Physiological and biochemical changes in imported litchi fruit

It is important to determine the appropriate conditions for maintaining postharvest quality of litchi fruit after arrival from overseas markets. The present study, therefore, aimed to detail the effects of different storage conditions on physiological and biochemical changes in aril and pericarp tissues of imported litchi fruit. Results from Chapter 4 were the first to fully detail the alterations of individual sugars, organic acids and total phenols in aril and individual anthocyanins in pericarp tissue as well as physiological changes under different storage temperatures in three cropping seasons. Results clearly demonstrated that higher concentrations of sucrose, malic, tartaric and total phenolic contents were maintained at 5°C in aril and higher anthocyanins and total phenolic concentrations in pericarp during 13 days as compared with 8, 10, 13 and 20°C storage. Fruit treated at 5°C also had lower weight loss, brighter red pericarp colour and higher total soluble solids as compared against those fruit stored in other temperatures. A temperature of 5°C was therefore proposed for litchi distribution and storage, and acted as the basis of this study.

Relative humidity (RH) and vapour pressure deficit (VPD) in a storage environment are also important parameters affecting postharvest life. A new system using different glycerol solutions was employed to achieve defined RH levels in the present study. Although effects of RH on postharvest changes in litchi fruit have been described in previous works, the recent study is the first report detailing the effects of different and controlled VPD on litchi postharvest alterations. Low VPD was required to maintain quality of imported litchi during 9 days storage. In addition to reducing both weight loss and respiration rate, storage at 95-100 %RH and 5°C (VPD = 0.000-0.084 kPa) successfully remained high levels of sucrose and malic acid content in aril, and tartaric acid, cyanidin 3-rutinoside and mannose in pericarp tissue. It was therefore recommended that storage conditions for litchi should not only focus on maintenance of the cool chain, but should also consider controlling a VPD of ≤0.068 kPa to attain improved conservation of visual appearance. Appropriate use of packaging materials can prolong shelf-life of assorted fruit and vegetables including litchi fruit. Imported litchi fruit were wrapped with either micro-perforated polypropylene (PP), PropaFreshTM PFAM (PF), NatureFlexTM NVS, CellophaneTM WS or kept unwrapped prior to storage at 13°C for 9 days. Predictably, packed fruit retained better quality during storage as compared with unwrapped fruit. Each film tested resulted in an altered gas composition in the packages and thus affected postharvest quality. PF significantly decreased hydrolysis of sucrose in aril and retained higher cyanidin 3-rutinoside levels in pericarp. PF film also limited fruit weight loss and maintained sugar and organic acids concentration in both aril and pericarp. Exogenous application of certain chemicals after harvest has been commercially used to control browning in litchi for many years. However, off-flavours may results and could potentially impact on consumer safety. These possible effects have enhanced demand for non-adulterated fruit on the market. Postharvest changes in pericarp and aril of non-adulterated and commercially-treated fruit as influenced by packaging films under different temperatures were detailed. Although commercially-treated fruit had higher aril organic acids and pericarp anthocyanins, sucrose hydrolysis in aril tissue was accelerated. The use of PF film at 5°C maintained higher sucrose and malic contents in aril tissue of non-adulterated and commercial litchi during 11 days as compared with unwrapped or PP regimes or 13°C storage. Results suggested that PF could replace PP as a new active film for the litchi industry and be a substitute for chemical treatment to maintain quality of litchi fruit.