Maturity, Ripening, and Quality Relationships 63
influence on the development of yellow coloration. Yellowing in broccoli is consid-
ered a major determinant of shelf life (Toivonen and Sweeney, 1997). As broccoli
florets mature, they become more susceptible to yellowing, presumably since the
florets are approaching a natural vegetative senescence stage (Watada et al., 1984;
Tian et al., 1995). A similar relationship has been found with postharvest yellowing
in cucumbers (Schouten et al., 1997), which are harvested as immature fruit but
begin to approach a normal ripening stage if harvested too late (Watada et al., 1984).
3.3 ASSOCIATION OF HORMONAL CHANGES
IN RELATION TO QUALITY3.3.1 ENDOGENOUS GROWTH HORMONES
Ethylene is the most studied endogenous hormone in postharvest research. Levels
as low as 0.005 μL L–1 of ethylene have been shown to have significant effects on
promoting premature senescence in strawberries, oranges, lettuce, beans, Chinese
cabbage, bak choi, choi sum, and gai lan (Wills, 1998). The reduction of shelf life
(as determined by consumer acceptability) was proportional to the logarithmic con-
centration of ethylene, whether the product was stored at 0°C or 20°C (Wills, 1998).
Removal of low levels of endogenously generated ethylene has been shown to
significantly enhance shelf life in strawberries in overwrapped punnets (Wills and
Kim, 1995). It appears that low levels of ethylene lead to premature softening and
decay in fruits and vegetables that are not climacteric and show little respiratory
response to ethylene (Wills and Kim, 1995; Wills, 1998).
Endogenously produced ethylene in climacteric fruits such as apples, pears, and
tomatoes eventually leads to an exponential rise in ethylene synthesis, resulting in
highly accentuated levels of ethylene (Figure 3.3). This dramatic rise in ethylene
production is associated with rapid biochemical changes in the fruit that lead to what
is generally referred to as ripening (Saltveit, 1999).
3.3.2 EXOGENOUSLY APPLIED HORMONES
3.3.2.1 Ethylene
The following discussion will highlight several points of interest in regard to the
effects of exogenously applied ethylene on quality deterioration in fruits and vege-
tables. More detailed discussion of effects and sources of exogenous ethylene can
be found in a review by Saltveit (1999).
Exogenously applied ethylene accelerates the ripening process (Abeles et al.,
1992) and hence would generally be considered as detrimental to product quality
retention (Saltveit, 1999). In many cases exogenous ethylene production is indeed
detrimental to quality, but in some cases the application of exogenous ethylene is
used to allow harvest of fruit such as bananas at a mature but unripe stage, and
ethylene exposure drives ripening just before delivery of product to the consumer
(Saltveit, 1999). This allows a normally damage-prone fruit to be shipped in a
relatively bruise-resistant, immature stage over long distances (Kader, 2003). Pears