|Abstract or Summary
- Volatile compounds of eight pear varieties were isolated and identified by
GC/MS. A total of 112 components were identified from headspace of intact fruits,
including 47 compounds reported for the first time in pear. The volatile profiles of
these pear varieties were characterized by esters, alcohols, hydrocarbons, aldehydes
and ketones. Qualitative and quantitative differences of compounds in the profiles are
discussed in terms of flavor differences between pear varieties.
Analysis of volatiles isolated from eating-ripe pears by direct sniffing GC
indicated that a group of nine esters including 2-methylpropyl-, butyl-, pentyl- and
hexyl acetates, butyl butanoate, ethyl hexanoate, methyl trans-2, cis-4-, ethyl trans-2,
cis-4- and ethyl cis-2, cis-4- decadienoates from 'Bartlett', 'Cornice' and 'Anjou' pear
possessed strong, specific pear-like aroma. A second group of esters was also
found to possess very positive aroma and can significantly affect overall sensation by
serving as 'contributory to flavor compounds'.
A newly developed, solid-phase microextraction technique was used to follow
changes of volatile production of 'Bartlett', 'Packham's Triumph' and 'Anjou' pears
during ripening. The major esters were found to increase at different rates and times
during fruit ripening. Highly significant correlations between specific groups of odor-active
compounds and pear flavor intensity during ripening indicated the significant
contribution of volatiles to overall pear flavor.
The effects of harvest maturity on volatiles differed between fruit varieties.
'Bartlett' harvested at all maturities, upon ripening produced similar amounts of flavor
constituents. Higher volatile production was observed in early harvested 'Packham's
Triumph'. Delayed harvest significantly reduced volatile production in 'Anjou'.
Prolonged storage of fruits resulted in severe reduction of volatile esters in
'Packham's Triumph' and 'Anjou'.
Alcohol acyltransferase, the ester synthesizing enzyme was isolated from
'Bartlett', partially purified, and its properties characterized. The enzyme was
estimated to have a molecular weight of 40 kD and was most active at pH 7.5 and
3O°C. The enzyme has Km= 0.6 mM and Vmax = 0.3 nmol/min for hexanol and Km= 53
μM and Vmax = 0.2 nmol/min for acetyl-CoA. Maximum activity of the enzyme was
obtained when hexanol and acetyl-CoA were used as substrates.