Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/18/2001
Publication Date: N/A
Interpretive Summary: Superficial scald is a serious storage disorder of apple and pear fruits. Symptoms include acute browning of large patches of the fruit surface, causing substantial losses in fresh-market sales. Scald is thought to be induced by oxidative reactions in the peel tissue involving an oily compound called alpha-farnesene. It is not known why fruit of some cultivars are very susceptible and those of other cvs are quite resistant to the disorder, but an abundance of natural antioxidants in the peel may be one factor that confers resistance. We have discovered a new group of fat-soluble antioxidant compounds, fatty- acid esters of coumaryl alcohol, in the surface wax of scald-resistant 'Gala' apples. These compounds could help prevent scald development in 'Gala' fruit and potentially may be beneficial in the human diet. This kind of information is needed to develop new means to reduce apple losses due to scald without potentially harmful chemical treatments, which ultimately will increase profits for the apple industry and ensure that fresh apples are safe and nutritious for the consumer.
Technical Abstract: During a study of alpha-farnesene accumulation and oxidation in the skin of cold-stored apple fruit, hexane extracts of epicuticular wax from 'Gala' apples were noted to have an unusual, 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 hydrolysis of the esters released two water-soluble phenolics separable by C18-HPLC. Electrospray ionization- MS gave a molecular mass of 150 for both, and proton-NMR plus UV absorbance spectra identified them as E and Z isomers of p-coumaryl alcohol (p-CAlc). Alkaline cleavage of the p-CAlc fatty-acid esters in the presence of methanol or ethanol resulted in partial derivatization of E-p-CAlc to the corresponding gamma- O-methyl or O-ethyl ether. Gradient HMQC-NMR of the HPLC-purified stearate ester of E-p-CAlc 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 mechanism of fatty-acid esterification and the physiological role of these compounds are presently unknown.