Milk permeate was separated at various temperatures by means of a hollow fibre ultrafiltration unit coupled to a stainless steel heat exchanger. Milk samples conditioned at 4°C were heated to 20, 40, 60, 80, 85 or 90°C prior to ultrafiltration. Ca, P, Mg, Na, K and citrate concentrations were measured in the permeate samples. Ca and P contents of the permeate decreased as the temperature increased. The pH was measured after cooling the permeate to room temperature. Smaller losses of Mg and citrate were also observed with increase in temperature. Na and K levels were not affected. A two-step time-concentration relationship was apparent for the species under study. An initial sharp decrease in concentration occurred in the first minute of holding time and was followed by a slower reaction. The possible occurrence of a two-step mechanism in the heat-induced salt balance changes is discussed. Dicalcium phosphate precipitation is believed to be coupled with tricalcium citrate precipitation upon heating.

Cold-stored milk lipolysis was enhanced from 5- to 50-fold when milk pH was adjusted to 7–8 with NaOH, while it was greatly decreased or stopped by an adjustment to pH 6–5·5 with citric acid. Small adjustments in pH (≤ 0·5 pH unit) also affected lipolysis, but the pH of native milk was not related to spontaneous lipolysis. The binding of lipoprotein lipase to cream was a pH-dependent process with an optimum near pH 7·0. Activity of the cream lipase on cold-stored milk fat continuously increased from pH 6·6 to pH 8·5. The activating effect of heparin on cold-stored milk lipolysis reached a maximum at pH 7·0–8·0 and a minimum in native milk, while addition of blood serum gave an opposite response.

Casein and other milk proteins in maternal colostrum and milk, the earliest food of the newborn, should not only be considered as a nutritional supply but also as a source of biologically active peptides. Some of them isolated from casein and lactotransferrin were active on platelet function. They inhibited both aggregation of ADP-treated platelets and binding of [125I] fibrinogen to ADP-treated platelets. Their behaviour was compared to that of fibrinogen peptides possessing similar effects: once more similarities between the milk and blood-clotting phenomena could be observed.

Tryptic phosphopeptides were obtained from whole bovine casein by chromatography on the anion exchange resin QAE-Sephadex A 25. Salt gradient elution of the column allowed separation of non-phosphorylated peptides from phosphorylated species. The preparations obtained contained at least seven distinct phosphopeptides of which the following casein fragments were identified: αs1(43–58):2P, αs1(59–79): 5P, αs2(46–70): 4P, β(1–28): 4P, β(2–28): 4P, and β(33–48): 1P. Fast protein liquid chromatography (FPLC) on Mono Q HR 5/5 resin showed that the phosphopeptides were eluted in the same order as from the QAE-Sephadex resin. However, on the analytical column HR 5/5 the fragments αs1(59–79): 5P and β(2–28): 4P, having the same net charge under the conditions of chromatography, co-eluted, whereas they were at least partly separated on the preparative column HR 16/10. Following enzymic dephosphorylation, the peptides eluted at lower salt strength in the gradient. FPLC on Mono Q resin thus permitted dephosphorylation to be monitored and intermediates between the parent species and the fully dephosphorylated peptide to be identified.

Concentrations of lactose, α-lactalbumin (α-la), glucose and glucose-6-phosphate were measured in milk obtained from rats under conditions where the rates of milk production varied from 0 to 2·6 g/h. In the milk from rats fed a low protein diet where milk production was 0·9 g/h, the concentration of α-la was 1·5 mg/ml, being significantly lower than that in the milk of control rats at 2·9 mg/ml and where the rate of milk production was 2·6 g/h. No differences were detected in the glucose concentrations. When rats were fed restricted amounts of the control diet and where milk production varied from 0 to 1·7 g/h during the course of the day, no differences in the concentrations of either α-la or glucose were detected. These results suggest that considerable caution must be used in interpreting the significance of milk glucose in the rat.

Artificial casein micelles were prepared at a casein concentration of 2·5% with 10–40 mM-Ca, 12–27 mM-phosphate and 10 mM-citrate and cross-linking of casein by colloidal Ca phosphate (CCP) was examined. No casein aggregates cross-linked by CCP were formed at 10 mM-Ca, 12 mM-phosphate and 10 mM-citrate, which are the approximate concentrations found in the soluble phase of bovine milk. Although the amounts of casein aggregates cross-linked by CCP and of colloidal inorganic phosphate increased with increasing Ca and phosphate concentrations, the effect was not uniform. The incorporation rates of individual casein constituents into casein aggregates cross-linked by CCP were in the order > > β-casein. This was the reverse of the order of dissociation rates during dialysis of casein aggregates cross-linked by CCP reported previously.

Milk which had been heated to 85°C for 40 min was examined for changes to milk salts taking place upon cooling to 4, 20, 40 and 60°C. The time-concentration curves showed a two-step reaction for Ca, Pi and pH. The effect of the heat treatment in milk heated to 85°C for 40 min was to produce greater changes in the second step of the reaction. Because of the variability of the results for other species (Mg and citrate), the reversibility was calculated only for Ca and P. It was 90–95% and 93–99% respectively depending on the cooling temperature. The reversibility of the heat-induced changes and especially of the possible dissolution equilibrium between Ca phosphate and citrate precipitates on cooling are discussed.

The term ‘hydraulic milking’ describes a new milking concept in which liner movement is restricted and the liner is flooded with milk beneath the teat. This condition, achieved with a multi-valve claw without air admission to the cluster, reduced milking time by 26% and increased milk flow rate by 20%. Four experiments describe the discovery of hydraulic milking and investigate its potential using equal or different levels of vacuum in the milkline and pulseline. Benefits from hydraulic milking include decreased lipolysis (≤36%) and milk foam (75%), improved teat condition and a high degree of protection against machine-induced infections. Evidence of increased milk yield is inconclusive. Cluster removal is impeded by hydraulic milking and the multi-valve cluster requires modification to facilitate the process. Pulsation characteristics and vacuum levels developed for conventional milking appear adequate for hydraulic milking. Unorthodox vacuum conditions may be needed, however, to exploit fully this novel milking concept.

The activity of milk plasmin, plasminogen and N-acetyl-β-glucosaminidase (NAGase) and the trypsin-inhibitor capacity (TIC) were monitored in 40 quarters during the first month of lactation. TIC and NAGase activity decreased rapidly and plasmili activity more slowly during the first week. Conversely milk plasminogen increased as time elapsed from parturition. When the quality of whey was analysed as a growth medium for mastitis pathogens, a slight inhibition in the growth of Escherichia coli, Staphylococcus aureus and Streptococcus agalactiae was seen at the day of parturition. There was a distinct stimulatory effect on the growth of Str. agalactiae during the second week of lactation. No relationship was found between in vitro bacterial growth and respective plasmin or plasminogen activity in milk.

Maternal milk should not only be considered as a nutrient, but also as a protecting agent against aggressions from the neonate's new environment. Breastfeeding facilitates transmission of a passive immunity by multifunctional factors which have a direct effect on the neonate's resistance to bacterial and viral infections. Among these factors are the main milk proteins, the caseins: during enzymic digestion of human and bovine caseins, immunomodulating peptides are released. Corresponding synthetic peptides stimulated in vitro phagocytic activity of murine and of human macrophages and exerted in vivo a protective effect against Klebsiella pneumoniae infection of mice. These data suggest that casein peptides may exert a stimulating function on the immune system of the newborn.

Results from a field trial involving 23 Norwegian dairy herds support the theory that deflector shields inserted into the teatcup liner can reduce the risk of intramammary infection. However, the effectiveness of this measure is questionable in cows already infected and in problem herds.

A convenient and sensitive method for determining γ-casein content of skim milk is described. It is based on quantitative separation in which the soluble fraction obtained from skim milk (1·5 ml) in a solvent system consisting of 50% (v/v) ethanol, 0·4 M-Na thiocyanate and 0·15 M-CaCl2 was applied to a small column of DEAE-cellulose and eluted with 0·02 M-Tris-HCl buffer (pH 8·0) containing 0·03 M-NaCl and 6 M-urea. Protein in this fraction (5 ml) was measured from its absorbance at 280 nm. The method is applicable also to heated skim milk. Its availability for investigating proteolysis in milk is expected from the fact that incubation of skim milk with porcine plasmin at 37 °C for 3 h increased γ-casein content, an increase that was lowered by addition of soyabean trypsin inhibitor. Incubation of raw skim milk with 0·02% (w/v) NaN3 resulted in increase in γ-casein with time and with increasing temperature. This was accompanied by a time lag, the length of which increased with decreasing temperature. The degree of proteolysis in skim milk, expressed as the amount of increase in γ-casein after 20 h of incubation at 37 °C, showed two pH optima, one at pH 8·0 and the other at pH 7·2–7·5. These results suggested that the present method.of γ-casein determination can be used for more precise studies of the milk plasmin system in which important factors, such as plasminogen activator and inhibitors, are involved.

A model heat-exchange apparatus was used to investigate the factors affecting deposit formation from milk on a stainless steel surface at 100 °C. The structure and composition of the deposits were determined by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and chemical analysis after solution in alkali. The effects of changing the pH, preheating and skimming of the milk were similar to those observed in a small-scale continuous ultra high temperature plant. The time course of deposit formation showed that a lag phase did not occur, but the deposit which formed after more than 45 min was more porous than that formed after shorter times. Most (50–90%) of the fresh deposit was readily removed by sonication, leaving a sublayer richer in minerais than the original. The results provide evidence for the two-layer model for deposit formation proposed by Tissier & Lalande (1986).

Measurements of the release of Ca, Mg and inorganic phosphate(Pi) from the casein micelles of bovine milk have been made, as functions of the pH, in the range 4·9–6·7, and at temperatures of 4, 20 and 30 °C. The results are in general agreement with earlier published studies in giving a value of 1·75–1·84 for the micellar Ca:Pi ratio. Mg appeared to behave similarly to Ca, although the amounts of micellar material were much smaller. The results on the acid-solvation of calcium phosphate are considered in relation to published quantitative studies of the pH-induced dissociation of the different types of caseins from the micelle, and of the micellar dissociation caused when micellar calcium phosphate is dissolved at neutral pH. It is evident from this that at present it is not possible to derive a universal relation between the dissociation of minerals and of caseins from the micelles at different temperatures and under different conditions.

Opioid antagonists were sought in the fragments of κ-casein which were obtained by chemical synthesis and enzymic digestion. A synthetic bovine κ-casein peptide (35–41), Tyr-Pro-Ser-Tyr-Gly-Leu-Asn (casoxin A) showed opioid antagonist activity at 200 μM in the guinea pig ileum assay. A synthetic peptide Tyr-Pro-Tyr-Tyr (casoxin B) which is found in bovine and human κ-casein, also showed opioid antagonist activity at 100 μM. Another opioid antagonist peptide (casoxin C) was isolated from tryptic digests of bovine κ-casein by reverse-phase HPLC. The structure of the peptide was Tyr-Ile-Pro-Ile-Gln-Tyr-Val-Leu-Ser-Arg, which corresponded to κ-casein (25–34). Casoxin C was active at 5 μM in the guinea pig ileum assay. Thus, bovine κ-casein contains three potential opioid antagonist sequences.

A method using low resolution NMR spectroscopy is described for investigating whey protein thermal denaturation. The method is based on measuring at 20 °C changes in water proton transverse (T2) relaxation parameter following the denaturing treatment. This parameter is shown to be sensitive to protein denaturation and not to other phenomena such as gelation. Examples are given for the qualitative study of protein thermal denaturation in whey protein concentratc, β-lactoglobulin, α-lactalbumin, bovine serum albumin and immunoglobulins aqueous solutions and for the quantitative determination of thermal denaturation in whey protein concentrate solutions.

The formation of galactose, lactulose and epilactose during heating of milk was studied. The concentration of these carbohydrates increased with pH and the severity of heat treatment. The presence of proteins reduced the amount of lactulose formed whereas it increased that of epilactose and galactose. The galactose in heated milks was derived from lactose degradation. Under the conditions studied, degradation of the Amadori rearrangement product did not result in the formation of galactose. The interaction between proteins and galactose increased the thermal stability of the monosaccharide.

The milk clotting activity of chymosin in combined rennets was determined as the difference between the total and residual activity after specific inhibition of chymosin with the purified immunoglobulin fraction of chymosin antiserum. The milk clotting activity of bovine pepsin A was assessed in a similar manner. In bovine rennets milk clotting activity of chymosin was measured directly after immunochemical inactivation of bovine pepsin A. Any method may be used for the determination of milk clotting activity. High correlation was found between the proposed method, which is simple and rapid to perform, and Chromatographic assay.

Subunit components of ovine, caprine and equine casein micelles were separated by gel chromatography using a TSK-G4000SW high-performance column and the subunit components of the fractions analysed and compared with bovine casein. Molecular weights of the casein complexes were determined by the combined use of high-performance gel chromatography and low-angle laser light scattering. The caprine and ovine caseins were separated into three peaks (F2, F3 and F4) which were similar to those of bovine casein with respect to composition and molecular weight (500, 100 and 23 K). These F2, F3 and F4 peaks consisted mainly of αs- and κ-casein, αs- and β-casein and β-casein respectively. The equine casein was separated into two components corresponding to F3 and F4 of bovine casein. These F3 and F4 peaks consisted mainly of αs- and β-casein and β-casein respectively. The molecular weight of equine F3 (850 K) was different from that of the other three species. The contents of F2 and F4 in these caseins were dependent on the contents of κ-casein and β-casein respectively.

Astringent off-flavours were observed in commercial samples of UHT-milk heated by direct-steam injection. The off-flavour components from these milk samples were isolated by the chloroform-methanol extraction procedure of Harwalkar & Elliott. The astringent-tasting components in these extracts were partly purified by isoelectric precipitation at pH 7·0 and by size exclusion chromatography on Sephadex G-50 columns. The extracted off-flavour components and the partly purified samples were further analysed by FPLC using a Mono-Q anion exchange column and PAGE. The FPLC and PAGE patterns of the extracts from astringent and non-astringent samples of milk showed marked differences and the astringent flavour correlated with one of the fractions resolved by FPLC. This fraction was found to be heterogeneous by PAGE. The astringent off-flavours in UHT-sterilized milk were γ-casein-like breakdown products of casein.