Pg. 42-46 Factors influencing SO2 residues on commercially fumigated HLH Mauritius and McLeans’s Red litchi fruit

In spite of considerable research being conducted on alternative postharvest tech­niques, sulphur fumigation of litchis presently remains the most effective preserva­tion and colour retention technique. It would further appear that a uniform world­ wide standard for sulphur residue limits is in the offing, partly due to lobbying by export countries. It is therefore of paramount importance that sulphur fumigation in South African litchi packhouses be optimally tuned to maintain acceptable residue limits in export fruit. The present study was conducted at packhouses in the Onderberg and Tzaneen districts and aimed to quantify the effect of environmental and fruit physiological parameters on sulphur residues. The results firstly confirmed previously shown trends that sulphur residues in McLean’s Red sulphur residues are considerably higher than in HLH Mauritius when fumigated under commercial conditions, especially when incorporating an acid dip. It was further revealed that more mature McLean’s Red fruit have an increased proneness to unacceptably high
aril sulphur dioxide residue levels compared to less mature fruit. It was also demon­strated that higher ambient temperatures in combination with low humidity leads to an increase in aril sulphur residues, especially when leaving the fruit overnight before fumigating. It is strongly advised that packhouse management take notice of these results and confer with the researchers especially with regard to fumigation of McLean’s Red.

Summary:

• Sulphur fumigation remains the most effective method for preserving litchis and retaining color despite research into alternatives.
• A uniform worldwide standard for sulphur residue limits on export fruit is anticipated, making optimized fumigation critical.
• The study was conducted in South African packhouses (Onderberg and Tzaneen districts) to assess environmental and physiological effects on sulphur residues.
• McLean’s Red fruit shows significantly higher SO₂ residues than HLH Mauritius under commercial fumigation, especially with an acid dip.
• More mature McLean’s Red fruit tend to accumulate unacceptably high levels of sulphur dioxide residues in the aril compared to less mature fruit.
• Higher ambient temperatures combined with low humidity increase aril SO₂ residues, particularly if fruit are left overnight before fumigation.
• Fruit from different producers showed variation in SO₂ residues correlating with maturity and firmness; more mature fruit had higher aril residues but lower peel residues.
• The current maturity standard for South African export litchis may need revision; comparison with other countries suggests varying sugar-to-acid ratio harvest criteria.
• Relative humidity and temperature have a significant impact: low humidity and high temperature increase sulphur residue levels.
• Fumigation time and conditions (humidity, temperature) must be carefully managed to keep residues within acceptable limits.
• Time delay between harvesting and fumigation causes increased SO₂ residues; minimizing this delay is crucial.
• Recommendations include installation of temperature control in fumigation rooms and better packhouse capacity management to avoid delays.