Excluding sunscald from long-term storage of ‘Granny Smith’ apples

Authors: MOGOLLON, RENE - Washington State University; MENDOZA, MANOELLA - Washington State University; LORENZO, LEON - Washington State University; Rudell, David; TORRES, CAROLINA - Washington State University

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2024
Publication Date: 6/8/2024
Citation: Mogollon, R., Mendoza, M., Lorenzo, L., Rudell Jr., D.R., Torres, C. 2024. Excluding sunscald from long-term storage of ‘Granny Smith’ apples. Postharvest Biology and Technology. 216. Article 113044. https://doi.org/10.1016/j.postharvbio.2024.113044.
DOI: https://doi.org/10.1016/j.postharvbio.2024.113044

Interpretive Summary: High orchard temperatures and sun exposure during apple production can lead to sunburn and during storage, sunscald. Sunscald compounds losses as apples are harvested and often placed in long-term storage where symptoms develop rendering the fruit unmarketable. To help mitigate these losses, we developed a sorting protocol that exploits the optical qualities of natural peel chemicals associated with sunburn symptoms and risk of development of sunscald symptoms during storage. Using hyperspectral imaging, we were able to image peel likely to develop sunscald symptoms at-harvest and before they develop months into cold storage. Using this information, a means to determine if imaged apples had sunburn or were at risk of developing sunscald was completed for ‘Granny Smith’ apples, a susceptible apple variety. This allow for apples that had been compromised by elevated heat and sun exposure to be removed from those entering long-term storage. This would allow for sunburn-free apples at-risk of developing sunscald to be marketed immediately after harvest and before sunscald develops.

Technical Abstract: Sunscald is a postharvest apple and pear peel disorder primarily caused by excessive light exposure during the latter part of the growing season. Symptoms are not apparent at harvest but develop during storage as superficial brown or gray discolored patches on the exposed portion of the apple. Sunscald results in considerable annual losses to susceptible cultivars such as ‘Granny Smith’. Mitigation relies on cultural controls such as shading or cooling exposed apples on the canopy periphery using shade cloth or evaporative cooling, respectively, which are not entirely effective. To reduce sunscald in the cold chain by eliminating at-risk apples from long-term storage, we present a novel protocol using hyperspectral imaging (HSI) to identify apples at harvest that are more likely to develop the disorder. Spectral bands attributed to sunscald risk using HSI were 400-550, 600-650, and 900-1,000 nm. From these bands, an index using reflectance values from chlorophyll (Chl, 430, 662 nm) and carotenoids (Car, 454, 549 nm) that summarizes the Chl to Car ratio was chosen. Index thresholds for indicating peel with sunburn symptoms and sunscald risk were chosen using calibration sets with sunburn (reject) and mild discoloration caused by high sunlight exposure (marketable but high sunscald risk), respectively. Spectra were summarized by region to reduce pixels, thresholds established, and a logic model identifying reject (sunburned), high (sunscald) risk, and low-risk apples from 2 populations selected for different sun exposure levels and 4 populations harvested from whole trees from different orchards. Using this protocol, nearly all those apples that later developed sunscald were placed in high-risk or reject categories. The frequency of high-risk apples without sunburn symptoms in the populations harvested from whole trees fell below 20%, although actual sunscald incidence within this category ranged from 55-81% among populations harvested from whole trees. This highlights other factors beyond elevated sun exposure contribute to sunscald incidence, rendering predictive modeling approaches inaccurate among orchards and seasons. Sorting by sunburn presence and sunscald risk allows for apples that have sunburn symptoms to be excluded and those that are marketable but most likely to develop sunscald to be pushed to market rather than storage. Calibrating the cutoff thresholds using select apples from each lot ensured that these thresholds were accurate among orchards/seasons. With the appropriate source/sensor array, this approach is readily adaptable to sorting lines or even robotic harvesters. Challenges include adapting this approach for other apple and pear cultivars and extending capabilities for additional uses, including sorting fruit into additional categories representative of relative sunlight exposure.