Effects of postharvest sulphur fumigation, steam and low pH treatments on polyphenol oxidase activity in litchi (Litchi chinensis Sonn.) fruit

Abstract

The effects of postharvest sulphur fumigation (0.6 kg elemental sulphur. 1000 kg⁻¹ of fruit), steam (95°C) for two seconds, or steam (95°C) for two seconds followed by subsequent immersion in a zero pH solution for four minutes, on polyphenol oxidase activity in litchi (Litchi chinensis Sonn.) fruit were determined at harvest, and at successive daily intervals until five days after harvest, during which period the fruit were stored at 28°C. On the day of harvest, polyphenol oxidase activity Was highest in sulphur-fumigated fruit (605 units. g⁻¹ fresh mass) but four days after harvest was highest in control fruit (ca 730 units. g⁻¹ fresh mass). In contrast, polyphenol oxidase activity was significantly lower in fruit treated with steam (95°C) for two seconds (215 units. g⁻¹ fresh mass), but was lowest in fruit treated with steam (95°C) for two seconds followed subsequent immersion in a zero pH solution for four minutes (ca 50 units. g⁻¹ fresh mass). Consequently, steam (95°C) and low pH both decreased polyphenol oxidase activity and the effect was additive.