Pg. 14-20 Determination of maturity cut-off points for South African export litchi fruit

A study is being conducted, aimed at the development of appropriate harvest window cut-off points for HLH Mauritius and McLean’s Red export litchis. It was found that sulphur fumigated HLH Mauritius litchis contracted relatively few fungal infections during export simulations when the TSS/TA ratio was lower than 30:1. The incidence of infection dramatically increased when the TSS/TA ratio became higher than 30: 1. In the case of McLean’s Red, it was more difficult to establish an exact cutoff value. The infection rate of this cultivar was found to increase linearly by approximately 2% for every 1 unit increase in the TSS/TA ratio. The over-mature McLean’s Red litchis also had significantly higher SO₂ residues in the aril of the fruit.

Based on the current results, it is recommended that sulphur fumigated South African HLH Mauritius and McLean’s Red litchis are exported between TSS/TA values of 20:1 and 30:1. Further research is to be conducted to establish to what extent the above parameters can be stretched. This will primarily involve the use of slow release sulphur dioxide sheets and controlled atmosphere (CA).

Summary:

• The study focused on developing appropriate harvest window cut-off points for two South African export litchi cultivars: HLH Mauritius and McLeans Red.
• Sulphur-fumigated HLH Mauritius litchis showed relatively few fungal infections during export simulations when the Total Soluble Solids to Titratable Acid (TSS:TA) ratio was below 30:1. Fungal infection incidence increased sharply above this ratio.
• For McLeans Red, it was harder to define an exact cut-off TSS:TA value. Fungal infection rates increased approximately 2% for every 1 unit increase in the TSS:TA ratio.
• Over-mature McLeans Red fruit also had significantly higher sulphur dioxide (SO₂) residues in the aril, indicating senescence effects.
• Recommended export TSS:TA ratio window for both cultivars: between 20:1 and 30:1 to minimize fungal infection, especially when sulphur fumigation is applied.
• Research used commercial fumigation, with fruit stored at 4°C for 28 days, followed by 7 days shelf life at 13°C, assessing fungal infection rates, mass loss, and quality parameters.
• HLH Mauritius showed a clear fungal infection increase beyond TSS:TA 30:1, making this an appropriate cut-off for exports.
• McLeans Red had continuous fungal infection increase with TSS:TA ratio increase; controlling this is more challenging.
• SO₂ residues increased in over-mature McLeans Red fruit, linking high TSS:TA ratios to senescent fruit with low shelf life.
• Further research is needed to extend harvest and export windows using slow-release sulphur dioxide sheets and controlled atmosphere storage.
• Regional variations in harvest parameters for other production areas (Onderberg for HLH Mauritius and Tzaneen for McLeans Red) are also recommended for future investigation.
• Overall, using the TSS:TA ratio as a maturity parameter provides a useful objective measure to decide export readiness for South African litchi fruit, aiming to reduce post-harvest fungal infections and improve shelf life.