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 glob al markets depends exclusively on postharvest treatments to suppress peel b rowning. The current standard treatment of litchi fruits in Israel includes fumigation with sulfur dioxide (SO2) followed by dipping the fruits in hyd rochloric acid containing the fungicide prochloraz. As part of the effort t o reduce the use of potentially hazardous chemicals in agriculture, we deve loped a new procedure that may enable SO2 to be avoided. Instead of fumigat ion, litchi fruits are sprayed with hot water while being brushed in a revo lving drum, after which the fruits are subjected to hydrochloric acid treat ment. Fruits that are processed in this way maintain a uniform red color fo r at least 35 days, without apparent deterioration in external or internal quality, or taste. Physiological studies demonstrate that polyphenol oxidas e (PPO) activity is reduced by the hot water brushing (HWB) procedure as co mpared with controls but not to the same extent as inhibition by SO2 treatm ents. 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 resu lt 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 PP O activity and maintain the anthocyanins in their red-pigmented form. (C) 2 000 Elsevier Science B.V. All rights reserved.