A convergence of sunlight and cold chain: The influence of sun exposure on postharvest apple peel metabolism

Authors: McTavish, Christine; Poirier, Brenton; TORRES, CAROLINA - Washington State University; Mattheis, James; Rudell, David

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/5/2020
Publication Date: 3/13/2020
Citation: McTavish, C.K., Poirier, B.C., Torres, C.A., Mattheis, J.P., Rudell Jr, D.R. 2020. A convergence of sunlight and cold chain: The influence of sun exposure on postharvest apple peel metabolism. Postharvest Biology and Technology. 164. Article 111164. https://doi.org/10.1016/j.postharvbio.2020.111164.
DOI: https://doi.org/10.1016/j.postharvbio.2020.111164

Interpretive Summary: High sunlight-related apple peel disorders that lead to appearance defects contribute up to 25% of total annual losses in many of the world’s highest production regions including Washington State. Many of these disorders only develop symptoms after harvest during the cold chain when product loss is more costly. In this study, we looked at chemical changes in the peel during cold air storage with respect to sunlight exposure in the orchard to find which systems are impacted and find chemical targets that define sun exposure. Changes in levels of different natural chemicals were reflective of whether they received more sunlight. These changes indicate that stress presented by the sun in the orchard is carried over into cold storage. The interaction between sunlight exposure and chilling may cause these disorders. Chemical changes also emphasize the degree to which cumulative sunlight may alter the way apples ripen and their overall eating quality for the consumer. Cumulative sunlight exposure could contribute to overall fruit inconsistency during the cold chain and at the retailer when apples that are bulked together once resided on different regions of the tree canopy where shading occurs. Finally, a particular group of these chemicals can be detected non-destructively and used to sort fruit into more consistent batches with respect to ripening or avoid losses due to disorders by managing stock according to risk.

Technical Abstract: Excess solar irradiation is responsible for a large percentage of annual apple fruit loss, and those losses are expected to increase in the majority of the world’s apple production regions. Losses are not limited to the orchard but are also represented by disorders that develop in the cold chain such as sunscald and elevated lenticel damage. Similarly, metabolism during storage would reflect changes and even diverge during ripening and cold storage depending upon relative amount of light exposure in the orchard. To determine and track these changes alongside changes of appearance, ‘September Wonder Fuji’, ‘Gala’, ‘Granny Smith’, and ‘Honeycrisp’ apples were selected for contrasting sun exposure on opposite aspects, stored in 1°C air, and peel from opposing sides sampled sequentially from 0 to 6 months. Additional sunburned peel was sampled at 0 and 6 months. Sun exposure provoked broad relative responses from multiple pathways indicative of solar stress response. Responses include accumulation of well-characterized photoprotective responses including accumulation of flavonol glycoside and carotenoid, compatible solutes, and primary metabolites. Others include previously unreported or lesser understood responses such as those potentially impacting membrane properties including changes in levels of monogalactosyldiacyglycerides and stigmasteryl glycosides, epicuticular surface metabolites including pentacyclic triterpenes and diacylglycerides, and production of volatile vinyl aldehydes indicative of residual solar stress during storage. Taken together, metabolic evidence indicates orchard light environment continues to impact not only appearance, but also rate of ripening and potentially fruit quality, even on the same apple. In this way, light exposure of any given apple could influence every cold chain management decision, and sorting apples at-harvest according to cumulative light exposure could better assure more consistent apples while potentially avoiding losses due to peel disorders.