Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 14, 1997
Publication Date: N/A
Interpretive Summary: Superficial scald is a serious storage disorder that affects many commercially important varieties of apples (e.g. 'Red Delicious' and 'Granny Smith') and pears (e.g. 'Anjou'). It occurs after long-term cold storage and results in browning or blackening of the peel, making the fruit unsalable. Scald has been linked to the production of large amounts of farnesene, an oily compound found in apple peel. Farnesene combines with oxygen to make conjugated trienols (CTs), which are toxic to the fruit tissues. The apple industry treats fruit with synthetic antioxidants to prevent oxidation of farnesene to CTs and thereby control scald. However, the safety of these chemical treatments has come into question, and their use in the future is uncertain. Certain apple varieties such as 'Gala' are highly resistant to scald, and several studies have indicated that these resistant fruit produce natural antioxidants. This report describes the extraction and chemical characterization of a new family of compounds from the peel of 'Gala' apples that are likely to have high antioxidant activity and therefore may help to prevent scald. Further research will determine if these compounds play a role in natural resistance to scald. Should this prove to be the case, other scientists could develop a genetic strategy to transfer the ability to make the natural antioxidants to scald-susceptible varieties, with the goal of controlling the disorder without unsafe chemical treatments.
Technical Abstract: Superficial scald is a storage disorder of apples that causes the peel to blacken. Susceptibility varies widely among cultivars. Scald is thought to be induced by oxidation products of farnesene and endogenous antioxidants may be involved in resistance. High 258 nm absorbance in hexane-dip extracts of apples correlates with low scald. Extracts from scald-resistant 'Gala' have a broad UV maximum at 259 nm. The 259 nm-absorbing compounds were partially purified and characterized. They were identified as a family of phenolic esters, with two maxima in hexane at 207 and 259 nm. Alkaline methanolysis yielded two free phenolics with similar UV spectra and methyl esters of even-chain saturated fatty acids ranging from C16 to C26. UV spectra of the two phenolics in MeOH resemble that of genistein (4',5, 7-trihydroxyisoflavone), an isoflavonoid. Preliminary NMR data indicate that the unknowns are flavonoids with a 4',5,7 hydroxylation pattern and one methoxy group. The two phenolics were baseline resolved by C18-HPLC, eluting at 3.3 and 4.2 min (genistein = 4.0 min). LC-MS gave tentative molecular masses of 296 and 294 for peaks 1 and 2, respectively. Elucidation of the structures of the two putative isoflavonoids is in progress. No prior report of isoflavonoid esters in the Rosaceae has been found. Hypothetically, they could serve as antioxidants that help to reduce scald and/or act as antifungal phytoalexins.