Postharvest treatments with MnCl(2) and ZnCl(2) reduce enzymatic browning and enhance antioxidant accumulation in soya bean sprout

Soya bean sprout is a nutrient-abundant vegetable. However, enzymatic browning of soya bean sprouts during storage remains a challenge. In this study, the effects of treatment with MnCl(2) or ZnCl(2) on the browning index, antioxidant nutrient accumulation, total antioxidant capacity and enzyme activities of phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT) were investigated in soya bean sprouts after storage at 4 °C and 90% relative humidity for 0, 7, 14 and 21 days. The results showed that postharvest treatment with 1, 2 and 10 mM MnCl(2) or ZnCl(2) profoundly retarded enzymatic browning in soya bean sprouts to different extents. Compared with the control, the 10 mM MnCl(2) and ZnCl(2) treatments drastically enhanced ascorbic acid, total thiol and phenolic content, and enhanced FRAP (ferric-reducing ability of plasma) antioxidant capacity in stored soya bean sprouts. Moreover, the MnCl(2) and ZnCl(2) treatments enhanced SOD, CAT and PAL but decreased PPO and POD activities compared with the control. In addition, the Mn and Zn content in soya bean sprouts significantly increased, by approximately two- to threefold, compared with the control. This study provides a new method for improving the nutrient quality of soya bean sprouts based on postharvest Mn or Zn supplementation.