Pg. 6-9 Preliminary evaluation of SO₂ sheets to prevent postharvest diseases of South African export litchi fruit

A study was conducted to establish the efficacy of SO₂ sheets and liners in terms of postharvest diseases and browning of HLH Mauritius litchi fruit. The study consisted of 3 trials. The first trial was aimed at establishing whether SO₂ sheets have the potential to replace conventional sulphur fumigation. The second and third trials were designed to determine whether the sheets can play a supplementary role when used in combination with sulphur fumigation. The results showed that it was not possible to obtain uniform bleaching when using the sheets on their own. However, the sheets were found to significantly reduce the incidence of fungal infections when placed in the bottom of boxes containing commercially SO₂ fumigated fruit. Residue analysis indicated the sheets not to increase SO₂ residues in the arif of the fruit. At this stage it is recommended that the sheets be used without liners after regular fumigation.

Summary:

• The study evaluated the efficacy of SO₂ (sulfur dioxide) sheets and liners in controlling postharvest diseases and browning in HLH Mauritius litchi fruit through three trials.
• Trial A aimed to determine if dual release SO₂ sheets could replace conventional sulfur fumigation and investigated the use of liners with these sheets. It found uneven bleaching when sheets were used alone, with the sheets reducing fungal growth but liners increasing fungal incidence by preventing drying.
• Trial B assessed the supplementary role of SO₂ sheets combined with sulfur fumigation, comparing slow release versus dual release sheets. Both types reduced fungal infections during shelf life, with no significant differences in external fruit appearance or SO₂ residue levels on the fruit.
• Trial C confirmed previous findings and tested reduced fumigation dosages combined with SO₂ sheets and different liners. Results suggested that plastic and paper liners increased SO₂ residues significantly but did not improve fungal control compared to fumigation alone. The fruit used in this trial were likely too mature for optimal export quality.
• Overall, SO₂ sheets alone cannot replace conventional fumigation due to uneven bleaching, but they can effectively supplement fumigation in reducing fungal infections without increasing residue risks.
• The study recommends using SO₂ sheets without liners after regular fumigation and calls for further research to refine the technology and develop appropriate maturity parameters for litchi export quality.
• SO₂ residue levels in fruit aril remained within acceptable limits (below 10 ppm), indicating safety for consumption after treatment with sheets.
• Maturity of litchi fruit is an important factor affecting postharvest quality and disease control, which needs more research focus in the South African export industry.