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University of Hawaii at Hilo
College of Agriculture, Forestry & Natural Resource Management
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Preliminary Research Report on Polyphenol Oxidase, PPO, Activity in Lychee Pericarp, Litchi chinensis Sonn., ssp. chinensis.

UHH Home > Academics > College of Agriculture, Forestry and Natural Resource Management > Research

By Norbert Furumo, Ph.D.

Postharvest browning of litchi has been attributed to several biochemical processes involving reactions catalyzed by enzymes as well non-enzymatic reactions. Based upon the current scientific literature, it appears that the key enzymes participating in browning of litchi are polyphenol oxidase (PPO; closely related to tyrosinase from mushroom) and anthocyanin-3-glucosidase (ACase; also known as anthocyanase, a type of b -glucosidase), an enzyme recently discovered in litchi, and to a lesser degree, peroxidase. When coupled together, PPO and ACase activity may lead to the formation of brown high molecular weight pigments in the pericarp of litchi. One possible sequence of events would have ACase catalyzing the release of glucose from typically red anthocyanin molecules resulting in the formation of anthocyanidin molecules. Next, PPO acts upon the anthocyanidin converting it from an ortho -diphenol to an ortho -diquinone which is then free to react with other ortho -diphenols commonly found in fruit. This non-enzymatic oxidative coupling reaction between ortho -diquinones and ortho -diphenols leads to the formation of large melanin-like polymers giving the fruit its brown color shortly after harvest.

Strategies to delay or prevent browning include treatment of fruit with agents that inhibit these biochemical processes either directly or indirectly. One indirect method involves a treatment first described by C. Kaiser, et al. in 1995. In their study, they found that if litchi fruit are exposed to steam for two seconds followed by a dip in approximately 1.0 M HCl (pH 0) followed by spraying with Vapoguard (or Wilt-Pruf), an antitranspirant, the fruit will maintain their red color for about 28 days when stored at 1 °C. We have tried this procedure with fresh rambutan (since litchi is not in season). Thus far (after 15 days) the fruit are holding up well; there has been no significant change in color of the treated fruit's skin or lentils (compared to a control sample). At this time, a similar trial is underway with litchi that has been frozen since June, 2005. These fruit were thawed at room temperature for two hours before treatment with steam, HCl and Wilt-Pruf. After three days the litchi have become quite dark in color indicating the treatment did not prevent browning (at least with previously frozen fruit).

A direct approach to prevent browning will involve treatment of litchi with compounds that have been shown in the literature to be inhibitors of PPO or ACase. Dipping the fruit into a solution containing one or more inhibitors may delay the onset of browning.

We have also performed basic biochemical analysis of litchi pericarp from previously frozen fruit. In these studies, we homogenized the pericarp in either a pH 4.0 or 6.8 buffer and then determined the protein content and activity of PPO and ACase. At pH 6.8, we found a low protein concentration of 0.035 mg protein/mL of extract while the pH 4.0 buffer yielded an extract with a protein concentration of 0.39 mg/mL, an order of magnitude greater. At pH 6.8 we measured about 10 mg of PPO per g of pericarp (using mushroom tyrosinase as the standard) while at pH 4.0 we measured 0.54 mg PPO per g of pericarp, or a 95% decrease. No ACase activity was observed in the pH 4.0 extract, the pH optimum for this enzyme. An ACase assay has not been performed on the pH 6.8 extract.