Relating Apple Volatile Biology with Aroma Perception During Fruit Maturation

Aroma volatiles from fresh apple and other fruit result from autonomous biological activity and biological activity following cellular disruption, as during mastication.  The lipoxygenase (LOX) pathway has been implicated in both autonomous and cell disruption-dependent processes.  We investigated odor-active volatiles from ‘Jonagold’ fruit resulting from cellular disruption throughout maturation to better understand the relationship between volatile biology and perceived apple aroma.  The study was conducted over 8 weeks and evaluations were twice-weekly.  On each date, internal ethylene, CO₂ production, and volatile profiles were measured for whole fruit and crushed slices of 1-methylcyclopropene (1-MCP) treated (‘non-ripening’) and control (‘normally ripening’) apples.  Only crushed slices were subjected to sensory analysis; sensory tests and volatile analyses of slices were performed approximately two minutes after crushing.  Unexpectedly, panelists were able to distinguish between non-ripening and normally ripening fruit more than two weeks before the onset of the ethylene and respiratory climacterics in the normally ripening fruit.  LOX-derived aldehydes cis-3-hexenal and hexanal were not detectable for whole fruit, consistent with dependence of LOX pathway activity on cellular disruption.  For crushed slices, however, aldehyde synthesis was, at first, extremely high for non- and 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 remained constant throughout the experiment for non-ripening fruit slices, but declined markedly for normally ripening fruit slices, coincident with autocatalytic ethylene formation.  Conversely, hexanal emissions increased in normally ripening fruit slices at this time, but not in non-ripening fruit slices. The autonomously-produced ester butyl acetate increased coincident with panel detection of treatment differences, although the concentration of butyl acetate at this early stage of development was well below the human olfactory threshold.  The data for autonomous and disruption-dependent aroma volatiles do not explain sensory analysis results.