SummaryThe report that addition of KI03 (0·1 mm) to milk before ultra high temperature (UHT) treatment induces extensive proteolysis during subsequent storage at 37 °C was confirmed. None was produced by addition of H202 KMn04 or K2Cr207. The pH optimum for KI03-induced proteolysis was between 7·0 and 8·0 and the temperature optimum 37—45 °C. β-Casein was particularly susceptible and the proteolysis pattern was similar to that caused by indigenous alkaline milk proteinase (MPA, plasmin). Addition of plasmin to milk before UHT treatment (140 °C/10 s) caused slight proteolysis during subsequent storage but addition of 0·1 mm-KI03 and plasmin caused extensive proteolysis which was prevented by addition of soyabean trypsin inhibitor, indicating the probable involvement of plasmin in KI03-induced proteolysis in UHT-treated milk. Equally extensive proteolysis occurred in serum protein-free casein micelle systems (SPFCM), with or without KI03, during storage at 37 °C following UHT treatment, indicating a role for whey proteins in KI03-induced proteolysis. Addition of β-lactoglobulin (β-lg) to a SPFCM system inhibited proteolysis, but extensive proteolysis occurred in a SPFCM system containing both β-lg and KI03. MPA-free Na caseinate (prepared by heating at 140 °C for 7 min) underwent extensive proteolysis when treated with plasmin before UHT treatment; proteolysis was inhibited by addition of °-lg to this system and KI03 reversed the inhibitory effect of β-lg. Plasmin proteolysis of isolated αs1-casein was inhibited by denatured β-lg (90 °C/30 min) at a level of 4 mg/ml but not by native β-lg. When denatured in the presence of KI03, β-lg had a lower free SH content than the control and was less inhibitory for plasmin in proteolysis of isolated αsl-casein. The results show that denatured β-lg inhibits plasmin proteolysis of caseins in UHT milk and that inhibition is prevented by KI03. This inhibition may occur via thiol–disulphide interchange, which is prevented if the SH group of ²-lg is oxidized by KI03, thus permitting the stimulatory effect of KI03 on proteolysis in UHT-treated milk.
Approximately 95% of available nitrogen can be precipitated from milk on adjustment to pH 4.6 after heating at 90°C × 15 minutes at its natural pH and pH 7.5, while 89% can be precipitated after heating at pH 10.0 at 60°C × 3 minutes. Non‐recovered protein includes some serum albumin, β‐lactoglobulin, α‐lactalbumin and proteose peptones. Protein isolates precipitated from milk heated at pH >7.0 are more soluble in the pH range 6.0–7.0 than those precipitated from milk heated at its natural pH. Whey proteins complex onto the casein micelles after heating milk at its natural pH, while on heating at pH >7.0 whey proteins appear to interact with k‐casein in the serum phase. When N‐ethylmaleimide is present in milk during heating the percentage protein recovered on pH 4.6 precipitation is decreased, confirming that disulphide linkage is involved in complex formation. However, addition of β‐mercaptoethanol to recovered isolates did not result in dissociation of the casein/whey protein complex, suggesting that forces other than disulphide bonding are also involved in maintaining the complex.
Surface activities at the air‐water interface and the emulsifying and foaming properties of sodium caseinate, conventional casein‐whey protein co‐precipitate prepared from milk heated at 90°C × 15 min at pH 6.6 and milk protein isolates prepared from milks heated at 90°C × 15 min at pH 7.5 or at 60°C × 3 min at pH 10.0 were determined. The surface activities of the four proteins at the air‐water interface were similar, while the emulsifying capacity and emulsion stabilizing ability of casein was less than that of the milk protein isolates or the conventional co‐precipitate. Fat surface areas formed on emulsification with the four proteins were similar and increased with increasing power input. Total protein adsorbed at the interface and protein load (mg protein/m2 fat) for the emulsions stabilized by sodium caseinate and the milk protein isolate prepared from the milk heated at 90°C × 15 min at pH 7.5 were similar and lower than those for emulsions stabilized by the other two proteins. Foam overruns followed the order: sodium caseinate > milk protein isolate prepared from milk heatedat90°C × 15min, pH 7.5 > milk protein isolate prepared from milk heated at 60°C × 3 min, pH 10.0 > conventional co‐precipitate, while foam stabilities followed the reverse order.
The hydration related properties of sodium casemate, conventional casein‐whey protein co‐precipitate preparedfrom milk heatedat 90°C × 15 min at pH 6.6 and milk protein isolates prepared from milk heated at 90°C × 15 min at pH 7.5 or at 60°C × 3 min at pH 10.0 were determined. Conventional acid‐precipitated casein and the acid‐precipitated protein isolates preparedfrom milks heated at pH >7.0 had similar solubilities and reconstitution properties which were better than those of a conventional co‐precipitate. Water sorption isotherms for all the proteins were similar. Viscosities followed the order: conventional co‐precipitate > milk protein isolates > sodium caseinate.
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