Hot water brushing:: an alternative method to SO2 fumigation for color retention of litchi fruits

Distribution of high-quality litchi (Lychee chinensis Sonn.) fruits to global markets depends exclusively on postharvest treatments to suppress peel browning. The current standard treatment of litchi fruits in Israel includes fumigation with sulfur dioxide (SO2) followed by dipping the fruits in hydrochloric acid containing the fungicide prochloraz. As part of the effort to reduce the use of potentially hazardous chemicals in agriculture, we developed a new procedure that may enable SO2 to be avoided. Instead of fumigation, litchi fruits are sprayed with hot water while being brushed in a revolving drum, after which the fruits are subjected to hydrochloric acid treatment. Fruits that are processed in this way maintain a uniform red color for at least 35 days, without apparent deterioration in external or internal quality, or taste. Physiological studies demonstrate that polyphenol oxidase (PPO) activity is reduced by the hot water brushing (HWB) procedure as compared with controls but not to the same extent as inhibition by SO2 treatments. In addition to its effect on PPO activity, HWB may lead to reduced pH of the pericarp, or more uniform distribution of the acid in it. This result suggests that HWB may act by bruising the external layer of the pericarp allowing the peel to be uniformly exposed to the acid which may inhibit PPO activity and maintain the anthocyanins in their red-pigmented form. (C) 2000 Elsevier Science B.V. All rights reserved.