Lipoxygenase-associated apple volatiles and their relationship with aroma perception during ripening

Several important odor-active volatiles are produced via processes initiated by cellular disruption brought about by cutting or mastication. Of these, six-carbon (C6) volatiles, including the aldehydes, cis-3-hexenal, its isomer trans-2-hexenal, and hexanal, as well as their corresponding alcohols, are produced from action of the lipoxygenase (LOX) pathway on substrates released by tissue disruption. We investigated the production of these and other odor-active volatiles from ‘Jonagold’ apple fruit for whole and disrupted tissues as a function of maturity/ripeness. The impact of ripening was determined by comparing ‘normally-ripening’ fruit with ‘non-ripening’ (1-methylcyclopropene-treated) fruit. The study was conducted over 8 weeks and evaluations were twice-weekly for two successive seasons. C6 aldehyde synthesis by crushed fruit was, at first, extremely high for non- and normally-ripening fruit such that the concentration of the aldehydes was several hundred times higher than their human odor thresholds in the containers used to present samples to the sensory panelists. cis-3-Hexenal, which remained constant throughout the experiment for non-ripening fruit, declined markedly for normally-ripening fruit, coincident with the onset of ripening. Conversely, trans-2-hexenal and hexanal increased in normally-ripening fruit as ripening progressed. Hexanol and hexyl acetate and other esters were produced in a ripening-dependent manner for disrupted as well as intact fruit tissues. The exception was cis-3-hexenyl acetate, which had same declining pattern as its aldehyde precursor, cis-3-hexenal. PCA determined that all volatiles except cis-3-hexenal, cis-3-hexenol and cis-3-hexenyl acetate, were highly associated with ripening. Also, cis-3-hexenal and cis-3-hexenyl acetate correlated negatively with all other variables (volatiles, ethylene, CO2 and sensory tests). Sensory tests revealed that panelists were able to distinguish between non-ripening and normally-ripening fruit approximately 2 weeks before the onset of autocatalytic ethylene, the respiratory climacteric, and ripening-dependent ester production of normally-ripening fruit. The data indicate a shift in LOX pathway activity occurs during ripening, but it is not clear if it is due to changing enzyme activities or changing substrate levels. The shift in pathway activity may be related to perceived changes in aroma for macerated fruit prior to autocatalytic ethylene production.

► Aroma changes begin before the onset of autocatalytic ethylene.
► cis-3-Hexenal from disrupted tissue declines during ripening, which has not been reported.
► Hexanal formation increases as cis-3-hexenal declines from disrupted tissue.
► Data suggest substantial changes in the fatty acid composition take place during ripening.
► An as-yet undiscovered isomerase is implicated in aldehyde metabolism during ripening.