Changes in Free and Lipid-Bound Fatty Acids As Related to LOX-derived Volatiles in Ripening ‘Jonagold' Apple Fruit

Aldehydes from the oxidation of free and lipid-bound fatty acids are important components of the aroma of apple fruit.  These aldehydes are produced during mastication and cellular disruption by the action of lipoxygenase (LOX) and other enzymes of the LOX pathway.  Principal among these aldehydes are cis-3-hexenal, its isomer, trans-2-hexenal, and hexanal.  Cis-3-hexenal is produced primarily from linolenic acid (18:3) and hexanal from linoleic acid (18:2).  Trans-2-hexenal is likely the product of an as-yet uncharacterized isomerase acting upon cis-3-hexenal. We previously reported that, for ‘Jonagold’ fruit, aldehyde synthesis was, at first, extremely high for normally-ripening fruit such that the concentrations of cis-3-hexenal and hexanal were several hundred times higher than their respective odor thresholds.  Cis-3 hexenal declined markedly for normally ripening fruit, coincident with autocatalytic ethylene formation, even as trans-2-hexenal and hexanal emissions increased.  We extended this work by analysis of free fatty acids and the fatty acid components of polar and non-polar lipids of ripening apple fruit to evaluate changes in the content of the substrates for LOX pathway reactions.  Tissue analysis was on fruit ripening over an 8-week period in 2009 and 2010; evaluations were twice-weekly.  On each date, internal ethylene, CO₂ production, and volatile profiles were also measured.  Of the free fatty acids, 18:1 (oleic) and 18:2 increased several-fold during ripening, but 18:3 content was exceedingly low and was unchanged during ripening.  For polar lipids, the 18:1 and 18:2 fatty acid content increased modestly, while the 18:3 content declined sharply as ripening progressed.  The increase in 18:2 free fatty acids is mirrored by the increase in emissions of hexanal and hexyl esters during ripening, however, there does not appear to be a similar relationship between 18:3 free fatty acids and cis-3-hexenal emissions.  Rather, the decline in cis-3-hexenal more closely follows the pattern in the 18:3-content of polar lipids.  Interestingly cis-3-hexenal and trans-2-hexenal were almost completely dependent upon cellular disruption for their formation, whereas hexanal was not. The data suggest that the hexanal, and possibly, hexyl esters, may originate from LOX action on 18:2 free fatty acids in intact fruit.  The data are also consistent with the production of hexanal from 18:2 free fatty acid and/or 18:2-containing polar lipids following tissue disruption.  Cis-3-hexenal and trans-2-hexenal may be lacking from the volatile profile in intact fruit due to a lack of 18:3 free fatty acid substrate and that their formation following cellular disruption is largely from 18:3-containing lipid precursors.