Produce Degradation Pathways and Prevention

64 Produce Degradation: Reaction Pathways and their Prevention

are an example of temperate fruit that requires chilling or ethylene exposure to
initiate and synchronize ripening and softening so that the product will be acceptable
to the consumer (Agar et al., 1999). However, exogenous ethylene exposure can also
be unintentional in postharvest handling systems, the two most common examples
being the production of ethylene by propane-powered fork-lifts in produce handling
areas and mixed loads containing ethylene-producing fruits and ethylene-sensitive
fruits and vegetables (Saltveit, 1999).
Exogenous ethylene exposure can have negative as well as positive effects on
fruits and vegetables. A good example of this point is the case of tomatoes, which
are often picked mature green and ripened with ethylene during transport to market
(Chomchalow et al., 2002). Tomato fruits exposed to exogenous ethylene treatments
to induce ripening have been shown to maintain better vitamin C content than fruit
left to ripen without treatment (Chomchalow et al., 2002). This was presumably due
to the fact that treated tomatoes reached the fully ripe eating stage faster than
untreated fruit, and hence less time was available for vitamin C loss. However,
mature green tomatoes exposed to exogenous ethylene have less vitamin C content
than fruit left on the vine to ripen (Soto-Zamora et al., 2001); this was believed to
be associated with higher levels of ascorbate oxidase and peroxidase activity in
ethylene-treated fruit. These two enzymes are believed to reduce vitamin C content
in fruit tissue (Soto-Zamora et al., 2001). These results with tomatoes show that the

FIGURE 3.3Phases of the climacteric period. (From Watada et al., HortScience, 19, 20,

1984. With permission.)

Rate of CO

or C 2

H 2

Production 4

Time

Preclimacteric minimum

Preclimacteric

Climacteric rise

Climacteric Peak

Postclimacteric