BIOPRESERVATION OF 'BIRGAH' EGGPLANT FROM POLYPHENOL OXIDASE ACTIVITY ASSAYED IN VITRO WITH ONION (ALLIUM CEPA L.) BY-PRODUCTS

Eggplant is a solanacea belonging to Solanum melongena L. species, widely cultivated in Sicily in open field, greenhouse or cold tunnel. Its fruits are largely consumed in many recipes as fresh and whole product or, more recently, as minimally processed vegetable (MPV), a rising commercial area in the fresh-cut industry. The most important problem for diced eggplant consists in the oxidation due to polyphenol oxidase (PPO, EC 1.14.18.1), an enzyme producing pigments responsible for the undesirable dark color which spoiled the product. Several methods such as the addition of antioxidants and the exclusion of oxygen as well as thermal processing have been used to inhibit the enzymatic browning. The aim of this work was to study fresh or heat-treated (100°C for 15 min) onion by-products (paste, juice and bagasse) in order to evaluate in vitro the PPO enzyme inhibition capacity in minimally processed eggplant cv. 'Birgah' (fruit large, round, light purple). The eggplants were grown in cold greenhouse in the Eastern coast of Sicily (Pachino, Italy) whilst the anti-browning extracts were obtained by processing onion wastes (residues and surpluses of bulbs) from Giarratana area (Ragusa, Italy), where this vegetable is traditionally cultivated. The heated onion juice was the most suitable anti-browning extract among all by-products tested, followed by pasteurized paste and fresh juice (54.2, 41.2 and 37.3% reduction in PPO activity, respectively), with a real perspective future scale-up, although the technological and stabilization processes applied may influence significantly the PPO inhibition capacity of onion extracts. Data also suggested that the thermal treatment enhances the effects of the extracts probably due to a synergic action between some endogenous active compounds present in onion and Maillard reaction products obtained during heating.