Submitted to: HortScience
Publication Type: Abstract Only
Publication Acceptance Date: March 28, 2001
Publication Date: N/A
Technical Abstract: Superficial scald is an oxidative stress-related storage disorder of apple fruit. It is largely controlled by postharvest application of diphenylamine, a lipophilic antioxidant. Endogenous antioxidants are thought to delay or reduce scald development, and several reports have linked UV absorbance at 258 nm in hexane-dip extracts of apple fruit with natural resistance to the disorder. Hexane extracts of epicuticular wax from scald-resistant 'Gala' apples exhibit a broad absorbance maximum at about 258 nm, which led us to isolate and identify the primary UV-absorbing compounds. Column and thin-layer chromatography steps yielded a fraction that gave a series of paired, 260-nm-absorbing peaks on C18-HPLC. These were shown to be a family of phenolic fatty-acid esters, for which retention times increased with increasing fatty-acid chain length and paired peaks were esters of two related phenolics with the same fatty-acid moiety. Alkaline cleavage of the esters released two water-soluble phenolics separable by HPLC. Electrospray MS gave a molecular mass of 150 for both, and proton-NMR plus UV absorbance spectra identified them as trans and cis isomers of p-coumaryl alcohol (p-CA). Proton-NMR of the HPLC-purified stearate ester of trans-p-CA indicated that fatty-acid esterification occurs at the gamma-OH rather than at the 4-OH on the phenyl ring. To our knowledge, this is the first report of fatty-acid esters of monolignols as a natural plant product. The purified p-CA esters showed moderate antioxidant activity in two different assays. This is consistent with a role in scald resistance in 'Gala' apples, and suggests that these compounds may be of some benefit in the human diet.