Anthocyanin decolorisation and its role in lychee pericarp browning

Mature red lychee fruit were stored at 3 different temperature and relative humidity regimes. Total anthocyanin concentration, pigment distribution, pH of the pericarp homogenate, Hunter a values (redness index), and visual colour were measured as a function of pericarp weight loss. Pericarp colour rapidly deteriorated during both ambient and high temperature storage, resulting in a uniform browning of the pericarp surface. The degree of tissue browning was proportional to the rate of pericarp desiccation. Although anthocyanin degradation occurred concurrently with tissue browning, visual colour and Hunter a values were not consistent with total anthocyanin concentration. Instead, a more significant correlation was seen between Hunter a values and the pH of the pericarp homogenate. Pericarp colour could be altered by external pH. Acidification of whole fruit increased pericarp redness, whereas alkaline treatment caused discoloration. Both colour responses occurred independently of anthocyanin synthesis and degradation and were completely reversible. These results question the current theory that browning is due to anthocyanin degradation. No evidence of browning was observed in the anthocyanin-containing mesocarp, and acidification of already brown tissue significantly increased pericarp redness independently of anthocyanin synthesis. We believe that anthocyanin pigments were progressively decolorised during ambient storage, possibly due to changes in pericarp pH. Once colourless, independent tissue browning became visual and was enhanced.