Phenolic fatty-acid esters from the peel of 'Gala' apples and their possible role in resistance to superficial scald

Synthesis and oxidation of alpha-farnesene are thought to play a central role in development of superficial scald in apple and pear fruits. In particular, oxidation products referred to as conjugated trienes (CTs), which exhibit UV absorbance maxima at 259, 269 and 281 nm, appear to be causally linked to the disorder. Based on the observation that an A(258 nm):A(281 nm) ratio greater than or equal to 2.0 in hexane-dip extracts of apples was closely correlated with scald resistance, it was recently proposed that a CT species with relatively strong A(281 nm) induces scald, while a CT with relatively strong A(258) nm does not. It is now known, however, that the CTs produced in apple peel are mainly two isomers with identical UV spectra. Thus, the A(258 nm) in apple skin extracts which correlates with a low incidence of scald is likely due to components other than CTs. In a study of scald variability among apple cultivars, it was noted that hexane extracts of the peel from scald-resistant 'Gala' fruit exhibited a broad absorbance maximum at 258 nm. The compounds responsible for this UV peak are identified here as a family of phenolic fatty-acid esters. After partial purification, their UV spectrum was shown to have a second maximum at about 206 nm and two small shoulders at about 294 and 305 nm. These features are typical of isoflavonoid spectra. C-18-HPLC of the intact esters with UV monitoring at 210 and 260 nm revealed at least 20 components. Alkaline methanolysis yielded two main phenolics that were methanol/water-soluble and separable by C-18-HPLC. The fatty-acid methyl esters produced were mostly even chained and saturated, and four major HPLC peaks yielded methyl esters of 16:0, 18:0, 20:0 and 22:0. Pending verification, this is the first report of isoflavonoid fatty-acid esters in any plant tissue. In light of the antioxidant activity of phenolics, and isoflavonoids in particular, further investigation of a possible role of these compounds in natural resistance to scald is warranted. (C) 1998 Elsevier Science B.V. All rights reserved.