Dual effects of whey protein isolates on the inhibition of enzymatic browning and clarification of apple juice

The inhibition was studied of enzymatic browning occurring in apple juice by whey protein isolates as compared to ascorbic acid and l-cysteine. A further comparison of different filter-aid pretreatments, such as whey protein isolates (WPI), pectinase and whey protein isolates - pectinase pretreatments, alone and combined with ultrafiltration was also made. The results indicated that WPI demonstrated a concentration-dependent inhibitory activity on polyphenoloxidase (PPO). At lower concentrations (0.005-0.01 g/l), the suppression effect of WPI on PPO activity was higher than that of both ascorbic acid and l-cysteine. WPI exhibited intermediate inhibition on PPO activity at the concentrations of 0.01-0.1 g/l, as compared to ascorbic acid and l-cysteine. The comparison of different clarification treatments suggested that WPI acted more effectively on clarification than pectinase. In addition, combined WPI-pectinase pretreatment significantly increased the clarity of apple juice, indicating a synergistic effect between WPI and pectinase. The subsequent ultrafiltration after WPI and pectinase pretreatments alone or combined could further improve the clarity and colour and reduce the turbidity of clear apple juice with non-significant influence on its typical characteristics.