Studies on the ripening of stilton cheese: Proteolysis

The pH of fresh Stilton curd, which 2 h after coagulation was ∼6·7, decreased to ∼4·8 in 4 day-old cheeses but increased gradually thereafter to ∼6 at the end of ripening (70 days). At the end of the normal ripening period, ∼60% of the total N (TN) was water soluble (WSN) and ∼58% of the WSN was soluble in 70% ethanol, while 40% was diffusible on dialysis against water; free amino acid N (phosphotungstic acid-soluble, PTA-N) reached ∼4·8% of TN. The levels of TNBS-reactive amino groups were highly correlated with the values of WSN and 12% TCA-N but showed a greater change in the PTA extract than the value of PTA-N. Dialysable WSN was resolved into five peptide fractions by chromatography on Sephadex G-10; these fractions were very heterogeneous and contained up to eight ninhydrin-positive peptides when examined by TLC. Chromatography on DEAE cellulose revealed a high level of heterogeneity in the 70% ethanol-soluble and -insoluble fractions and this was confirmed by the wide range of N-terminal groups. The free amino acids (FAA, as measured by ion exchange chromatography) represented 6·8% and 9·9% of TN in the 70 day-old experimental cheeses and in commercial samples, respectively. Valine, leucine, lysine and glutamic acid accounted for ∼50% of total FAA throughout the ripening period.