Soft scald development in new apple cultivars

Soft scald and bitter pit have a significant impact on the South African apple industry. ‘Scifresh’ was used in this study and is particularly susceptible to soft scald and bitter pit. The main aim of the study was to understand the factors that influence disorder susceptibility and propose strategies to minimize the risk.

Four trials investigated the effects of 1) different cooling protocols and storage durations; 2) regular and controlled atmosphere (CA) storage, storage times, and 1-methylcyclopropene (1-MCP); 3) forced air cooling (FAC) after packing, and 4) harvest maturities on post-storage quality of ‘Scifresh’ apples. Fruit quality evaluations and biochemical analyses were done at harvest, after different storage periods and after shelf-life.

Less mature fruit [52 % starch breakdown (SB)] were more susceptible to soft scald development. Immediate room cooling (coolest protocol) at -0.5 °C reduced bitter pit but increased soft scald development. Preconditioning (warmest protocol) and stepwise cooling (intermediate protocol) reduced soft scald but increased bitter pit expression because it increased ethylene levels in the fruit. To reduce bitter pit, fruit is best stored for longer periods (4 months) or, if the period is less than 4 months, fruit needs to be treated with 1-MCP. Fruit harvested post-optimum (64 % SB), stepwise cooled, CA- stored for 6-8 months and 1‑MCP treated had an increased risk of bitter pit. There was not a risk in fruit that was cooled immediately and stored under the same conditions. Fruit quality was similar for fruit that was FAC or passively cooled after packing in both seasons. Fruit harvested above 55 % starch breakdown and larger fruit (diameter > 65 mm and mass > 146 g) had a higher risk of bitter pit. Harvesting ‘Scifresh’ apples with a starch breakdown (SB) below 30 %, resulted in low TSS levels which could result in eating quality issues in the market.

Reduce the risk of soft scald in ‘Scifresh’ apples by stepwise cooling (ambient for overnight after harvest, thereafter, decrease pulp temperature to 12 °C within 24 h, subsequently decrease temperature with 1 °C every 24 h until a pulp temperature of 0.5 °C is reached) the fruit after harvest. To reduce the risk of bitter pit, fruit harvested at optimum maturity and which was not treated with 1-MCP should be considered for long-term CA storage while fruit intended for RA storage must be treated with 1-MCP and be stored for 4 months before chamber opening. If fruit surface temperature does not exceed 10 °C during packaging, room cooling, and not FAC, must be used. The recommended harvest maturity is SB of 30–40 %, TSS > 12 %, firmness > 8 kg with a 40 % blush colour and F4 colour intensity (‘Scifresh’ colour swatch)

Fruit of post-optimum maturity will result in higher bitter pit incidence. Stepwise cooling should be performed to limit the incidence of soft scald. Store fruit for longer than 3 months to reduce bitter pit expression. Blush colour alone should not be used as a maturity parameter.