Heat stability and renneting characteristics of milk systems containing bovine casein micelles and porcine or bovine β-lactoglobulin

Abstract

This work studied the effect of the absence of a free thiol (-SH) group in β-lactoglobulin (β-lg) on the heat stability of a synthetic bovine milk system and the rennet coagulation characteristics of a similar system following heating. The heat coagulation time (HCT) versus pH profiles of serum protein-free bovine casein micelles (SPFCM) dispersed in synthetic milk salts buffer containing added porcine β-lg did not exhibit a maximum-minimum typical of bovine milk, whereas a similar system containing added bovine β-lg had a typical maximum-minimum; this was attributed to the lack of formation of a disulphide induced complex between SPFCM and porcine β-lg as porcine β-lg does not have a free thiol (-SH). The non-sedimentable nitrogen (NSN) and N -acetylneuraminic acid (NANA, an index of κ-casein) levels were determined in the ultracentrifugal (UC) supernatants recovered from SPFCM systems, with or without added porcine or bovine β-lg, which were heated at pH 6.7 or 7.1. The results indicated that porcine β-lg induced more protein to dissociate from the bovine casein micelles at both pH values than bovine β-lg; however, the κ-casein content of the dissociated protein was lower in the heated porcine β-lg systems than in the heated bovine β-lg systems. The HCT-pH profiles of the dispersed UC pellets recovered from the heated systems indicated that the dissociation of micellar κ-casein may not be the only critical factor leading to the rapid coagulation of milk at the pH minimum on heating. A large increase in the rennet coagulation time (RCT) occurred following heating of milk or SPFCM systems containing bovine β-lg and a relatively small increase in RCT occurred following heating of SPFCM alone or SPFCM systems containing porcine β-lg. These results support the suggestion that the increased RCT of bovine milk systems following heating is due to the formation of the disulphide induced complex between bovine β-lg and κ-casein.