Pg. 47-52 Litchi (Litchi Chinesis Sonn) Pericarp Colour Retention – Alternatives to Sulphur

The South African litchi industry is predominantly export orientated and pericarp (skin) colour retention is of primary importance, where export markets demand red fruit. Until now, gaseous sulphur treatments were thought to play a major role in the maintenance of this colour by firstly, fixing the anthocyanins, and secondly acting as a fungicide thus preventing browning reactions. Sulphuring of fruit however, results in undesirable aftertastes, constitutes a potential health problem and is ineffective against some fungi. In addition, since sulphur-fixed anthocyanins are colourless, the sulphuring process is sometimes reversed by dipping the fruit in low pH solutions. The pH of the solution is thus critical and detailed investigations have shown that a pH of 0,0, following blanching of the fruit in boiling water at 98°C, is most effective in retaining the red colour of the litchi pericarp for up to five weeks in storage at 1°C. Internal browning of the fruit flesh however, was observed and attempts at eliminating these were investigated.

Summary:

• The South African litchi industry relies heavily on fruit with retained red pericarp color, traditionally achieved by gaseous sulphur treatments that fix anthocyanins and act as fungicides.
• Sulphur treatments, however, can cause undesirable aftertastes, health concerns, and are ineffective against some fungi; also, sulphur-fixed anthocyanins are colorless until reversed by low pH dips.
• Research showed that dipping fruit in boiling water (98°C) followed immediately by immersion in a very low pH solution (distilled water adjusted to pH 0.0) effectively fixed the red color for up to five weeks in storage at 1°C.
• Alternative chemical treatments such as ascorbic acid, citric acid, fruit waxes, and various coatings were ineffective in retaining red color.
• Treatments with sodium metabisulphite and low pH dips reduced browning but carried risks of sulfur residues and aftertaste, prompting moves away from sulphur-based treatments.
• The mechanism of color retention is linked to stabilizing anthocyanins in the pericarp through intermolecular or intramolecular co-pigmentation and maintaining acidic conditions to prevent pigment breakdown.
• Heat treatment at 98°C for 30 seconds to 1 minute breaks down cell membranes allowing anthocyanins to interact with the low pH solution, fixing the red color and preventing browning.
• Longer heat treatment or delayed low pH dipping can cause internal browning of the fruit flesh and pericarp browning during storage.
• Non-ionic detergents like Triton X-100 show some promise in solubilizing membranes but require long treatment times and are less practical.
• The study emphasizes that fixing anthocyanins requires vacuolar and cell membrane breakdown, acid fixation, and possibly inhibition of polyphenol oxidase enzyme activity.
• Internal flesh browning, particularly around the pedicel, remains a challenge after successful color fixation, requiring further research to identify causes and solutions.
• The findings suggest that a combination of heat treatment followed immediately by dipping in very low pH water can be a sulfur-free alternative for maintaining attractive red color in export-quality litchis.
• Future work should focus on minimizing internal browning and optimizing storage conditions to extend shelf life without the drawbacks of sulphur treatments.