LAUSR

BACKGROUND During the thermal processing of fruit, it has been observed for phenolic compounds to either degrade, polymerize or transfer into macromolecules. In this study, the bound and free phenolic compound composition, content and phenolic related enzyme activity of lychee pulp were investigated to determine whether the free phenolic had converted to bound phenolic during heat pump drying (HPD). RESULTS It was found that after heat pump drying, when compared with the fresh lychee pulp (control), the content of bound phenolic of dried lychee pulp had increased by 62.69%, whereas the content of free phenolic of dried lychee pulp decreased by 22.26%. It was also found that the antioxidant activity of bound phenolic had also increased after drying. With the use of HPLC-MS/MS, it was identified that (+)-gallocatechin, protocatechuic aldehyde, isorhamnetin-3-O-rutoside, 3, 4-dihydroxybenzeneacetic acid and 4-hydroxybenzoic acid were newly generated during HPD, when compared with the control sample. After drying, the contents of gallic acid, catechin, 4-hydroxybenzoic acid, vanillin, syringic acid and quercetin in bound phenolic had also increased, while polyphenol oxidase and peroxidase still showed enzyme activity, which could be related to the conversion of free phenolic to bound phenolic. CONCLUSION Overall, during the thermal processing of lychee pulp, the free phenolic was found to be converted into bound phenolic, new substances were generated, and antioxidant activity was increased. Due to these findings, it was concluded that heat pump drying improved the bound phenolic content of lychee pulp, thus, providing theoretical support for lychee processing industry. This article is protected by copyright. All rights reserved.