Flow separation and cavitation effects in a simple homogenizing valve have been studied visually and recorded photographically. The degree of separation depended upon the profile of the seating inlet boundary, the pressure loading and several other interdependent factors. Cavitation did not occur independently of separation; conditions would be suitable for microscale cavitation to occur within the separated flow itself, but this was not discerned with certainty. The suppression of separation and any cavitation, assessed visually, did not influence the homogenizing characteristics of the valve. It was concluded that hydrodynamic forces of cavitation are not responsible for fat globule disintegration in drop-and-lift or poppet types of high-pressure homogenizing valve.

The effect of varying periods of energy undernutrition on milk yield and composition was determined in 30 British Friesian cows receiving a typical winterstall diet. An absence of steaming-up, and underfeeding from the beginning of lactation, depressed both milk yield and solids-not-fat (SNF) content, especially in early lactation. Initially the reduction in milk SNF content was accounted for mainly by a loss in lactose content, but as lactation progressed this effect lessened and an effect on milk protein content was established. Protein content was similarly depressed by underfeeding in mid-lactation. The restoration of normal feeding in mid-lactation after varying periods of underfeeding was associated with only small responses in SNF and protein contents.

The effect of the type of basal diet and supplementary concentrates on the extent of the increase in SNF and protein contents in response to improved feeding was examined in 2 further experiments. Responses were in all instances small and it was concluded that with the diets commonly in use in Great Britain the provision during the winter period of supplementary concentrates, whatever their composition, is unlikely to give a marked improvement in milk SNF or protein contents.

A cubed molassed sugar-beet product with added urea, dicalcium phosphate, trace elements and vitamins was prepared containing 17% crude protein and up to 0·53% P. The product was evaluated in 3 changeover trials with lactating cows and the treatments involved comparisons with a low protein (10%) concentrate and one supplemented with groundnut plus cottonseed meals. When the product was included at 35–62% in milk production diets the mean daily yield of about 15 kg was about 0·5kg less than when full supplementation was as vegetable protein. This was considered more likely to be associated with inefficient utilization of urea rather than because of an energy difference between molassed sugar-beet pulp and barley.

The effects on rumen fermentation and milk-fat secretion of a dietary supplement of protected tallow given to 4 Friesian cows established on a low-roughage ration and with depressed milk fat is reported. The ratios of acetate to propionate in the rumen were unaffected by the supplement and remained typical of those associated with low-roughage diets in that the proportion of propionate was increased. The supplement produced almost complete recoveries in yield and content of milk fat without any increase in intramammary fatty-acid synthesis. The recoveries were due to transfer of about 20% of the total fatty acids of the tallow supplement. These results are discussed in relation to the effects of low-roughage diets on milk-fat secretion and it is concluded that in the ‘low-fat syndrome’ the capacity of the mammary gland to absorb preformed fatty acids is not impaired.

Ultraviolet difference spectral measurements and some circular dichroism data are reported on the systems (β-lactoglobulin (BLG)/soya-lysolecithin (sll)), (κ-casein (KC)/sll) and (BLG/KC/sll). Sll destabilizes the native conformation of BLG and KC and promotes the interaction between BLG and KC. The influence of sll: protein ratio and time of heating on the complex formation has been investigated.

The rates of production of protozoa in the rumen of buffalo calves have been estimated using a single-injection isotope dilution technique. The calves were fed at 2 levels of crude protein, namely, 13% lower and 19% higher than that recommended by the National Research Council. The animals were given their rations at 2-h intervals for 3 weeks. Thereafter 14C-labelled rumen protozoa were injected into the rumen of each calf in a single injection. Samples of the rumen liquor were drawn at various time-intervals for 10 h and were analysed for the concentration of protozoa and radioactivity. The decline in the specific radioactivity of protozoa cells in the rumen as a function of time was used for calculating half life (t½). A mathematical equation was used to calculate the rate of production of protozoa. The average t½ was 1077 min for both groups and the rates of production of protozoa were 73·9 and 92·1mg/min in groups fed on low and high planes of crude protein respectively, showing that the production of protozoa is significantly (P < 0·01) higher in animals consuming a ration high in crude protein.

Two experiments were carried out with Friesian cows grazing improved winter pastures. In expt 1, 12 cows were fed 6·6kg/d of a barley-based concentrate containing either no N supplement, or supplements of casein, formaldehydeprotected casein or urea which provided 48 g N/d. In expt 2, 12 cows were fed 2·6kg/d of a barley-based concentrate containing either casein, formaldehyde-protected casein or equal mixtures of both, all of which provided 96g N/d.

In expt 1, although the production of fat-corrected milk was 7·1% higher and that of total solids 6·1% higher on the protected casein than on the casein diet, production of milk and milk constituents on the control and urea diets were similar to values obtained for cows fed protected casein. In expt 2, milk production and composition were unaffected by treatments.

The concentrations of amino acids in jugular plasma and mammary uptake per litre of plasma were not affected by the experimental diets in expt 1, and were unrelated to milk yield in either experiment. In expt 2 the concentrations of glycine, serine and threonine in jugular plasma were significantly higher in the diet in which none of the casein was protected. Mobilization of muscle protein may have been responsible for the elevated levels of glycine in cows fed this diet. Mammary uptakes of essential amino acids in expts 1 and 2 were respectively 47 and 42% of those available, whereas the uptakes of non-essential amino acids were respectively 24 and 17% of those available.

An attempt was made to determine the amino acids most likely to limit milk production. Milk:uptake concentration ratios were obtained using our values for amino-acid uptakes and values published previously for the concentration of amino acids in milk. The results of these calculations suggest that the amino acids most likely to limit milk production are threonine, phenylalanine and methionine.

Studies were undertaken to determine the influence of dietary proteins on the rennin and pepsin contents of preruminant calf vell. Three groups of 12 Friesian calves were each fed either milk proteins, whey proteins or a 50:50 mixture of these 2 diets. They were slaughtered at a constant weight of 150kg and their vells collected and dried. Another group of vells was obtained from 8 animals that had been fed milk proteins in a separate experiment. The extraction of the abomasal enzymes was carried out at acid pH, and the extracts were quantitatively analysed for rennin and pepsin by DEAE-cellulose column chromatography. Qualitative analyses were also performed by agarose-acrylamide gel electrophoresis. The only enzymes observed using this last method were rennin and bovine pepsin II. Statistical analysis of the quantitative enzyme determinations indicated a trend for the vells from calves fed diets containing casein to be richer in total activity and in rennin, while the level of pepsin remained approximately constant. It seems that casein may induce the secretion of rennin. However, further experiments will be necessary to confirm this. Important differences were observed between the 2 groups of veils from calves given the same diet, but grown in slightly different conditions.

The relative importance of β-hydroxybutyric acid (BHBA) and acetate as precursors for milk-fat synthesis was studied in lactating goats by infusing separately tracer quantities of [3−14C]DL-BHBA and [1−14C]acetate into the jugular vein, and [1−14C]butyrate into the portal vein. The concentrations and specific radioactivities of blood plasma constituents, the yields and specific radioactivities of individual milk fatty acids and the relative radioactivities of individual carbon atoms of milk fatty acids were determined.

The infusion of [1−14C]butyrate resulted in the appearance of labelled BHBA in the blood plasma which behaved almost identically with infused [14C]BHBA as a precursor for milk fatty acids.

The relative radioactivity of carbon atoms of the fatty acids of milk fat following the infusions provided direct evidence that BHBA had provided an intact 4-carbon unit at the methyl end of each fatty-acid chain. Acetate provided 2-carbon units both for the elongation of the 4-carbon units and for complete de novo synthesis. BHBA also provided 2-carbon units which behaved in a similar fashion to those from acetate.

Acetate and BHBA together accounted for all of the C4–C12 acids of milk fat, about 75% of the C14, 45% of the C16 and 10% of the C18.

The total contributions of the various precursors to the fatty acids of milk fat were: acetate 42%, BHBA 9·4% and other plasma precursors (by difference) 48·6%.

A survey of the composition of the milk of individual cows in 6 commercial herds in the North of England was undertaken from October 1968 to July 1969. In all herds there was a decline in solids-not-fat (SNF) content; in 4 of the herds this was mainly the result of a fall in protein content, although in all herds there was a decline in lactose content in association with an increase in udder infection and an advance in average stage of lactation. Between-herd differences in the minimum value for lactose content were related to differences in udder infection, stage of lactation and lactation number.

Two of the herds were selected for further study from October 1970 to April 1971. After a preliminary period each herd was divided into 2 matched groups. One group received the usual management, and the other was subjected to remedial changes of supplementary feeding and antibiotic therapy and improved udder hygiene. In both herds, treatment reduced udder infection and improved lactose content and in one of the herds there was also an increase in protein content.

A series of experiments is described, outlining a study of 5 factors likely to influence the efficiency and reproducibility of a standardized method used for the electronic counting of somatic cells in milk. Samples containing cells covering 5 ranges from 250000 cells/ml to 1500000 cells/ml were used and the results analysed statistically.

The residual error was given special consideration, and was found not to contribute to any significant degree to the accuracy of the method. Sample volumes of 0·5 ml and 0·1 ml used for counting were compared, and there was a decrease in accuracy using the smaller volume. The influence of individual technician skill was studied with 4 workers having differing degrees of experience, and no significant addition to the overall error was observed. The effect of agitating bulk tank milk for between 0 and 2 min before sampling was examined. Variations were considerable for agitation times of less than 1 min, but the recommended mixing time of 2 min was found to be adequate with milk quantities between 450 l and 2250 l (100 and 500 gal), and at the 5 cell-count levels chosen.

The distributions of the cell counts at the 5 levels were found to be normal with the marginal exception of samples tested at 250000 cells/ml.

All aspects of the technique, sample preparation, use and calibration of the Coulter Counter, etc, should be closely followed to ensure that no inaccuracies will result from inadequate maintenance of the machine or other equipment employed in the test.

Two methods are described for the separation of diacetyl and acetoin in milk by steam distillation. In the first method, acetoin is measured in the residue remaining in the extraction flask after steam distillation while diacetyl is measured in the first 10 ml of distillate which contains 35% of the acetoin and 100% of the diacetyl. Increases in the volume of residue taken for analysis decreased colour development by acetoin. In the second and preferred method, diacetyl is measured in the first 10 ml of distillate and acetoin in the second 10 ml of distillate which contains about 24% of the added acetoin. Diacetyl in both methods is measured by a modification of the method of Prill & Hammer (Walsh & Cogan, 1974) and acetoin by the Westerfeld (1945) procedure.

Both the rate of formation and the total amount of dimethylglyoxime formed from diacetyl in the presence of hydroxylamine were increased on omission of phosphate from the Owades & Jakovac (1963) hydroxylamine reagent. Sodium acetate was also unnecessary and slightly decreased the rate of dimethylglyoxime formation. The optimum concentration of hydroxylamine lay between 0·5 and 0·75 M. The heating time necessary for formation of dimethylglyoxime can be reduced from 60 to 20min at 75°C. Based on these findings a shortened and more sensitive method for the estimation of diacetyl is recommended.

The heat produced per colony-forming unit (CFU) of growing bacteria was found to depend on the species and was greater the younger the culture. With the exception of 2 determinations out of 13, with 6 species the results show that it should be possible to detect < 5×105 CFU/ml by the heat produced. A seventh species, a micrococcus, could be detected at a lower level of CFU presumably because the unit was a clump of several cells. Gross variations which occurred with fullcream milk await investigation.

It has been claimed by Stewart, Puppione & Patton (1972) that milk is a valuable source of cell membrane in a ‘relatively pure’ state originating from shed mammary secretory cell microvilli and golgi vesicles. The morphological basis for this has been re-examined in this paper. It is shown that the vesicular and elongated membranous structures found in skim-milk are identical to structures present as part of the initial milk fat globule membrane (MFGM) of alveolar and expressed milk, and that both structures survive extraction with chloroform–methanol. It is therefore suggested that a major part of the skim-milk membrane is material shed from the freshly secreted milk fat globule and consists of a specialised derivative of the secretory cell plasmalemma, the initial MFGM.

The influences of different operating parameters upon the distribution of pressure across a simple homogenizing poppet valve have been investigated. Regions of contracted and separated flow were defined by detailed probing of static pressures and found to correlate well with the results from previous visual and photographic observations.

Direct confirmation was obtained of the conclusion, first reported by Loo & Carleton (1953), that fat globules in milk are disrupted chiefly in a zone between the valve and seating surfaces which is close to the seating inlet boundary. Progressive coarsening of the globule suspension was detectable in the flow downstream from this zone.

A method for the quantitative partition of the albumin fraction of milk serum proteins is described. The partition, which is carried out by chromatography on a column of Sephadex G-100 (2·5×90cm) at 4°C in the presence of 0·02 m-sodium phosphate buffer (pH 6·7) containing 0·1 m-NaCl, yields 4 fractions. By gel electrophoresis it was shown that the fractions represent, in order of elution from the Sephadex, the following: fraction 1, a mixture of the largest proteins present, probably comprising γ-globulins, proteose-peptones and lactoferrin; fraction 2, bovine serum albumin and bovine serum transferrins; fraction 3, β-lactoglobulins; and fraction 4, α-lactalbumins. The protein contents of the various fractions were determined by nitrogen analysis and the results showed that good agreement was obtained between values from duplicate fractionations, and also that the recovery of material was satisfactory. The partition of the albumin fraction was not complete, but at least 90% of the material was found in fractions which represent essentially single proteins. The influences of some aspects of the pretreatment of samples, buffer composition and column preparation are described.

The effects of cystine-reducing and sulphydryl-alkylating agents on the extent of the primary phase of rennin action on unheated and heated milk samples were studied. Alkylating agents alone had a negligible effect, but heated samples, reduced with tri-n-butyl phosphine prior to alkylation with a mono-functional agent (sodium iodoacetate) did not show the usual inhibition of the primary phase. Heated samples, reduced and alkylated with a bi-functional agent (ethylene dibromide) which can re-cross-link the broken disulphide bonds, however, did give the normal inhibition. These results suggest that effects on the primary phase due to heat treatment of milk are determined to a large extent by rearrangement of the disulphide cross-links, and if these are permanently broken, heat appears to have little effect.

The results differ in some important respects from those obtained on the complex formed between isolated β-lactoglobulin and isolated κ-casein, and therefore throw doubt on the generally held view that there is a similar type of interaction in whole milk.

Coagulation time–pH curves with a coagulation-time minimum around pH 6·8 (type A curve) could progressively become type B (no minimum) as the heating temperature was decreased from 150 to 130°C. The short coagulation time that most milks have when pH is around 6·8 was found to be the result of a ‘premature’ coagulation, probably caused by calcium phosphate deposition on the larger caseinate micelles. This is followed by a second coagulation, not visually detected, that coincides with the coagulation time that would be expected if no coagulation-time minimum existed on the coagulation time-pH curve. The coagulation time of milks giving type A and type B curves may therefore not be comparable.

Forewarming milk for 30 min at 80°C can introduce or accentuate a coagulationtime minimum when the milk is subsequently heated at a higher temperature. The effects of adding β-lactoglobulin, copper and N-ethylmaleimide on the heat stability of milk were examined and explanations proposed for these effects.

Micelles reconstituted from urea-treated milk by exhaustive dialysis against bulk milk were similar to native micelles with respect to colloidal phosphate: casein ratio, ethanol stability, heat stability and susceptibility to first-stage rennin action. Reconstituted micelles were considerably smaller than native micelles as indicated by turbidity, sedimentation and viscosity, had shorter second-stage rennet coagulation times, were unstable to [Ca2+] > 20 mM and had reduced base-binding capacity. It is suggested that the induced Ca sensitivity is due to unfavourable alterations in the micellar κ-casein coat arising from the decrease in average micelle size and can be offset by increasing the κ-casein complement of the system or by increasing the degree of aggregation by slowly raising the [Ca2+] level.