Valorization of onion extracts as anti-browning agents

The enzymatic browning, whose main responsible is polyphenol oxidase (PPO, EC 1.14.18.1), is involved in the phenolic oxidation and colour alteration of minimally-processed fruits and vegetables. Currently, the research of new strategies to inactivate PPO is moving towards replacing synthetic additives such as organic acids and sulphites with natural inhibitors. The present study is focused on investigating the anti-browning performance of juices and distillates obtained from three onion varieties (white, yellow, and red) and Borettana onion wastes (inner layers). Their inhibitory activity on a commercial mushroom tyrosinase and some plant PPOs has been evaluated by spectrophotometric and electrophoretic analysis. The in vivo trials has been also carried out by monitoring over time at room temperature the colour change on potato slices under accelerated browning conditions. The effectiveness of onion samples in limiting enzymatic browning was affected by not only the enzyme source but also inhibitor type. Although distillates had higher anti-PPO capacity as confirmed by in vitro assays, juices showed better in vivo effectiveness. Hence, onions and their wastes can be valorised as a natural source of anti-browning agents to control PPO activity thus preserving sensory, antioxidant and nutritional properties of agro-food products.