Pg. 15-16 SO₂ Residues in Litchi Fruit after Post-Harvest SO₂ Treatments (1990)

During the 1990 season, S0₂ residues in litchi fruit, cv Mauritius and cv Madras, were determined immediately after commercial S0₂ treatment, and after four weeks of cold storage. In both cultivars, S02 concentration in the pulp decreased during the four-week storage period and a big variation in the S0₂ residue concentrations occurred. In 79 per cent of the Madras samples analysed, the S0₂ residues in the pulp exceeded the tolerated 10 ppm level after storage, while only 22 per cent of the Mauritius samples fell into this category.

Summary:

• The study measured sulfur dioxide (SO₂) residues in litchi fruit cvs. Mauritius and Madras immediately after commercial SO₂ fumigation treatment and after four weeks of cold storage to simulate export conditions.
• SO₂ fumigation is used to control browning in South African export litchis, but there is concern about residues exceeding the tolerated level of 10 ppm in the fruit pulp.
• After storage, SO₂ concentration in the pulp decreased in both cultivars but showed large variation between samples.
• 79% of Madras samples exceeded the tolerated 10 ppm SO₂ residue level after storage, compared to only 22% of Mauritius samples.
• Madras fruit also has a problem with a sulfur taste, which is less prevalent in Mauritius fruit.
• The difference in residue levels between cultivars may be due to differences in skin permeability.
• No consistent relationship was found between fruit maturity (measured by TSS and acidity) and SO₂ residue levels.
• Post-harvest sulfur treatments that leave residues are becoming less acceptable in export markets.
• Alternative methods to control browning without sulfur residues, such as shrink-wrapping, gas-packaging, or marketing the naturally brown ripe litchi, are recommended.
• SO₂ was applied at rates of 1.0 to 1.5 kg per ton of fruit in fumigation rooms at packhouses.
• Samples were analyzed for SO₂ residues by freezing and laboratory analysis.