Superficial scald on Granny Smith - understanding the mechanisms and limitations of the storage protocols used by industry and assess risk indicators in storage for scald development

Superficial scald is a postharvest, oxidative stress disorder. It consists of an induction (pre-symptomatic) period, that is characterized by irreversible physiological changes (upregulation of genes, ethylene biosynthesis, oxidation of volatile compounds), followed by symptom expression, namely superficial browning of the peel, after prolonged storage. Further investigation of the mechanism involved in superficial scald development is needed to predict the occurrence of this disorder and to adjust storage times accordingly. Therefore, this study aimed to understand the biochemical changes that take place during superficial scald induction and symptom expression and to build a statistical model to help predict superficial scald incidence and severity for different storage treatments, over different seasons.

‘Granny Smith’ apples were subjected to 12 different storage treatments. Biochemical variables and superficial scald incidence were evaluated after 4, 8, 16, 24, and 33 weeks (W) storage + 6W regular atmosphere (RA) shipment period + 10 days at 20 °C (10d shelf-life (SL)). Biochemical variables thought to have an associative role in superficial scald induction and symptom development, including farnesene metabolism (α-farnesene, CTols and MHO), ethylene biosynthetic compounds (internal ethylene, O₂, CO₂, ACC, and MACC) and redox systems (ROS, AOC, total phenolic compound, extractable condensed tannins, non-extractable condensed tannins) were measured. Partial Least Square-Regression (PLS-R) was employed to minimize the sample prediction error and to seek linear functions of the predictors that explain as much as possible variation in each response.

A strong, positive correlation between increased internal ethylene, ACC and MACC and superficial scald incidence following prolonged storage. A good correlation between MHO in fruit peel and superficial scald incidence during early storage (4, 8 and 16W storage +6W RA +10d SL), however MHO did not correlate to superficial scald incidence following prolonged storage (24 and 33W + 6W RA + 10d SL). It is proposed that these compounds are associated with scald induction, while phenolic compounds, including condensed tannins, are related to symptom expression and play a possible role in the characteristic browning observed in the fruit peel.

Results indicate that ethylene biosynthetic components (internal ethylene, CO₂, ACC, and MACC) could possibly be used to predict superficial scald incidence during prolonged cold storage. However, the predictive capacity of the PLS-R model must be verified.

Superficial scald is associated with fruit ripening, therefore, inhibiting ethylene action, using 1-MCP in combination with  low-oxygen storage treatments, significantly reduce superficial scald incidence.