citric acid monohydrate (64 parts) and potassium citrate monohydrate (36 parts) in a concentration of 2 per cent. In this way reliable results can be obtained without standardising acids or determining pH. values. This method may be used for any pH between 3 and 6.