Advances in understanding of enzymatic browning in harvested litchi fruit
Introduction
Litchi (Litchi chinensis Sonn.) is a subtropical to tropical fruit of high commercial value in international trade. The fruit typically has a bright red peel colour and is sweet, acidic, juicy and soft but with crisp pulp (Nakasone & Paull, 1998). Harvested litchi fruit are highly perishable. They can rapidly lose their bright red skin colour and turn brown within 1–2 days at ambient temperatures (Huang & Scott, 1985; Jiang & Fu, 1998a; Zhang & Quantick, 1997). Post-harvest browning of litchi fruit has been attributed mainly to degradation of red pigments in association with oxidation of phenolics by polyphenol oxidase (PPO) and/or peroxidase (POD) enzymes (Huang, Hart, Lee, & Wicker, 1990; Zauberman et al., 1991; Zhang & Quantick, 1997). Li and Yan (1963) first discerned the relationship between PPO activity and litchi peel browning. Significant progress in purification and characterisation of PPO and its substrates in litchi pericarp tissue has since been made. Nonetheless, enzymatic browning is still the major practical limitation to litchi fruit storage (Jiang, Yao, Lichter, & Li, 2003). This paper reviews enzymatic browning of litchi fruit after harvest, with an emphasis on recent advances.
Section snippets
Polyphenol oxidase
Litchi pericarp tissue browning is mainly due to the oxidation of phenolics and degradation of red pigments by polyphenol oxidase. This oxidase is also referred to as catechol oxidase, tyrosinase, catecholase or o-diphenol oxygen oxidoreductase. PPO has been isolated and purified from litchi fruit peel. Its pH and temperature optima are 6.5 and 70 °C, respectively (Jiang, Zauberman, & Fuchs, 1997b; Jiang, Zauberman, Fuchs, & Fu, 1999). The enzyme can be inhibited by antioxidants, such as
Pigments and browning substrates
Compared with the literature on PPO and POD enzymes, there are very few publications relating to the role of anthocyanins in litchi pericarp browning. Prasad and Jha (1978) and Rivera-Lopez, Ordorica-Falomir, and Wesche-Ebeling (1999) identified anthocyanins as the red pigments present in litchi pericarp. Lee and Wicker (1991) subsequently reported that litchi pericarp contains seven types of anthocyanins (cyanidin-3-rutinoside, cyanidin-3-glucoside, cyanidin-3-galactoside,
Peroxidative activity and membrane lipids
Oxidative enzymes and their substrates are in different subcellular compartments in red intact litchi fruit pericarp (Liu, Jiang, Chen, Zhang, & Li, 1991). Accordingly, compartmentation limits mixing that results in enzymatic browning (Liu et al., 1991). Peroxide content and malondialdehyde (i.e. a product from peroxidated membrane lipids) concentrations increase in aging litchi fruit. Conversely, superoxide dismutase activity, associated with the anti-oxidant capacity of litchi pericarp
Concluding remarks
Impetus for research on litchi fruit deterioration in China and elsewhere has come in conjunction with increased production and demand around the world. The major producer, China, seeks to identify domestic and international markets for this unique and popular fruit. Post-harvest browning of litchi fruit skin is the main limitation to market acceptance. The biochemistry of enzymatic browning has not yet been fully elaborated (Jiang et al., 2003; Peng, 1998). However, it is proposed that
Acknowledgements
This work was supported by the International Foundation for Science (Grant No. E2265/3F) and the National Science Foundation of China (Grant No. 39900102).
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