Volatile ester-synthesising capacity throughout on-tree maturation of ‘Golden Reinders’ apples

Volatile compounds emitted by apple (Malus × domestica Borkh.) fruit change during maturation and ripening, and hence maturity stage at harvest is a key factor determining fruit aroma. In this work, production of aroma volatile compounds and some related enzyme activities were monitored weekly in ‘Golden Reinders’ apples during 7 weeks prior to commercial harvest. Three maturity stages (immature, mature unripe and ripe) could be defined by the variables studied. Esters were quantitatively prominent among the compounds identified in the volatile fraction, and in most cases their production was higher at later stages of fruit maturation. Hexyl acetate, hexyl propanoate and hexyl 2-methylbutanoate, together with ethyl 2-methylbutanoate, 2-methylbutyl acetate and butyl 2-methylbutanoate, were found to be important contributors to the characteristic aroma of ‘Golden Reinders’ apples. Little variations in alcohol o-acyltransferase (AAT) activity levels were found throughout the experimental period. Higher lipoxygenase and hydroperoxide lyase activities were found in the skin than in the flesh tissue, suggesting involvement of this tissue in volatile ester-forming capacity during fruit maturation. The enhancement of volatile ester production by ‘Golden Reinders’ apples at late maturity stages may have arisen mainly from greater availability of substrates, thus pointing out the relevance of regulating points located upstream of AAT in the pathway.

► Biosynthesis of aroma volatile compounds was monitored during on-tree maturation.
► Most esters increased at later stages, without large variations in AAT activity.
► Enhanced volatile ester-forming capacity arose from improved supply of precursors.
► Noticeable differences in activity levels were found between skin and flesh.