Phenolic fatty-acid esters from the peel of `Gala' apples and their possible role in resistance to superficial scald
Bruce D. Whitaker
Postharvest Biology and Technology Vol: 13 Issue: 1 Pages: 1-10.
1998
บทคัดย่อ
Synthesis and oxidation of -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 A258
nm:A281 nm ratio
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 A281
nm induces scald, while a CT with relatively strong A258 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 A258 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. C18-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 C18-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.