Sunburn of Apples: Types, Effects, and Impact On Consumer Acceptance of Fruit

Sunburn is a physiological disorder of apple fruit with progressively increasing importance. The damage can be caused by excessive light or temperature conditions or by the combination of these factors, corresponding to the various types of the damage: photooxidative sunburn, sunburn necrosis, and sunburn browning. This study summarizes research results on apple sunburn obtained at the Ohio State University in the last three years. The damage formation is closely related to changes in the pigment pattern of the peel (decreased chlorophyll content and reduced photosynthesis, decreased anthocyanins, increased phenolics content). Pigment composition of undamaged and sunburned fruit peel showed a close relationship to its spectral reflectance features in the UV and visible ranges. Increased electrolyte leakage and modified reflectance features in the NIR region assumed increasing cell damage and decreasing water content in the order of undamaged fruit, photooxidative sunburn, sunburn browning, and sunburn necrosis. Mineral nutrient content of the peel and underlying cortical tissues showed a gradient from shade to sun, and to sunburned fruit, caused by severe temperature gradient within the fruit. Scanning electronmicroscopy imaging revealed that the wax cuticle consists of an organized crystal structure in non-damaged fruit, while apples with sunburn browning or sunburn necrosis have a melted-like amorphous wax surface. Crystal structure of the wax can still be found on fruit with photooxidative sunburn. Structural damage of the epidermal and hypodermal cells is characterized for sunburn necrosis only, however, disorganization of the chloroplast and other cell organelles was observed for sunburn browning and photooxidative sunburn as well. Untrained consumer panels with over 100 attendees in each session carried out sensory assessments of undamaged and sunburned ‘Granny Smith’, ‘Golden Delicious’ or ‘Fuji’ fruit. For appearance, significantly lower rating scores were given to sunburned fruit for all three cultivars compared to undamaged shade or sun fruit. Due to higher soluble solids content of sunburned fruit, damaged fruit samples were rated for taste higher than sun or shade samples of ‘Granny Smith’ and ‘Golden Delicious’. Because ‘Fuji’ is characterized by sweet taste, panelists were not able to differentiate in taste between sunburned and undamaged fruit. Storage potential of sunburned apples is also discussed in this paper in a comparison with undamaged shade and sun fruit.