1. Twenty cows were used in an experiment to compare the effect of five different diets on the fat and solids-not-fat content and yield of milk.

2. During initial and final control periods all the cows received 18 lb. hay daily and about 4 lb. of a balanced concentrate mixture (crude protein 16·5%) per 10 lb. of milk produced. In the experimental period, which lasted 7 weeks, a control group of four cows remained on this diet. Two similar groups received daily only 4 lb. hay and about 6 lb. extra concentrates; in these groups the concentrates contained 22·3% crude protein. The remaining two groups received similar treatment except that the concentrates contained only 11·6% crude protein. In addition, 10 1b. macerated delignified straw pulp (containing 1 lb. crude fibre) were given to half the cows receiving each type of concentrate. At the end of the experiment the cows were turned out to grass.

3. With the diets low in hay and high in concentrates there was a fall in the fat content of the milk, with some variability in the response of individual cows (see 4 below). With the concentrates high in protein, the fall appeared to be rather slower than with those low in protein, but by the sixth and seventh weeks of experimental treatment it was estimated to be 0·67% fat with high and 0·72% fat with low protein concentrates. The experiment showed, therefore, that serious falls in fat content may occur in cows early in lactation with diets of 4 lb. hay and concentrates containing 22·3 or 11·6% protein. The addition of a limited amount of crude fibre as straw pulp brought about no recovery in milk fat content.

4. The extent of the depression brought about by the change to diets low in hay diminished with advancing stage of lactation.

1. Four comparable groups of Shorthorn cows were used for an investigation of the effects of various amounts of hay, given with concentrates, on the fat content and yield of milk.

2. During the initial and final control periods all the cows received 16 lb. hay daily and about 4 lb. of a balanced concentrate mixture (flaked maize 50%, weatings 35% and decorticated groundnut cake 15%) per 10 lb. of milk produced. During the experimentaltreatment period of 7 weeks the groups received 12, 8 or 4 lb. hay, or 8 lb. coarsely ground hay, per cow daily respectively, with sufficient of a low protein concentrate to compensate for the reduction in hay, and the balanced mixture as in the control periods.

3. The change from 16 to 12 lb. hay did not affect the fat content of the milk, a small decline being attributable to the advance in stage of lactation, but the mean values for the last 2 weeks of treatment, adjusted for the differences in the initial control period, showed that the milk fat percentage for the groups receiving 8 and 4 lb. hay and 8 lb. ground hay had fallen by 1·16,1·12 and 1·72 respectively by comparison with that for the 12 lb. hay group.

4. A group of Friesian cows was included in the experiment, and received 4 lb. hay during the treatment period. The fall in the fat content of their milk was greater, but not significantly different from, that observed with the corresponding group of Shorthorn cows.

5. In the treatment period the milk yields tended to decline more rapidly than in the control periods, and this taken in conjunction with the falls in fat content was reflected in considerable falls in the yield of milk fat, ranging from 28·4 to 55·2% with diets containing less than 12 lb. hay. Both the fat percentage and fat yields recovered in the final control period.

A manually operated tympanometer pressure recorder of simple construction designed for recording the changes in pressure in the lower cistern of the mammary gland of the cow during let-down and milking is described. The instrument makes possible the measurement of the internal milk pressure without the need for direct access to the milk cistern. The measurements are made with a tympanometer unit which is held firmly against the outside wall of the udder.

Measurements of intramammary pressure changes in the sow following intravenous injection have shown that the milk-ejecting action of vasopressin and acetylcholine is similar to that of oxytocin. Doses up to 0·4 mg. of histamine produced no measurable effects within the mammary gland.

The injection of small amounts of oxytocin into the lactating sow has shown that the minimum quantity required to evoke a response in a normal adult animal is of the order of 0·05 i.u. If the injection is prolonged, the dose must be increased to produce a response. Large doses (0·8–1·0 i.u.) given at widely different rates produce multiple ejection pressure graphs with two or more peaks.

Methods for measuring the reflectance of milk are described. The effect of heating is to reduce the reflectance of whole and separated milk progressively towards the blue end of the visible spectrum. The reflectance of cream is almost unaffected. The measurement of reflectance at the blue end of the visible spectrum is shown to be a useful method for the determination of the brownness of milk. The variation of rate of browning with temperature and with pH and the effect of storage on brownness are given.

It has been shown that human milk, like cow's milk, increases in fat content during suckling. The increase is not associated with any systematic change in fat globule size distribution. Human milk fat globules, like those of the cow, have been shown to cluster at body temperature. These facts support the theory that the increase in fat content during suckling is due to the fine ducts filtering out the larger globule clusters.

An inverse relationship has been demonstrated between the titratable acidity and formol titre of raw separated milk heated at temperatures of 60, 70 and 80° C. suggesting that the combination of lactose and protein, with the elimination of basic amino groups attached to protein, is one of the main reactions responsible for the heat-induced acidity. At temperatures of 90 and 100° C., when thermal decomposition of protein takes place, heat-induced acidity develops very rapidly and is accompanied by a marked increase in the number of basic amino groups and in the number of these groups which combine with lactose. Below 80° C. the development of acidity was not accompanied by discoloration of the millk, but at 80° C. and above, browning of the milk occurred. The relation between heat-induced acidity and browning is discussed.

The loss of moisture per pound of free moisture in a cheese is shown to depend on the moisture movement in the cheese and hence on a function of the group (diffusion rate × time/(height)2). The results of a series of experiments were correlated by means of the above expression. The variables correlated were time of storage, moisture content, fat content, air humidity, air temperature, size and shape of the cheese. Total moisture affects the free moisture value, and air humidity affects the free moisture via the equilibrium moisture. The main effect of temperature is on the rate of moisture diffusion, and the fat content also affects the rate of moisture diffusion. All experimental results for close-bodied cylindrical cheeses were approximately correlated by the expression.

Titration curves of calcium-containing and calcium-free solutions resembling milk serum indicated that precipitation of calcium from such solutions was greatly impeded by citrate. In the absence of citrate, precipitation of tricalcium phosphate was complete at pH 6·0, but, in solutions containing citrate, precipitation of tricalcium phosphate occurred gradually throughout the titration and was not complete at pH 10.

In some solutions precipitation of calcium phosphate ceased at about pH 9·7, even though the base added had been insufficient to neutralize tertiary hydrogen equivalent to the known calcium content. Precipitation of dicalcium phosphate must therefore have occurred.

The observed stability of calcium in these solutions was much greater than that predicted from the accepted solubility and dissociation constants. It is therefore concluded that detailed studies of these constants, and of the factors controlling precipitation of dior tricalcium phosphate, are needed.

A high proportion of the group ‘N’ streptococci, isolated from Cheddar cheese curd, produced volatile acids and acetoin, in varying degrees, in milk culture. The active acetoin-producing strains closely resembled type strains of Str. diacetilactis (Matuszew-ski) and Str. citrophilus P.R.M. but differed from them in producing acetoin in lactose broth. The weak acetoin-producing strains resembled Str. cremoris HP in this characteristic, but in their morphological and cultural reactions, mor e closely resembled Str. lactis. The titratable acidity in milk cultures varied inversely with the production of volatile acids and acetoin.