Internal Fruit Quality Affected by Sunburn Browning In Apples

Sunburn of apples is a physiological disorder caused by heat and light stress. Sunburn results in large economic losses to growers in several regions of the world.  We previously identified three types of sunburn (sunburn necrosis, sunburn browning, and photooxidative sunburn), but consider sunburn browning to be most important commercially.  Recently, we found several postharvest changes in internal fruit quality of apples with sunburn browning.  Apples of five cultivars (‘Gala’, ‘Golden Delicious’, ‘Jonagold’, ‘Granny Smith’, and ‘Fuji’) harvested at commercial maturity were sorted into five classes ranging from no sunburn (Sb-0) to severe sunburn browning (Sb-4) and placed in cold storage.  Flesh firmness, soluble solids concentration (SSC), and titratable acidity (TA) were measured on flesh tissue beneath the sunburned area of all apples at harvest and after 3 and 6 months of regular atmosphere cold storage.  Flesh firmness and SSC increased significantly, but TA decreased in all cultivars as sunburn severity increased from Sb-1 to Sb-4.  These trends persisted during cold storage, but TA declined markedly over time in apples with more severe sunburn.  This resulted in a marked increase in the SSC to TA ratio (SAR) in all five cultivars during cold storage.   A rapid decline in TA has been reported to shorten storage life and to affect acid taste of fruit.  A second study compared, only at harvest, the sun, shade and shoulder sides of the same five apple cultivars used previously with five classes of sunburn browning (Sb-0 to Sb-4). .  As severity of sunburn increased, flesh firmness, SSC, and SAR increased more on the sun side than on shade or shoulder sides.  With increasing sunburn severity, starch index increased in the shade and shoulder sides, but changed little on the sun side of sunburned fruit.  Hence sunburned apples were more mature than non-sunburned apples. These findings suggest that internal fruit quality of apples with sunburn browning is affected not only on the sun side, but also on shade and shoulder sides of sunburned fruit.   We suggest that the common commercial practice of combining some sunburned apples with non-sunburned fruit in premium grades of apples for marketing should be re-evaluated.