Effect of plasmin and somatic cell enzymes on proteolysis in aseptic starter and rennet free model cheeses

Aseptic, rennet and starter free (ASRF) cheeses were prepared from milk of low and high SCC. The coagulant used, pepsin, was inactivated by a high pH cooking stage during manufacture, thimerosal was added to prevent bacterial growth and gluconic acid-δ-lactone was used to acidify the curds to a range of pH values. Curds prepared from high SCC milk had higher overall levels of proteolysis than low SCC curds over a range of pHs from 6.4 to 4.8 and had accelerated breakdown of β- and α_s1-caseins. The pattern of proteolysis was not consistent with a system dominated by cathepsin D and plasmin alone. The mastitic curds also had an accumulation of hydrophobic bitter material over 10 weeks ripening at 11 °C, which, together with the accelerated breakdown of α_s1-casein may indicate a role for somatic cell proteinases in influencing cheese ripening, and flavour and texture of cheese in particular. This may be linked to similar problems associated with cheese prepared from late lactation milks of elevated SCC.