Journal of Dairy Research 

About: Journal of Dairy Research is an academic journal published by Cambridge University Press. The journal publishes majorly in the area(s): Casein & Lactation. It has an ISSN identifier of 0022-0299. Over the lifetime, 4724 publications have been published receiving 131472 citations.

Proteinases in normal bovine milk and their action on caseins.

Abstract: Native proteolytic enzymes in good quality normal bovine milk readily hydrolysed the caseins during incubation or storage, producing the gamma-caseins, proteose-peptone components 5 (PP5) and 8-fast (PP8F) and a considerable number of other unidentified fragments, many of which were also subsequently found in the proteose-peptone fraction. The rate of casein hydrolysis was greater in pasteurized than in raw milk, with beta-casein being slightly more susceptible to attack than alpha S1-casein. Measurements of gamma-casein and proteose-peptone formation have been made and it was found that PP5 was an intermediate product that was subject to further proteolysis while PP8F was a stable end-product. With the exception of component 3 (PP3), virtually all constituents of the proteose-peptone fraction increased during storage and appeared to be products of the action of proteolytic enzymes. Further evidence was obtained from the effects of various inhibitors that the principal proteinase of normal milk is plasmin, but slight differences were apparent between the protein breakdown patterns induced by storage and by added plasmin, which was consistent with the presence of more than one proteinase. Incubations in the presence of soya bean trypsin inhibitor to prevent plasmin action clearly revealed that another enzyme(s) was also involved.

176. The Determination of the nitrogen distribution in milk

Abstract: Details are given of a semi-micro Kjeldahl method for the determination of the total, casein, albumiij, globulin, proteose-peptone, and non-protein nitrogen of milk. The method is accurate, rapid, and particularly suitable for the determination of the smaller nitrogen fractions.The various nitrogen fractions are separated by the improved procedures described in the preceding paper.

Angiotensin I-converting enzyme inhibitory properties of whey protein digests: concentration and characterization of active peptides.

Abstract: The aim of this study was to identify whey-derived peptides with angiotensin I-converting enzyme (ACE) inhibitory activity. The bovine whey proteins α-lactalbumin and β-lactoglobulin were hydrolysed with pepsin, trypsin, chymotrypsin, pancreatin, elastase or carboxypeptidase alone and in combination. The total hydrolysates were fractionated in a two step ultrafiltration process, first with a 30 kDa membrane and then with a 1 kDa membrane. Inhibition of ACE was analysed spectrophotometrically. The peptides were isolated by chromatography and identified by mass and sequencing analysis. The most potent inhibitory peptides were synthesized by the 9-fluorenylmethoxycarbonyl solid phase method. Inhibition of ACE was observed after hydrolysis with trypsin alone, and with an enzyme combination containing pepsin, trypsin and chymotrypsin. Whey protein digests gave a 50 % inhibition (IC50) of ACE activity at concentration ranges within 345–1733 μg/ml. The IC50 values for the 1–30 kDa fractions ranged from 485 to 1134 μg/ml and for the <1 kDa fraction from 109 to 837 mg/ml. Several ACE-inhibitory peptides were isolated from the hydrolysates by reversed-phase chromatography, and the potencies of the purified peptide fractions had IC50 values of 77–1062 μM. The ACE-inhibitory peptides identified were α-lactalbumin fractions (50–52), (99–108) and (104–108) and β-lactoglobulin fractions (22–25), (32–40), (81–83), (94–100), (106–111) and (142–146).

Association of denatured whey proteins with casein micelles in heated reconstituted skim milk and its effect on casein micelle size

Abstract: When skim milk at pH 6.55 was heated (75 to 100 degrees C for up to 60 min), the casein micelle size, as monitored by photon correlation spectroscopy, was found to increase during the initial stages of heating and tended to plateau on prolonged heating. At any particular temperature, the casein micelle size increased with longer holding times, and, at any particular holding time, the casein micelle size increased with increasing temperature. The maximum increase in casein micelle size was about 30-35 nm. The changes in casein micelle size were poorly correlated with the level of whey protein denaturation. However, the changes in casein micelle size were highly correlated with the levels of denatured whey proteins that were associated with the casein micelles. The rate of association of the denatured whey proteins with the casein micelles was considerably slower than the rate of denaturation of the whey proteins. Removal of the whey proteins from the skim milk resulted in only small changes in casein micelle size during heating. Re-addition of beta-lactoglobulin to the whey-protein-depleted milk caused the casein micelle size to increase markedly on heat treatment. The changes in casein micelle size induced by the heat treatment of skim milk may be a consequence of the whey proteins associating with the casein micelles. However, these associated whey proteins would need to occlude a large amount of serum to account for the particle size changes. Separate experiments showed that the viscosity changes of heated milk and the estimated volume fraction changes were consistent with the particle size changes observed. Further studies are needed to determine whether the changes in size are due to the specific association of whey proteins with the micelles or whether a low level of aggregation of the casein micelles accompanies this association behaviour. Preliminary studies indicated lower levels of denatured whey proteins associated with the casein micelles and smaller changes in casein micelle size occurred as the pH of the milk was increased from pH 6.5 to pH 6.7.