Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

CHANGES IN NUTRITIONAL QUALITY OF FRUITS AND VEGETABLES 445
been found that in some temperate fruits such as pears, ethylene synthesis can be induced
by low temperature.
The precursor for ethylene biosynthesis is methionine, which is converted to S-adenosyl
methionine (SAM) in the presence of the enzyme methionine adenosyl transferase. Next,
the enzyme ACC synthase converts SAM into 1-aminocyclopropane-1-carboxylic acid
(ACC), which is the immediate precursor of ethylene. The enzyme 1-aminocyclopropane-
1-carboxylic acid oxidase (ACO) converts ACC into ethylene. At low temperature, accumulation
of the enzyme ACO is induced (Lelièvre et al., 1997). During modified and controlled
atmosphere (CA) storage, the level of oxygen also affects the rate of ethylene production.
At low levels of oxygen (1–3%), ethylene production is reduced because oxygen is a cosubstrate
for ACO (Lelièvre et al., 1997). Studies have demonstrated that ethylene controls
several organoleptic quality changes that occur during ripening (Tian et al., 2000; Zhu et al.,
2005; Gao et al., 2007). Binding of ethylene to its receptors accelerates the process of fruit
ripening and senescence. It has been proposed that after binding to its receptors, ethylene
releases calcium from storage compartments, which initiates phospholipid degradation by
binding of PLD to plasma membrane (Pinhero et al., 2003).
Several methods have been developed for the inhibition of ethylene biosynthesis and
its action. Aminoethoxyvinylglycine (AVG), marketed commercially as ReTain R○ ,isaninhibitor
for ACC synthase, which is an important step in ethylene biosynthesis. Delayed fruit
ripening and reduced fruit drop was noted in AVG-treated apples and pears (Rath et al.,
2006). 1-Methylcyclopropene (1-MCP) inhibits ethylene action by preventing its binding to
the ethylene receptors, which enhances fruit shelf life and firmness. It is marketed commercially
as SmartFresh TM and is used for postharvest treatments in apples and tomatoes. This
treatment is effective at low concentration, and no residue has yet been detected in treated
fruits. Its affinity to the receptors is 10-fold greater than that of ethylene (Blankenship and
Dole, 2003). The use of 1-MCP for different horticultural crops has been approved in several
countries.
The optimally effective concentration and exposure time for 1-MCP vary with the
commodity and treatment temperature. For apples, the effective concentration to delay
ripening is 1 ppm. It has been reported that exposure to a lower concentration of 1-MCP for
a longer duration has the same effect as the exposure to a higher concentration for a short
duration. In cut carnations, the effect of exposing to 250–300 nL/L of 1-MCP for 5 min
was the same as that of an exposure to 0.5 nL/L for 24 h (Blankenship and Dole, 2003). It
has been observed that the effect of 1-MCP in preventing loss of fruit firmness is decreased
at lower temperature. This could be because of lower affinity of 1-MCP binding at low
temperature (Blankenship and Dole, 2003). It was noted that apples at 3 ◦ C required 9 h of
1-MCP treatment to maintain fruit quality during prolonged CA storage; however, this time
requirement was reduced at higher temperature. Moreover, treatment time and temperature
also depend on cultivars. In order to get the same physiological effects of 1-MCP at the
same concentration, “Empire” apples needed less treatment time than “Cortland” apples
(DeEll et al., 2002).
21.4 Fruit quality enhancing metabolic pathways
Several metabolic pathways are involved in the synthesis of components that determine the
organoleptic qualities during fruit ripening. Respiration is increased during ripening, which