Endogenous enzymes and their role in browning reactions of fruit juices. Christian Molitor, Alberto Zavarise
Institute of Chemical, Environmental and Bioscience Engineering
Technical University of Vienna, Austria
Abstract
Fruits are the most important crops after grains and are therefore of enormous economic relevance, with fruit juice being one of the oldest, most effective and much liked forms of the conservation of fruits. Juices and nectars produced from red fruits and berries show a short shelf life, concentrates and purees have to be frozen during storage and transportation in order to prevent colour loss and undesired browning reactions. The fruit juice industry is therefore interested in increasing stability, since a longer shelf life would reduce production, transportation and storage costs significantly. Whereas 15 years ago low colour stability was explained by strictly chemical reactions, to date there are indications that the incomplete inactivation of endogenous enzymes during heating is responsible. The addition of Nethyl- maleimide as an enzyme inhibitor increased colour stability, underpinning the role of residual enzyme activity. It is unclear which enzymes are responsible for the colour change. Endogenous enzymes (polyphenol oxidases (PPO), peroxidases (POD), ascorbic acid oxidases, B-glucosidases) are the presumed key players for colour degradation as many of them seem to survive current preservation methods that are optimized rather for the inactivation of microorganisms than of enzymes. By using fully ripe or overripe frozen strawberries the stability of strawberry nectar was significantly increased (storage of 12 months). Reasons for the positive effect on colour stability by freezing fruits at -18 C over several months prior to processing are not clear to date. It has been shown that the activity of PPO and POD is significantly reduced by slow freezing. We show how latent and active forms of PPO could be involved in the browning reactions and explain how structural biology can help to solve the problem of juice browning.
Keywords: PPO, POD, fruit juice, red fruits, freezing, thermal treatment, colour stability
