Time/temperature profiles were obtained for commercial ultra high temperature (UHT) and sterilized milk processing plants and for a pilot UHT plant operating under nine different conditions. Samples of milk from each process were analysed for lactulose by an enzymic method, yielding concentrations of lactulose of 4 to 118 mg/100 ml. The measured lactulose concentration could be derived from the corresponding time/temperature profile by assuming an Arrhenius relationship with an activation energy of 152 kJ mol-1, for the whole range of UHT and in-container processes examined.

A study of the content of water soluble vitamins in the 12 baby milk formulae available in the UK in April 1983 showed that levels of vitamins in the individual brands were generally in excess of those declared by the manufacturer. For cows' milk based formulae the mean excess of vitamins over the declared level was 94%. Vitamin levels in prepared formulae averaged three times the minimum recommended level. The average level of B vitamins in prepared cows' milk based formulae was six times higher than in mature human milk. For the two soya based products, the levels of vitamins were generally in excess of the declared level and levels of vitamin B12, biotin and, in one sample, vitamin B6 were considerably in excess of those found in mature human milk.

The 31P nuclear magnetic resonance spectrum of liquid milk was examined. Of the three peaks observed, the two larger were assigned to inorganic phosphate (Pi) and the seryl phosphate (SerP) residues of casein; the third peak was assigned to a phosphodiester, which is probably glycerophosphoryl choline. The pH-dependences of the chemical shifts of the Pi and SerP were measured with and without added EDTA, and the results confirm the assignments. The width of the Pi peak in milk is significantly greater than in similar solutions lacking casein, probably because of binding to, or chemical exchange with, the casein micelle. Most of the SerP residues in milk are not sufficiently mobile to have been detected in these experiments but a significant fraction of SerP residues is able to move freely and can be titrated.

Six dairy cows were treated before milkings with either oxytocin (Pitocin, 20 i.u.) or ACTH (Synacthen, 150 i.u.), principally to determine their effect on the ratio of citrate:lactoferrin concentrations in the milk. With ACTH treatment, after 3 d milk yield and citrate concentration decreased significantly, lactoferrin and bovine serum albumin (BSA) concentrations increased significantly. Somatic cell counts (SCC) increased temporarily in the milk of three of the cows which previously had > 100000 cells/ml. Lactoferrin yield remained fairly constant but citrate yield was significantly reduced. The citrate: lactoferrin molar ratio decreased from 1373 to 606. With oxytocin treatment, after 4 d milk yield first increased and then significantly decreased, citrate concentration decreased significantly while there were no significant changes in lactoferrin or BSA concentration or in the yield of any other milk constituents. The citrate: lactoferrin molar ratio decreased from 1621 to 1301. There were no significant changes in SCC either during treatment or 4 d after treatment but there was a significant rise at 16 d after treatment. It was concluded that in lactating cows both hormones affected citrate and lactoferrin concentrations in the direction that would improve the antibacterial properties of milk, but that this was accompanied by adverse effects on milk secretion. The extent of the change was not sufficient to be likely to produce inhibition of coliform bacteria.

The cheesemaking properties of recombinant chymosin from Escherichia coli were compared with those of standard rennet in parallel trials with three single strain starters. For each pair of cheeses the cheesemaking characteristics, recoveries of milk solids in the curds and compositions were similar. The rates and types of proteolysis and texture development during ripening between cheeses made with the two coagulants were not significantly different.

Preheating milk at 140 °C for 1 min at pH 6·6, 6·8, 7·0 or 7·2 shifted the heat coagulation time (HCT)/pH profile to acidic values without significantly affecting the maximum stability. Whey proteins (both β-lactoglobulin and α-lactalbumin) co-sedimented with the casein micelles after heating milk at pH < 6·9 and the whey protein-coated micelles, dispersed in milk ultrafiltrate, showed characteristic maxima–minima in their HCT/pH profile. Heating milk at higher pH values (> 6·9) resulted in the dissociation of whey proteins and κ-casein-rich protein from the micelles and the residual micelles were unstable, without a maximum–minimum in the HCT/pH profile. Preformed whey protein–casein micelle complexes formed by preheating (140 °C for 1 min) milk at pH 6·7 dissociated from the micelles on reheating (140 °C for 1 min) at pH > 6·9. The dissociation of micellar-κ-casein, perhaps complexed with whey proteins, may reduce the micellar zeta potential at pH ≃ 6·9 sufficiently to cause a minimum in the HCT/pH profile of milk.

The increase in levels of free fatty acids (FFA) in morning and afternoon milk after 48 h cold storage at 5 °C was determined at two stages of lactation in milk from 96 cows. Lipolysis was positively correlated to day of lactation (r = 0·6, P < 0·001), while the correlations between lipolysis and milk yield and fat content respectively were due to these factors being related to day of lactation. In afternoon milk the level of FFA increased almost three times above that in morning milk. Lipolysis was closely related to the lipoprotein lipase (LPL) activity associated with the cream fraction (r = 0·8, P < 0·001). Afternoon milk had higher LPL activity in the cream fraction than morning milk. Lipase activity in the cream increased with increasing stage of lactation. Cooling of milk increased LPL activity associated with the fat phase. This effect was greater in afternoon milk from late lactation than in morning milk from early lactation. When heparin was added to milk the LPL activity associated with the fat increased substantially; again, this effect was greater in afternoon milk from late lactation than in morning milk from early lactation. Lipolysis was higher in heparinized milk than in normal milk, and there was a close correlation between lipolysis and LPL activity associated with the fat (r = 0·82, P < 0·001). Characteristics of the milk fat globule influencing the attraction of LPL were found to be fundamentally important to lipolysis.

Enzymic digests of samples of heated skimmed milk were found to contain small amounts of S-carbamyl-L-cysteine. This material was concluded to have arisen from reaction of a cysteinyl residue in βlactoglobulin (β-lg) with cyanate derived from the native urea of bovine milk. After heating at 75 or 90 °C for 20 min skimmed milk contained 0·009 and 0·029 μmol/ml respectively of protein-bound S-carbamyl-cysteine. Evaporated milk contained 0·058 μmol/ml. The S-carbamylcysteine contents of samples of β-lg heated under similar conditions in equimolar 0·17 mM-KCNO at pH 6·6 were 0·016 and 0·062 μmol/ml respectively. At 60 °C no formation of S-carbamyl-cysteine was detected. Samples of evaporated milk and skimmed milk which had been heated for 20 min at 60, 75 or 90 °C were examined, after acid hydrolysis, for the presence of homocitrulline. None was found. Similarly αs1- and β-caseins heated at 60 and 75 °C in 4 mM-urea at pH 6·6 yielded no homocitrulline, but after treatment at 90 °C for 20 min αs1-casein contained 68 μg/g and β-casein contained 73 μg/g protein-bound homocitrulline. This was concluded to have been the product of reaction of cyanate and ε-amino groupings of lysyl residues of the caseins. The relative importance of carbamylation involving ε-amino groups or sulphydryl groups in milk is discussed.

The kinetics of the epimerization of lactose to lactulose were determined for two ‘model milk’ solutions, one containing lactose and the other lactose and lysine. The lysine was added to lactose to determine whether amino groups of proteins act as base catalysts promoting the production of free lactulose. The presence of lysine actually reduced the rate of production of free lactulose, presumably by the formation of amino-sugar complexes. The rate of production of free lactulose over the range of processing conditions was shown to be first order for both systems. Thermal effects on lactose were shown to be complex. The epimerization of lactose to lactulose was reversible and both sugars were thermally decomposed to their monosaccharides. The difference in the rate of production of free lactulose between the lactose, and the lactose and lysine solutions was also observed in processed milk and its ultrafiltrate (protein free).

A set of 12 fractions was isolated from homogenized cows' milk by a method of successive centrifugation steps. The fractions were analysed in terms of fat and protein contents and the diameters of the particles in them were also measured. Detailed comparisons of the turbidities of the samples and their light scattering properties allows comparison between them. It is concluded that the separation is less effective than a similar technique applied to casein micelles, but nonetheless that the different fractions do contain particles with different properties.

The inhibitory effect of five disinfectants on acid production by Streptococcus lactis and Str. cremoris Cheddar cheese starters was compared using two techniques, a laboratory scale fermentation tube test, and pilot scale cheese manufacture. Threshold inhibition concentrations (partial, total) of disinfectants (mg/1) on acid production observed in the fermentation tube were: peracetic acid/H2O2 (0–5, > 600) iodophor (75–220, > 400), quaternary ammonium compound (QAC), (20–30, > 200), acidic anionic (60–120, 600–1200), and Na hypochlorite (400–1000, > 1000). Threshold inhibition concentrations (partial, total) observed in the cheese vat were (mg/I); peracetic acid/H2O2 (0–5, 5–35), iodophor (15–30, 75–200), QAC (10–20, > 200), acidicanionic(30–120, > 600), and Na hypochlorite (35–400, > 400). Flavour threshold concentrations in the cheese at 4 weeks were (mg/l): peracetic acid/H2O2 5, iodophor 8, QAC 20, acidic anionic 12, and hypochlorite 50. It is concluded that problems in cheese manufacture are unlikely to occur with hypochlorites, iodophors, and acidic anionics, but that care should be taken when using QAC and peracetic acid as sanitizers. A correlation coefficient of 0918 was computed between the two methods of determining partial acid inhibition (P < 0·005).

The increase produced by formaldehyde (HCHO) in the heat stability of milk did not occur when milk was treated with HCHO at temperatures up to 60°C followed by dialysis at 5°C. However, the minimum in the heat coagulation time (HCT)–pH curve was irreversibly removed if the milk was preheated at 80–C for 10 min in the presence of 5 mM-HCHO. Although this treatment blocked the ε-amino groups of lysyl residues, the stabilizing mechanism is considered to be due to the cross linking action of HCHO which reduced the level of non-sedimentable, κ-casein-rich protein dissociated from the micelles on heating. The specific crosslinking agent, dimethyl suberimidate, modified the HCT-pH profile of milk in a manner similar to preheating at 80°C for 10 min with 5 mM-HCHO, supporting the crosslinking hypothesis. The results of this study appear to lend some support to the proposal of Kudo (1980) that the minimum in the HCT-pH curve of milk is due to the dissociation of κ-casein from the micelles on heating at high temperatures at pH values greater than 6η7.

The first stage in acid casein manufacture is the isoelectric precipitation of casein from skim milk. This stage controls the particle size of the casein curd which is important in the later washing, dewatering and drying stages. Casein was precipitated batchwise from 9% total solids skim milk using 0·3 M-H2SO4 at temperatures from 25 to 53 °C and at pH (measured in the whey) from 3·6 to 5·1. A wet sieving method has been developed for measuring the particle size distribution of the casein curd. Ferric ammonium sulphate was added to the curd/whey mixture to toughen the curd particles and to reduce their stickiness. For given precipitation conditions the particle size distribution could generally be modelled by a normal distribution. The mean particle size generally increased with increasing temperature and pH. The increase in particle size with pH was pronounced at 53 °C, while at 25 and 35 °C particle size varied little with pH. The calcium contents of washed casein curd samples were low at all precipitation pH at a precipitation temperature of 35 °C, but increased markedly above pH 4·4 at precipitation temperatures of 45 and 53 °C.

Pseudomonas strain AFT 21 produced three heat stable extracellular proteinases in milk and nutrient broth at 7 or 21 °C, but the proportions depended on medium and cultivation temperature. The three proteinases were EDTA- and o-phenanthroline-sensitive metalloenzymes and were not inhibited by N-ethylmaleimide or phosphoramidon. Proteinases I and II showed maximum activity at pH 7–7·5 and proteinase III at pH 8·5. All three enzymes showed maximum activity at 45–47·5 °C, but had relatively high (19–27% of maximum) activity at 4 °C. They were unstable at 55 °C in phosphate buffer, pH 6·6, or synthetic milk ultrafiltrate (SMUF) containing 12 mmol Ca2+, but were stabilized by short preheating at 100 °C. They were extremely heat stable in both phosphate buffer and SMUF, pH 6·6, at 70–150 °C. Their D-values at 140 °C were 69, 54 and 80 s respectively. The Z-values for Pseudomonas AFT 21 proteinase III in phosphate buffer and SMUF were 29·7 and 30·3 °C respectively; the corresponding activation energies for inactivation were 8·7 × 104 J mol-1 and 9·2 x 104 J mol-1.

Bovine milk cream (40% fat) was ripened under different time/temperature/inoculum conditions with Streptococcus lactis subsp. diacetilactis DRC-1 before converting by heat treatment into ghee (Indian clarified butterfat). Biochemical changes in cream, in terms of titratable acidity, volatile acids, diacetyl and acetaldehyde and flavour changes in the ghee made from it in terms of free fatty acids (FFA), total carbonyls (TC) and flavour score, were studied. Of the different combinations of ripening conditions, 30 °C, 18 hand 3% inoculum produced optimum changes in the cream which resulted in the development of the most desirable flavour in the ghee, being comparable to that of desi ghee (made by the indigenous method). The flavour characteristics as determined by FFA (% oleic acid), TC (μmol/g fat) and flavour score (out of 10) of the best ripened cream ghee were found to be 0·54, 6·756 and 8·83 respectively. The flavour of ghee became impaired after a particular level of acidity developed in cream after 18 h incubation. The optimum conditions of ripening standardized under laboratory conditions were also extended to a semi-industrial scale.

The effects of κ-casein and β-lactoglobulin (β-lg) genetic variants on cheesemaking were examined in two laboratory scale experiments. Variants of κ-casein had highly significant (P < 0·001) effects on milk citrate content (A > B) and rennet clotting time (A > B). When curds were cut at constant firmness there were only minor effects of κ-casein variants on milk component recoveries and cheese yield and composition. A tendency to higher fat recoveries from milks containing κ-casein B was reflected in significantly (P < 0·05) higher fat contents in cheese after 3·5 months of ripening. Genetic variants of βlg had highly significant (P < 0·001) effects on casein number (B > AB > A) and protein recoveries (B > AB > A) and significant (P < 0·05) effects on yield of cheese dry matter (B > AB > A). Green (unripened) cheeses made from the different milks also differed in fat, protein and moisture contents, probably in part due to differences in fat/casein ratios of the cheesemilks. The compositional differences were reflected in differences in the extent of proteolysis during ripening (A > AB > B). In the experiment with β-lg genetic variants the effect of time of cold storage of the cheesemilk was also investigated. No differences were found in milk composition and component recoveries or cheese yield, composition and quality when milks were cold-stored for 24, 48 or 120 h before cheesemaking.

Cheddar cheeses were made from milks concentrated 3- to 6-fold by ultrafiltration. Modifications to the conventional cheesemaking process were designed to obtain the correct composition and pH value in the final curd, improve the fat and moisture retention and cheese texture, and increase the ripening rate. The compositions of all cheeses were similar to those of controls made with unconcentrated pasteurized milk. The curd and cheese structures and the textural properties of the cheeses were closer to those of the controls when made from milk concentrated 5-fold than when made from milk concentrated 3-fold. Light homogenization of the milk before concentration led to improved fat recovery and decreased graininess and crumbliness in the cheese. Lowering the coagulation temperature of concentrated milks gave improved curd structure and less grainy and more sticky cheeses. Addition of the bacterial proteinase, Neutrase, with the salt, stimulated proteolysis and flavour production in the cheeses and made them more sticky. Neither reduction of the mineral levels in the milk by acidification before concentration nor reduction of the lactose concentration by diafiltration had any significant effect on the composition or textural properties of the cheeses.

The effect of chelating agents on extracellular proteinase production by Pseudomonas fluorescens 32A was examined. Increasing concentrations of orthophosphate slightly stimulated growth while inhibiting proteinase synthesis. Fifty percent inhibition was found at 35 and 28 mM-orthophosphate at 5 and 20 °C respectively. Extracellular protein concentration was reduced by 30% when cells were grown with 100 mM-orthophosphate. Polyacrylamide gel electrophoresis of the cell-free supernatants suggested that reduced enzyme synthesis had taken place as evidenced by the decrease in staining intensity of the protein band corresponding to the proteinase. Other phosphate compounds could replace orthophosphate as an inhibitor. Extent of inhibition was related to chain length; polyphosphates with 4–6 or 13–18 phosphorus atoms were the most effective inhibitors. EDTA (0·5 mM) completely inhibited proteinase synthesis. This inhibition could be partly reversed by Ca2+ and, to a lesser extent, Mn2+. Proteinase production at 5 °C in skim milk was completely inhibited by phosphate glass (P13–P18). Control experiments showed that loss of activity with chelators was not due to inhibition of preformed enzyme. The results suggest a possible role for polyphosphates in controlling proteinase production in stored milk.