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.

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.

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.

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 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.

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.

The design and operation of a laboratory-scale ultra-high-temperature sterilizer capable of treating 0·5-l quantities of milk are described. Temperature control, sporicidal efficiency and change of composition of the milk during processing are discussed.

Diacetyl, acetoin, 2,3-butyleneglycol, 2-butanone and 2-butanol concentrations have been determined in ripened Cheddar cheese made using Streptococcus diacetilactis DRC1 or Str. cremoris AM2 or HP as starter culture. Diacetyl content was found to be initially higher in the DRC 1 cheese than in the other 2 cheeses, presumably because this starter readily ferments citrate. The disappearance of acetoin appears to be related to the formation of 2,3-butyleneglycol in DRCl and AM2 cheeses, but not in HP cheese. Only trace amounts of 2-butanone and 2-butanol were observed in the 3 cheeses even though the concentration of a postulated precursor, 2,3-butyleneglycol, was present in considerable concentration in the DRC1 and AM2 cheeses. Analysis of a number of mature AM2 and HP cheeses selected at random indicated the presence of 2-butanone in only a few of the AM2 cheeses. It is suggested that this compound may have arisen from the presence of a micro-organism which entered the cheese vat at the time of cheese-making.

Cheddar cheeses made using single strains AM2 and HP of Streptococcus cremoris as starter were analysed for methyl ketones during ripening at 7°C. The level of odd-numbered C3–C15 methyl ketones increased very slowly in all cheeses and at maturity represented only 15–20% of the level potentially available from the degradation of esterified β-ketoalkanoic acids of milk fat in cheese. The potential for formation of the methyl ketones in unripened cheese curd was similar to the sum of free and potentially available ketones in mature cheese which suggests that the oddnumbered ketones once formed are not reduced or otherwise degraded by the normal cheese microflora. Significant variations were observed in the level of 2-butanone, which, in some cases, far exceeded the quantity which could be formed by breakdown of β-ketovaleric acid. Organoleptic evaluations and comparison of the levels of methyl ketones present in mature cheese with published threshold data suggest that while the methyl ketones may contribute to flavour they do not in their own right impart a typical mature Cheddar flavour to cheese.

Comparisons are made between the sulphur-containing volatiles of 4 Cheddar cheeses, using a gas chromatograph equipped with a flame photometric detector.

Two of the cheeses were made aseptically from whole milk, with and without starter, the third was made from whole milk with starter in an open vat, and the fourth was made from skim-milk also in an open vat. Only the cheeses made from whole milk with a starter culture developed normal Cheddar flavour; the cheese made without starter and the skim-milk cheese had little or no flavour.

Considerable differences were found in the chromatograms of the 4 distillates and in particular methanethiol was found only in the distillates from the cheeses having Cheddar flavour.

Concentration of milk by reverse osmosis is severely hindered by the formation of deposits on the membrane. Electron micrographs of this deposit show a thin (11nm) continuous electron-dense layer adjacent to the membrane. An electronlucent zone (10–15nm thick) separates this layer from the main bulk of the deposit which is 30 μm thick and consists largely of granules with some vesicles in a diffuse matrix. The density of packing of the granules increases with proximity to the membrane. Their appearance, size and digestion with pronase suggest that these are milk protein granules. The order and nature of these various layers could explain the pattern of flux decline.

Arterio-venous (AV) blood plasma concentration differences of amino acids across the mammary glands of 2 lactating goats were measured at intervals throughout a day. One gland of each animal had been transplanted to the neck for experimental purposes. The variation throughout the day in arterial concentration of all amino acids was similar. The variation in AV differences was slight for most essential amino acids, greater for glutamic acid and proline and very marked for aspartic acid, alanine, glycine and citrulline. There was no statistical difference between the AV difference of any amino acid measured simultaneously across the 2 glands of either goat. The arterial concentrations of certain pairs of amino acids were significantly correlated. The implications of the results for estimation of rates of amino-acid synthesis in the mammary gland using the intra-arterial infusion technique are discussed.

Experiments were performed on 3 lactating goats, in which mammary arterial plasma amino-acid concentrations were elevated by the infusion of a solution of essential amino-acids into the carotid artery supplying a transplanted mammary gland. In 2 experiments there were marked elevations in the arterial concentrations of most essential amino acids, but in one case only did this result in significantly increased uptake of amino acids by the gland, the arterio-venous difference being significantly correlated with arterial concentration for all except one amino acid. In the experiment in which increased amino-acid uptake was observed, infusion also resulted in a significantly increased milk yield and increased milk protein yield. The results are discussed in relation to data from other laboratories and lead to the suggestion that milk protein synthesis may be limited by the availability of either methionine or tryptophan.

Three experiments were carried out on 2 lactating goats, in which mammary arterial plasma amino-acid concentrations were elevated by the infusion of a solution of non-essential amino acids into a carotid artery supplying a transplanted mammary gland. In a fourth experiment a solution of glucose was similarly infused. In some cases the increased arterial concentrations of amino acids resulted in their increased mammary uptake, and in a depression of glucose uptake. However, infusions of neither amino acids nor glucose resulted in increased milk protein yield. Infusion of [U-14C]glutamic acid in one experiment demonstrated gluconeogenesis from glutamate carbon within the mammary gland. The results are discussed in relation to data obtained in another laboratory, from which it has been claimed that non-essential amino acid supply may limit milk protein synthesis. The present results provide no confirmation for the claim.

The addition of small amounts of phosphate salts reduced the amount of deposit formed from heated milk, both in laboratory experiments and during the operation of a practical ultra-high-temperature heat-exchanger. Sodium pyrophosphate was much more effective than the other phosphates tested. Reasons are suggested for these observed effects.

Milk gels varying widely in firmness were examined by both scanning and transmission electron microscopy, and a close correlation was found between their ultrastructure and firmness. In gels containing 40 and 50% total solids (14 and 17% protein respectively), casein micelles appeared as individual entities linked by some bridging material. At 60% total solids (20% protein), the micelles were fused and offered a considerably higher resistance to a penetrometer probe. Likewise some additives to gels, such as ammonium persulphate and CaCl2, caused the micelles to fuse. These chemicals appreciably increased firmness of such gels. In gels containing hexametaphosphate, a known inhibitor of gelation, most of the casein micelles were disintegrated; these gels were soft.

Intracellular dipeptidase of Streptococcus cremoris NCDO 924 has been investigated as a marker enzyme for detecting the release of starter enzymes into Cheddar cheese during maturation. Dipeptidases were produced extra- and intracellularly by Str. cremoris in broth cultures, but the intracellular enzyme was distinguishable by its relatively high activity against L-alanylglycine at low temperatures. The presence of dipeptidase activity of intracellular origin was demonstrated in extracts of Cheddar cheese freed from whole starter cells, opaque material of high molecular weight and free amino acids by a combination of centrifugation and gel filtration on Sephadex G200. The enzyme was not detected in cheeses made without starter (δ-gluconic acid lactone cheeses). In fresh curd with a high viable count of starter the dipeptidase activity was low and mostly extracellular. As the count declined the proportion of intracellular enzyme activity increased, reaching a maximum when 90% of the starter cells had died. The total dipeptidase subsequently decreased until the level remained 30% higher than that of the original curd, and changed little on further ripening up to 120 d. There remained a significant proportion of extracellular dipeptidase activity throughout the maturation period studied.

Rennin acts on a specially sensitive bond in αs1 B-casein to produce a basic peptide containing residues 1–23 of the original protein. At pH 6·4 and 30°C, the action is specific and rapid, the kinetic constants being Km 4·5×10−4M, Kcat 3·8 s−1, and kcat/Km 0·85×104 s−1 M−1. Pepsin, and a protease impurity in the acid phosphatase from wheat germ, have a similar action.