Forewarned is forearmed

What is not known, is what causes the condition.

Scientists used to suspect that alpha-farnesene and its oxidative products were the culprits. However, recent studies indicate that these compounds are associated with the induction phase and can be used as an oxidative marker, but not much more. The cause of superficial scald has been ascribed to a more complex metabolic cascade caused by oxidative stress responses during cold storage. A 2020 study found, for instance, that superficial scald induction is related to cumulative oxygen exposure after harvest, during cold storage and throughout the post-storage chain.

Despite not fully understanding the causes of the disorder, the industry has developed treatments and protocols that reduce oxidative stress and maintain fruit quality. These include postharvest treatments with the synthetic antioxidant diphenylamine (DPA) or 1-MCP, an ethylene inhibitor that binds to and blocks the ethylene receptor. Low-oxygen storage treatments, such as controlled atmosphere (CA), repeated low-oxygen stress, ultra-low oxygen, and dynamic controlled atmosphere-chlorophyll fluorescence, further reduce oxidative stress products.

Notwithstanding all these control measures, superficial scald still causes significant damage and annual losses, especially when protocols are not followed to the letter.

To put the industry more on the front foot in the battle against superficial scald, a study was designed to understand the biochemical changes that take place during the induction and symptom-expression phases of the disorder. The aim was to use this information to build a statistical model to help measure the risk of superficial scald incidence for different storage treatments and over different storage seasons.

The project was led by Dr Elke Crouch from Stellenbosch University (SU) Department of Horticultural Sciences and conducted in collaboration with Dr Anel Botes from the Agricultural Research Council (ARC), Dr Ian Crouch and Daniël Viljoen from ExperiCo, Drs Ashwil Klein and Lizex Husselmann from the University of the Western Cape, and SU PhD student Monja Gerber.

The study

Conducted between January 2018 and August 2022, the study had three objectives.

1. Determining the efficacy of treatments
   to mitigate scald by applying and evaluating different storage treatments over three seasons.
2. Evaluating the mechanism of action for different storage protocols, individually or in combination.
3. Establishing the link between meta-bolites/proteomics measured and when they occur, compared to the time when scald develops.

In conducting the study, 'Granny Smith' apples harvested at pre-optimum maturity was used, given the high risk this maturity level holds for the development of superficial scald. The fruit was subjected to 12 different storage treatments over three seasons. Biochemical variables and superficial scald incidence were evaluated after four, eight, 16, 24 and 33 weeks of storage, plus a six-week regular-atmosphere shipment period and 10 days at shelf-life conditions.

Results, conclusions, and the way forward

This study found strong correlations – using the partial least squares regression (PLS-R) model – between biochemical indicators during the induction phase of the disorder, and different biochemicals during its expression. However, these need to be further tested with a larger pool of samples to assess their robustness and to determine if they can be used on larger scale commercially.

The take-home message for the industry is that superficial scald is associated with the ripening of fruit. Therefore, inhibiting ethylene action by using 1-MCP (as soon as possible after harvest and again at the end of storage) in combination with low-oxygen storage treatments, significantly reduces the risk of the occurrence of the disorder.

Featured Image: Superfi cial scald on ‘Granny Smith’ after 12 weeks of cold storage at 1 °C, with no treatments to prevent the disorder