Postharvest control of litchi (Litchi chinensis Sonn.) pericarp browning by cold storage at high relative humidity after enzyme-inhibiting treatments

Abstract

Enzyme inhibitors were studied as adjuvant treatments in the control of litchi pericarp browning by protection against desiccation during reefer transport. Various organic acids (acetic, malic, citric, and oxalic acid) and inorganic salts (NaCl, CaCl2) were investigated for in vitro inhibitory effects on the peroxidases (POD) and phenol oxidases (including laccase) extracted from litchi pericarp. Promising inhibitors were tested on ‘Hong Huey’ litchi fruit for their capability to prevent pericarp browning during cold storage (21 d, 5°C, 90% relative humidity) with and without foil wrapping. An enzyme assay had been optimized for measuring the in vitro activities of phenol oxidases toward (−)-epicatechin, being the natural phenolic key substrate in litchi. Phenol oxidase activities were chiefly pH-dependent and completely inactivated at pH≤3.5 by the organic acids used for buffering, whereby chelating agents performed best, especially oxalic acid. POD activity was stable over wider pH and ionic strength ranges, with inhibition being maximal (84%) in 0.25M oxalic acid buffer (pH 3.5). CaCl2 (0.25molL−1) decreased POD activity by 68%, while rising doses increased the initial lag phase up to 2.5 min. In contrast to these in vitro enzyme-inhibiting effects, postharvest fruit treatments with these phenol oxidase and peroxidase inhibitors did not improve color retention during cold fruit storage, but proved ineffective or even favored pericarp browning compared to the control fruit. Pericarp color retention was maximal (96–97%) throughout cold storage of fruit in gas-permeable but moisture-retaining foil bags for at least two weeks, whether the fruit had been dipped into cold water (control) or into citrate (25mmolL−1) solution. Consequently, pericarp color retention only required an intact pericarp at harvest and postharvest protection against desiccation. Preventing water loss through preservation of cell compartmentation thus proved to be crucial and sufficient for the control of enzymatic browning under reefer conditions.