Effects of cysteine addition and heat treatment during non-fat dry milk processing on the plasmin enzyme system

The objective of this study was to measure the effects of cysteine addition and heat treatment on plasmin (PL) activity and plasminogen (PG) activation in reconstituted non-fat dry milk (NFDM). Cysteine was added to a final concentration of 0.16 mg·mL −1 to raw, defatted milk and pasteurized at 72 °C for 15 s (low heat, LH) or 100 °C for 30 s (medium heat, MH) prior to spray drying. Both MH fluid milk and NFDM had no PL activity or PG activation, regardless of cysteine addition. For both LH fluid milk and NFDM, there was a significant decrease in PL activity upon cysteine addition ( P < 0.0001 for both), as well as in PG activation ( P = 0.0003 for fluid milk, P < 0.0001 for NFDM), likely due to the polymerization of PG or binding of PL and PG with other milk proteins. Upon storage of reconstituted powders at 21 °C and 4 °C, PL activity increased with a complimentary decrease in PG-derived PL activity in LH samples, though these changes slowed in samples with added cysteine. Urea-PAGE showed proteolysis of β-casein consistent with PL activities in LH samples. No PL or PG-derived PL activities or proteolysis were observed in stored MH samples. These results suggest that the addition of cysteine prior to the pasteurization and manufacture of LH-NFDM significantly decreases the potential for proteolysis during storage of reconstituted powder caused by the PL enzyme system.