Degradation of water-soluble polysaccharides in pulp of litchi during storage.

Water-soluble polysaccharides (WSP) were extracted from the pulp of litchi. Its main component was identified as arabinogalactan. The dominant monosaccharide constituents were arabinose and galactose. Galactose and mannose accumulated at the end of storage. ATP, ADP and AMP levels declined with increasing pulp breakdown index. WSP depolymerized which was characterized by a decrease in its content and molecular weight, while its structure remained stable during storage. Polygalacturonase and pectate lyase (PL) were active at the early storage time, and ß-galactosidase (GAL) and α-l-arabinofuranosidase followed thereafter. Except for some pectin methylesterase (LcPME), LcPL, LcGAL and LcPME gene expression was downregulated. It was deduced that depolymerization of polysaccharides was mainly caused by the rupture of the branched side chain and glacturonic acid backbone to smaller repeating units, and both cell wall-degrading enzymes and nonenzymatic factors, such as energy level, participated in the degradation of polysaccharides, and consequently pulp breakdown of litchi.

Metabolic variations in the pulp of four litchi cultivars during pulp breakdown.

Fruit of four litchi cultivars were stored at 25 ± 1 °C. The shelf life changed from long to short respectively was "Feizixiao (FXZ), "Jingganghongnuo (JGHN)", "Huaizhi (HZ)" and "Nuomici (NMC)". During pulp breakdown, marketable fruit and total soluble solids (TSS) decreased significantly, while respiratory rate increased significantly. After metabolomics analysis, a total of 179 metabolites were detected in litchi pulp, including 56 primary metabolites, 79 volatile compounds, 28 free amino acids and 16 hydrolyzed amino acids. Compared with other litchi cultivars, FZX pulp was rich in volatile alcohols and amino acids, NMC pulp was rich in soluble sugars and sesquiterpenes, and JGHN and HZ pulp were rich in sesquiterpenes. During the postharvest storage, most of volatiles and amino acids were induced in NMC pulp, while most of volatiles were reduced in JGHN and HZ pulp. The specific metabolites accumulated in a litchi pulp might be related to its shelf life and fruit quality. The increased metabolites during pulp breakdown might be also related to the resistance of litchi pulp.

Effects of hydrogen water treatment on antioxidant system of litchi fruit during the pericarp browning.

Litchi fruit were exposed to 0.7 PPM hydrogen water (HW) before storage at 25 ± 1 â„ƒ. HW treatment delayed the pericarp browning and maintained the total soluble solids (TSS) of litchi fruit. Then, a total of 25 antioxidant system-related characters were determined to evaluate the effects of HW on antioxidant system during pericarp browning. Compared with control pericarp, the pericarp of HW-treated litchi fruit exhibited higher levels of superoxide radical (O2-·) scavenging activity, glutathione (GSH), monodehydroascorbate reductase (MDHAR), polyphenol oxidase (PPO) and total flavonoids during whole storage, higher levels of hydrogen peroxide (H2O2), catalase (CAT), glutathione disulfide (GSSG), ascorbate oxidase (AAO) and total phenols only on day 1, and higher levels of ascorbate peroxidase (APX), total anthocyanin, glutathione reductase (GR) and glutathione peroxidases (GPX) at later stage of storage. Those HW-induced antioxidant system-related characters might directly or indirectly enhanced the antioxidant capacity and delayed the pericarp browning of litchi.

Changes in pericarp metabolite profiling of four litchi cultivars during browning.

The pericarp browning is an important physiological index during the postharvest storage, which seriously shortens the shelf-life of litchi fruit. In this study, the browning index of four litchi cultivars were compared, and the shelf-life, from longer to shorter, was 'Feizixiao (FXZ)', 'Jingganghongnuo (JGHN)', 'Huaizhi (HZ)' and 'Nuomici (NMC)', respectively. Then, comparative metabolomics were performed in the pericarp of four litchi cultivars during browning. Finding results showed that a total of 119 kinds of metabolites were detected in litchi pericarp, including 30 kinds of primary metabolites, 44 kinds of volatile compounds, 29 kinds of free amino acids and 16 kinds of hydrolytic amino acids. After ANOVA and OPLS-DA, 52 kinds of metabolites were important with predictive VIP > 1 and p < 0.05. In FZX pericarp, the contents of many amino acids increased significantly, which might be related to the yellow-green pericarp and play an important role in delaying browning. In the pericarp of JGHN, NMC and HZ, a great number of soluble sugars and some free amino acids were induced during browning, which was negatively correlated with the browning speed of three red pericarp cultivars. The browning induced a large number of sesquiterpenes in the pericarp of FZX, NMC and HZ, which was positively correlated with the browning index. In addition, the correlation analysis showed that the amino acids were negatively correlated with the volatile compounds, suggesting that pericarp browning could induce the conversion of metabolic products from amino acids to terpenes.