Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

HEAT TREATMENT FOR ENHANCING POSTHARVEST QUALITY 247
Ethylene (uL g/h)
9
8
7
6
5
4
3
2
1
0
0 5 10 15
Days at 15°C
Control
38°C 2 day
Fig. 11.1 The ethylene production from mature green tomatoes held at 15 ◦ C. Control tomatoes were placed
directly in storage, and heated tomatoes were held for 2 days in 38 ◦ C air before storage. (Adapted from Lurie and
Klein, 1992.)
1-aminocyclopropane-1-carboxylic acid (ACC) oxidase activity decreased 90% compared
to untreated apples, which correlated well to the inhibition of ethylene produced by the
heated apples, while ACC accumulated to higher levels in heated apples than in control apples
(Klein, 1989). This indicated that a 38 ◦ C heat treatment inhibited ACC oxidase more
than ACC synthase. However, ACC synthase was inhibited by a 38 ◦ C heat treatment, but
more slowly than ACC oxidase in both apples (Roh et al., 1995) and kiwifruit (Antunes and
Sfakiotakis, 2000). The inhibition of the ethylene pathway recovered slowly during apple
storage after a heat treatment, and upon removal from storage ethylene production was often
higher than from untreated fruit (Klein, 1989; Lurie and Klein, 1991). The heat-induced
inhibition of ethylene synthesis was due both to direct inhibition of enzyme activity and to
reduced synthesis of new enzyme. The abundance of mRNA of ACC oxidase was strongly
depressed at 38 ◦ C (Lurie et al., 1996).
The inhibition of ripening due to lack of ethylene is reversible if the heat treatment is
not too extended and does not cause damage. Volatile production inhibited by 38 ◦ C hot
air treatment of apples, or 42 ◦ C hot water immersion of tomatoes recovered following the
treatment, particularly if the fruit were stored for a period of time (Fallik et al., 1997;
McDonald et al., 1998). Lycopene synthesis, inhibited by heat treatment of tomatoes, also
recovered, and heat-treated tomatoes ripened normally (Lurie and Klein, 1992; McDonald
et al., 1998). In fact, if the fruits were stored at low temperatures, the heat-treated tomatoes
ripened normally (Fig. 11.2) while control fruits decayed as a result of chilling injury
(Lurie and Klein, 1992). Polygalacturonase, an enzyme involved in digestion of cell walls
leading to fruit softening is induced by ethylene. High temperature inhibited the activity of
this enzyme and affected fruit softening in both mango (Ketsa et al., 1998) and tomatoes
(Mitcham and McDonald, 1992). High temperature also inhibited β-mannanase and α- and
β-galactosidase activities in tomatoes (Sozzi et al., 1997). Again the inhibition of these
ripening processes of color development, volatile evolution, and softening, as with ethylene
synthesis, was at the level of both enzyme activity and gene expression.
Nonclimacteric fruits also show effects of reduced softening rate and color development
following heat stress. Strawberries, either hot air heated for 3 h or hot water heated for
15 min at temperatures from 40 to 50 ◦ C and then held at 20 ◦ C, had delayed color development
and reduced firmness loss compared to unheated berries (Garcia et al., 1995; Civello