The metabolism of alcohols by apple fruit tissue

Abstract

The interconversion of short-chain aliphatic alcohols, aldehydes and esters by apple fruit has been studied by gas-liquid chromatographic analysis of the products when substrates (mainly alcohols) were supplied as vapours to fruit slices at 20°C, and to whole fruit during storage at 3°C. In preliminary experiments with fruit from various sources after different periods of storage, cortical tissue slices formed corresponding aldehydes and acetate esters from alcohols; esterification was more rapid with higher alcohols. Esterification by peel tissue slices was more rapid initially but quickly declined. Metabolism was further investigated with a batch of apples during storage under different conditions. Both cortex and peel tissue were capable of acetylating butanol and 2-methyl propanol at all stages of maturity, from the pre-climacteric state at normal harvest time, when endogenous ester levels were low, through 58 days ripening at 12°C. The tissues showed constant hydrolytic activity towards butyl acetate throughout this period. Whole apples in air and 2% 0 2 at 3°C took up ethanol but formed little ethyl acetate and retained very little free ethanol. In both atmospheres whole apples metabolised approximately 40 % of added hexanol to hexyl acetate and up to 8% to hexanal. The low levels of esters in apples from low oxygen atmospheres (and the absence of esters in unripe apples) are, therefore, a consequence of low rates of alcohol synthesis.

Biochemistry of apple aroma: A review

“…Alcohols are the second most important compounds that contribute to the aroma of ripe apples aft er esters (33), the most abundant being 2-methyl-1--butanol, 1-butanol, 1-hexanol, 1-propanol and 2-methyl--1-propanol (14,(34)(35)(36). They are direct precursors of esters (6), which may occasionally be fragmented to their corresponding alcohols and fatt y acids by the action of esterases (37)(38)(39)(40)(41). It has been reported that apples synthesize more than 40 alcohols in apples (6).…”

“…In ripe apples, alcohols constitute 6 to 16 % of the total volatiles (8), whereas esters represent, depending on the apple variety, from 80 % in Golden Delicious and Golden Reinders (14,45) to 98 % in Starking Delicious (36). Esters can have linear or branched chains, and their concentration depends on the availability of precursors (37,46,47) and on the selectivity and activity of the enzymes involved (48)(49)(50)(51). Butyl acetate, hexyl acetate, 2-methylbutyl acetate and ethyl 2-methyl--butanoate are the most important esters due to their high content and impact on apple aroma of several varieties (6,52).…”

“…However, storage conditions may reduce volatile compound biosynthesis in apples (184,187) depending on the type of storage atmosphere employed and the length of the storage period (184,188). Modifi ed atmospheres, especially those with ultra-low oxygen (p(O 2 )<1 kPa), cause a reduction in the biosynthesis of linear chain volatiles due to a reduction in the concentration of alcohols and their esters (37,179), except for ethanol and its derivatives, which are produced in apple under anaerobic conditions (10,187). The synthesis and degradation of fatt y acids also decline (15,17,184) due to the reduction in β-oxidation, LOX activity and ethylene biosynthesis (22).…”

“…During postharvest, concentration of esters may decrease due to their hydrolyzation by carboxylesterases to form their respective acids and alcohols, and by their diffusion in the environment (37)(38)(39)(40)(41). However, a low ester content can be caused rather by the lack of alcohols as precursors than by esterase activity, or diff usion (15,37,191).…”

Identification of Genes with Potential Roles in Apple Fruit Development and Biochemistry through Large-Scale Statistical Analysis of Expressed Sequence Tags

“…Ester formation in apple is largely confined to the fruit tissue and, more specifically, the skin (Knee and Hatfield, 1981). It has been demonstrated that the LOX and b-oxidation pathways permit the catabolism of fatty acids during fruit ripening to supply precursors for ester formation (Rowan et al, 1999).…”