Pg. 57 An Overview of Post-Harvest Storage of Mauritius Litchi Fruit

Litchi fruit, although of tropical origin, is non-climacteric and therefore able to endure low storage temperatures. Unfortunately, however, the litchi rind is relatively thin and lacks a thick, durable cuticle. Consequently, desiccation is a major factor during post-harvest storage life. As a result of desiccation, litchi fruit rapidly lose red rind colour and unless treated immediately after harvest, will turn an unsightly brown colour. Presently, the South African industry uses sulphur fumigation to prevent browning, but sulphur is undesirable for several reasons, viz. it causes aftertastes, is ineffective against some fungi and constitutes a potential health problem. Furthermore, overseas markets have lowered the tolerance level of sulphur to 10 mg/kg sulphur in the fruit flesh. Consequently, it was imperative that research aimed at eliminating sulphur fumigation be undertaken.

Summary:

• Litchi fruit is non-climacteric and tolerates low storage temperatures, but its thin rind lacks a durable cuticle, making desiccation a major issue during storage.
• Desiccation causes rapid loss of red rind color and browning unless treated immediately after harvest.
• Sulphur fumigation is currently used in South Africa to prevent browning but has downsides: aftertaste, ineffectiveness against some fungi, health concerns, and lowered market tolerance levels for sulphur residues.
• Research aimed to eliminate sulphur fumigation by fixing anthocyanins in situ to preserve fruit color.
• A 30-second dip in boiling water (98°C) followed by four minutes in a zero pH solution fixed pericarp color for 28 days at 1°C, but caused heat damage to the fruit pulp.
• Substituting two seconds of steam at 95°C followed by four minutes in zero pH solution preserved red color, maintained clean pulp, and retained taste after 28 days at 1°C.
• Studies showed steam treatment caused membrane breakdown and disrupted the fruit cuticle, increasing electrolyte leakage and causing fruit shriveling and higher soluble solids.
• To counter shriveling, an artificial wax coating (1% Vaporgard) was applied, which preserved firmness and soluble solids similar to untreated fruit during storage.
• Chemical analyses found higher chlorine levels in steam and zero pH treated fruit, which is safe, and sulphur was mainly found in sulphur-fumigated fruit.
• Rind color in untreated fruit became lighter and changed from red to reddish orange during storage at 30°C; steam plus zero pH treatment enhanced and maintained red color longer.
• Anthocyanin levels decreased in untreated fruit but were better retained or increased briefly after steam and zero pH treatment.
• Polyphenol oxidase, the browning enzyme, was strongly inhibited by steam and even more so with subsequent zero pH treatment, explaining color retention.
• Semi-commercial trials in South Africa with steam + zero pH post-harvest treatment showed successful preservation for Mauritius and Hong Huay litchis.​
• This summary captures the key findings and methods from the article regarding post-harvest storage techniques aimed at preserving Mauritius litchi fruit quality without relying on sulphur fumigation.