60 Produce Degradation: Reaction Pathways and their Prevention
higher production of aroma volatiles, and hence better flavor quality (Fellman et al.,
2003). Flavor (aroma) generation is tightly associated with respiration and ethylene
production during the climacteric stage of ripening in fruits such as apples (Song
and Bangerth, 1996; Fellman et al., 2003). Knee and Hatfield (1981) found that
mature but unripe apples did not synthesize acetate esters; this was due directly to
inherently low rates of alcohol synthesis in mature but unripe fruit. If substrate
alcohols were supplied, then acetate esters were produced by apples at all states of
ripeness. Production of 2-methylbutyl acetate has been associated with “red apple
aroma” and as such has been suggested as an indicator for physiological maturity
in some apple cultivars (Mattheis et al., 1991; Young et al., 1996).
Generation of aroma volatiles is known to be inhibited in apples when they are
placed in controlled atmosphere storage for extended periods of time, and it can
take some time before volatile generation capability recovers (Girard and Lau, 1995;
Fellman et al., 2003). Maturity at harvest has an influence on this process. For
example, the more mature fruit of ‘Redchief Delicious’ apples are at harvest, the
more rapid the aroma volatile regeneration after removal from controlled atmosphere
storage (Fellman et al., 2003). Generation of mango aroma volatiles is similar to
that of apples: greater amounts are produced at more advanced maturity (Lalel et
al., 2003a,c). In addition, final eating quality is optimized if the fruit is harvested at
a mature-green stage (Lalel et al., 2003a). In litchi fruit, harvesting at a more
advanced state of ripeness tends to produce greater amounts of ethanol and acetal-
dehyde in modified atmosphere packages (Pesis et al., 2002). Increased production
of ethanol and acetaldehyde was associated with greater rates of deterioration in
litchis, since their accumulation was also associated with greater rates of decay in
the fruit once placed under poststorage shelf conditions (Pesis et al., 2002).
The relationship of at-harvest soluble solids (sugars) to soluble solids after
storage is somewhat confusing for apples. In ‘Cox’s Orange Pippin,’ ‘Jonagold,’
‘Gala,’ and ‘Braeburn’ apples, the final storage soluble solids content was similar
no matter what the initial soluble solids content was when the fruit was picked,
whereas ‘Fuji’ apples show an increase in poststorage soluble solids with later
harvests (Girard and Lau, 1995; Plotto et al., 1995; Fan et al., 1997b). In addition,
sensory panels indicated that late-picked ‘Fuji’ apples had improved overall flavor
characteristics (Plotto et al., 1995). The reason for the differences found for ‘Fuji’
compared with ‘Cox’s Orange Pippin’ and ‘Gala’ is likely related to the ripening
characteristics of the cultivar. Jobling and McGlasson (1995) demonstrated that
‘Fuji’ ripened in a different pattern than ‘Gala’ and suggested that maturity indices
used for ‘Fuji’ be based on different criteria than those for ‘Gala’ and other apples.
Titratable acidity declines with more advanced maturity in ‘Cox’s Orange Pip-
pin’ apples, and this leads to an apparent increase in sweetness of the apples, even
though sugar contents do not change substantially (Knee and Smith, 1989). A similar
pattern is seen with other cultivars such as ‘Jonagold,’ ‘Gala,’ ‘Braeburn,’ and ‘Fuji,’
except that soluble solids do increase somewhat with later harvests (Girard and Lau,
1995; Plotto et al., 1995). While later-harvested apples have lower titratable acidity,
the relative rate of decline in titratable acidity during storage does not change with
maturity (Plotto et al., 1995).