Pg. 37-41 Research on Alternative Postharvest Treatments for Litchi Fruit

A study was conducted to evaluate a range of potentially effective postharvest treatments for HLH Mauritius and McLean’s Red litchis. In the search for an effective dye to improve the colour of export fruit, a water based dye used in food and cosmetics was evaluated. However, the dye was found to be ineffective on unsulphured fruit as browning of the peel still took place during storage. In sulphured fruit it was difficult to attain sufficient dye absorption into the peel. The fruit were therefore first steam treated and this increased the absorption to an acceptable level after 1.5-2 hours of soaking in the dye solution. Prior bleaching of the litchi peel with hydrogen peroxide resulted in deterioration of the aril and a higher degree of mass loss. In addition to the above treatments, two antioxidants (electrochemically activated water and methyl jasmonate) and two experimental litchi coating formulas (Agricoat formulas 2 & 3) were evaluated. Both antioxidants were found to be ineffective while the two coating formulas retarded pericarp browning to an extent, especially when acidified with citric acid to pH 3. Further developmental research is, however, required with these coatings. The third set of experiments involved the use of controlled atmosphere (CA) storage of HLH Mauritius and McLean’s Red. With HLH Mauritius, the experiment entailed the use of six CA atmospheres in combination with five sulphur fumigation regimes. Only the most promising atmosphere and standard fumigation treatment was used in the McLean’s Red trial. In the HLH Mauritius trial it was found that sulphuring at levels that are lower than the commercial standard can retard fungal growth effectively when stored under CA. However, the effect was negated during the ensuing shelf-life phase and the S0₂ residues were found to be considerably higher than those of the controls. In the McLean’s Red trial, the results attained with CA were quite promising in that the fungal infection was significantly reduced, while the proportional increase in sulphur residues was less pronounced than in HLH Mauritius.

Summary:

• The study evaluated various alternative postharvest treatments for HLH Mauritius and McLean’s Red litchi cultivars aiming to find sulphur-free options for export quality preservation.
• A water-based food dye (Sicovit) intended to improve color was ineffective on unsulphured fruit due to peel browning; steam treatment improved dye absorption but is impractical for commercial use.
• Hydrogen peroxide bleaching improved appearance but caused aril deterioration and increased mass loss, despite inhibiting fungal growth.
• Methyl jasmonate treatments did not improve color retention and accelerated peel browning but showed some inhibition of fungal growth and slowed aril deterioration.
• Electrochemically activated water treatments accelerated browning, increased mass loss, and reduced firmness and titratable acid; only some treatments inhibited fungal infection.
• Two experimental litchi coating formulas from Agricoat retarded pericarp browning to some extent, especially when acidified to pH 3, with formula 2 showing better results than formula 3.
• Controlled atmosphere (CA) storage trials with various CO₂ and O₂ combinations demonstrated effective fungal growth inhibition and reduced mass loss, mainly at high CO₂ levels.
• Lower sulphur fumigation levels under CA still effectively controlled fungal growth but resulted in higher SO₂ residues, which could be problematic during shelf life.
• CA storage showed promise especially in McLean’s Red litchis by reducing fungal infection with a less pronounced increase in SO₂ residues than HLH Mauritius.
• The use of controlled atmosphere combined with reduced sulfur fumigation could improve export quality, but care is needed regarding fumigation time and residue management.
• Full-scale export trials with controlled atmosphere are recommended to validate these promising results.