Summer and winter milk-fat samples from 14 dairies in Sweden were fractionated by crystallization from acetone solution (1:8) at 15 °C. The composition of the major fatty acids of the parent milk fat and of the acetone insoluble fraction were examined by GLC, and the gross triglyceride pattern by TLC on plates of silicic acid treated with silver nitrate.

The fatty acid composition of the milk fat was similar to that of milk fat from other countries and varied according to season and also, to a smaller extent, from region to region. Four fractions, representing 33–45, 41–34, 18–14 and 7–6 % of the fat and which contained progressively smaller proportions of saturated acids, were obtained by TLC.

The acetone insoluble glyceride (AIG) fraction was characterized by a smaller content of short-chain fatty acids and unsaturated fatty acids, and a larger content of saturated long-chain fatty acids, than the parent milk fat. AIGs from summer milk fat contained a larger proportion of C18 acids and a smaller proportion of C6–C16 acids than AIGs from winter milk fat.

Four fractions representing 62–70, 15–8, 16–15 and 7 % of the AIG fraction were obtained by TLC. The distribution of the triglycerides in the AIG fraction differed from that in the parent milk fat, mostly in the relative amounts of glycerides in the 2 most saturated TLC fractions. The seasonal variation was largely confined to these 2 fractions.

A Coulter counter was used to measure the diameters and numbers of fat globules in serial samples taken during a single milking of each of 8 cows. Milk fat percentages of samples were estimated by the Babcock method. The average globule diameter, globule number and fat percentage all increased during milking. Regression analyses indicated that fat percentage was a stronger function of fat globule diameter than of globule number. The results are discussed with relation to the rise in fat percentage that occurs during milking.

The release, by the action of rennet, of peptides which are soluble in trichloroacetic acid (TCA) has been studied using milk samples from individual cows. This release showed an initial marked increase with time of rennet action. After the milk clotted there was no further increase for at least 30 min. There was considerable variation in the total amount of N and of sialic acid released from milk samples taken from individual animals at different times during the lactation.

For a given milk sample treated with rennet, the amount of sialic acid in the filtrate obtained after precipitation with 2% TCA was similar to that obtained with 10% TCA, whereas the amount of nitrogen was much greater with the 2 % TCA. It appeared that the peptides containing sialic acid were released at a slower rate.

On average, about half the total sialic acid in the casein was recovered in the TCA filtrate after rennet action, which suggests that either a proportion of the κ-casein was not accessible to enzyme action or the technique used did not permit full recovery of the sialic acid-containing peptides.

In order to obtain more information on the importance of the milk ejection reflex in the lactation of normal ewes, experiments were conducted to determine whether the neuroendocrine reflex is able to modify the parameters which characterize the flow rate of the milk under normal milking conditions.

Ewes were allowed to suckle their lambs freely during the 3 days following parturition. They were then milked with a machine and the individual milk production was measured volumetrically at each milking. Different fractions obtained during milking (‘machine milk’, ‘machine strippings’ and ‘hand strippings’), and the milk flow, were measured using a sensitive recording system. In further experiments, machine milking was performed after intravenous administration of oxytocin, during general anaesthesia, and after unilateral or bilateral denervation of the mammary gland.

The ewes fell into 2 categories: those giving most of their milk in 1 rapid emission, and those giving it in 2 quite separate emissions. The results indicated that the second emission found in some of the ewes was the milk emptied from the acini by the neuroendocrine ejection reflex. Disappearance of the second emission resulted from the administration of oxytocin before milking—which induces passage of the milk from the acini into the mammary cistern—and also from general anaesthesia or denervation of the mammary gland.

The flavour stability of casein and co-precipitated milk proteins has been examined organoleptically. Differences in flavour stability were correlated with processing conditions and the early stages of the non-enzymic browning reaction were implicated in the off-flavour development. The ‘gluey’ flavour present in stored casein could be removed and the flavour stability improved by treatment with activated carbon or Sephadex. However, a better solution to the problem was to avoid the development of off-flavour by careful choice of conditions of preparation, drying and storage to minimize heat damage and browning. Conditions were established under which milk protein could be expected to have flavour stability during at least 6 months storage at ambient conditions.

Compounds associated with the ‘gluey’ off-flavour in stored casein have been isolated and identified by a combination of gas chromatography and mass spectrometry. Most of the flavour was found in the non-acid steam-volatile fraction which contained many compounds including n-alkanals, n-alkan-2-ones, furfural and other furan derivatives, benzaldehyde, dimethyl disulphide and benzthiazole. No single compound had the typical gluey flavour, which must be a complex sensation in which the quantities of minor components may determine the specific gluey character. o-Aminoacetophenone was present in the casein in subthreshold amounts. This compound has some gluey flavour and may have a synergistic effect in enhancing the total flavour.

The quality of fresh and stored butter made from chilled cream collected over 3 days and stored either by accumulation in a vat or by separate storage of each batch was compared with that of butter made from unchilled cream collected over one day.

Provided each batch of cream after separation at 85 °F was cooled to 55 °F before addition to the vat, accumulation of cream at either 40 or 48 °F gave butter similar in quality to the control butter. Butter from cream cooled to 55 °F and held at this temperature, however, was slightly inferior in quality. The addition of the cream to the vat with no cooling, or cooling only to 70 °F, after separation gave butter of inferior quality due to rancid or stale flavours.

Butter from cream cooled either to 70 or 50 °F before storage of each batch in separate containers at either 40 or 50 °F was only slightly inferior in quality and in keeping quality to the control butter.

Defects in the chilled cream butters appeared to be due to lipolysis caused either by activation of the natural lipase of the cream or by lipolytic bacteria or both.

A lactating mammary gland of a sheep and a goat were perfused for several hours in the presence of [U-14C]D-fructose and received adequate quantities of acetate, glucose and amino acids.

In both experiments, there was a small incorporation of 14C in the expired CO2. Smaller radioactivities were measured in milk citric acid, lactose, casein and fat, the activities decreasing in that order. The specific activities of the amino acids from one casein hydrolysate were determined. The highest radioactivities were found in alanine and serine; methionine, glutamic acid and aspartic acid showed a smaller incorporation of 14C.

These results indicate that fructose is metabolized only to a very limited extent by the mammary gland. The relative distribution of 14C observed among the different substances isolated may be explained by a direct splitting of fructose into two C3-fragments, glycolysis and metabolism via the Krebs cycle.

Curd was made in a continuous curd-making apparatus (Berridge & Scurlock, 1969) and immediately separated from its whey. Its temperature was maintained at a high level (41–48 °C) as it was moved continuously in the form of a thin layer through a set of rollers. The curd then had a moisture content similar to that of normal curd at the end of scalding in the traditional process. After a suitable period of cheddaring the pH value and the moisture content of the continuously made curd were the same as those normally reached at the same stage in the traditional process.

In correctly operated continuous heat-treatment plant the processing temperature is maintained at a sufficiently elevated level to give the desired thermal death rate for any micro-organisms present before treatment. This paper shows that if through imperfect operation of the plant, the processing temperature fluctuates about the required level, the chances of survival for bacteria and spores will theoretically be increased. Tabulated results show the magnitude of the effect under varying conditions and the compensating increase in processing temperature to counteract it.

The nature of the αs-casein complex of bovine milk has been investigated by cation-exchange chromatography on columns of sulphoethyl Sephadex C-50 of purified preparations of αs-casein and of whole acid-precipitated casein. Three main fractions were separated from each. Two behaved as single homogeneous proteins as judged by starch-gel electrophoresis; they were the main constituent of the complex, αs1-casein, and a closely related phosphoprotein, designated αs0-casein, which was present in small amounts only. The third fraction also constituted only a small part of the total complex. It was heterogeneous on starch gel electrophoresis and contained 2 major and 2 minor components. This fraction, while similar to other members of the αs-casein complex and to β-caseins hitherto described in that it contained phosphorus, nevertheless differed significantly from these since 3 at least of its components contained sulphur either in the form of disulphide bonds or of sulphydryl groupings.

Information is presented on the composition of the 3 fractions including C-terminal end group analysis.

Effects of age and agitation of milk, temperature of separation, heat treatment and cooling, homogenization and ageing on the body of double cream were studied. Homogenization and temperature of heat treatment gave rise to the greatest effect. The effects of most treatments were inter-related to some extent, but this was most pronounced with cooling and ageing.

Using bacterial endotoxin as a tracer material, it has been shown that a jet of liquid impinging on the end of the teat for 1 min after milk flow had ceased during machine milking could force material past the barrier of the streak canal. Positive results were obtained when the maximum pressure at the centre of the impact area was about 0·25 bar, with a considerably higher frequency of positives at 0·3–0·5 bar. It seems possible that such impact pressures might occur during milking and cause transfer of infective material to the teat sinus. Impact force could arise when milk returned to the teatcup liner as the result of local or general conditions of instability in the milking machine vacuum.

Loss of milk clotting activity when rennin was treated with urea at several concentrations and at different pH values was compared with the change in UV absorption of the protein. The change in absorption at 287 and 293 nm did not correlate with loss in activity, indicating that the loss of enzymic properties was not related to the extent of unfolding of the molecule but was related to the reaction of susceptible groups to the environmental conditions.

Reactions of modifying agents, for example N-bromoacetamide, 2-hydroxy-5-nitrobenzyl bromide and mercuric ions with tryptophan groups and the relationship of these changes to enzyme activity suggested that tryptophan was not specifically associated with enzyme action but was important in maintaining tertiary structure.

Rennin was not inhibited by the pepsin inhibitors p-bromophenacyl bromide and α-diazo-p-bromoacetophenone which suggested that the active site did not possess a reactive carboxyl group as in pepsin. However, the reversible inhibition with 1-cyclohexyl-3(2morpholinoethyl)-carbodiimide metho p-toluenesulphonate together with the results of oxidation by N-bromoacetamide indicated that a tyrosine group or groups was important in enzyme activity.

The fact that rumen micro-organisms have a modifying effect on the utilization of food nitrogen by ruminants has been recognized for very many years (references to early work are given in Hungate, 1966) but until about 1947, when this subject was reviewed by McNaught & Smith (1947), many of the processes involved were not well understood. Since that time a large amount of work has been done in this field and more recent reviews (Chalmers & Synge, 1954; Blackburn, 1965; Hungate, 1966; Waldo, 1968) present a much clearer picture. Most of the work covered by these authors will be mentioned only briefly in the present review and attention will be directed mainly to developments reported in the last few years. Few references to work published before about 1964 will be given as they may readily be found in later publications.