Pg. 50-54 The effect of temperature management on retention of litchi colour and quality

The majority of litchi fruit produced in South Africa is treated postharvest with sulphur to decrease fruit browning and control postharvest diseases. However, 97% of exported fruit are sold to the European Union, which has deemed sulphur to be unacceptable. Alternative forms of fruit quality maintenance are thus necessary. The primary cause of fruit postharvest browning is believed to be desiccation. Previous work using PVC bag packaging has, however, resulted in enhanced postharvest fungal decay, due to excessive condensation. In this study fruit was packaged in polypropylene bags, with specific anti-mist and gas exchange properties. Static and hydrocooling methods were tested and for postharvest decay control, a bacterial compound was tested. Fruits were stored at 1 °c and 5.5°C for 40 days, with evaluations at 10 day intervals. Packing in bags significantly reduced fruit water loss and enhanced retention of fruit colour for up to 20 days. There was no advantage of hydrocooling over standard static cooling, however, most water loss occurring in the first 10 days of storage. Total soluble solids of packaged fruit decreased with storage but remained high (15 to 20° Brix) while acidity was poor at below 0.40%. The TSS: acid ratio showed good eating quality as it remained between 31:1 and 60:1. Fruit that was hydrocooled and stored at a temperature of 5.5°C without a Biosave treatment showed a statistically superior colour which remained good for up to 40 days of storage. The higher storage temperature showed better colour retention, however, there was greater incidence of disease.

Summary:

• Sulphur treatment is commonly used postharvest in South Africa to reduce litchi fruit browning, but the EU prohibits sulphur, necessitating alternative quality maintenance methods.
• Fruit browning postharvest is mainly due to desiccation, leading to rapid loss of red color and irreversible phenol degradation by enzymes.
• Packaging litchi fruit in polypropylene bags with micro-perforations significantly reduces water loss (<2%) and helps maintain fruit color up to 20 days.
• Both static cooling and hydrocooling were tested; no significant advantage of hydrocooling for water loss control, though hydrocooling showed slight improvement.
• Most water loss occurs in the first 10 days postharvest, suggesting that rapid cooling soon after harvest could extend shelf life.
• Fruit stored at 5.5°C retained better color and anthocyanin content over 40 days than fruit stored at 1°C, likely due to less rind damage at higher temperature.
• However, storage at 5.5°C led to more postharvest disease incidence, requiring better biological pathogen control.
• Use of Biosave, a bacterial treatment, did not significantly affect water loss or color retention but was tested for decay control.
• Total soluble solids (TSS) decreased during storage but remained commercially acceptable (>15 Brix); acidity was low (<0.4) but TSS/acid ratio indicated good eating quality.
• The study suggests shelf life extension to 40 days is possible by combining polypropylene bag packaging and storage at 5.5°C, though better disease control is needed at this temperature.
• Further research is needed to optimize storage temperature, cooling regimes, and integrate biological postharvest disease control.
• The techniques developed offer a promising sulphur-free alternative for enhancing litchi fruit quality for export to the EU and local markets.