170 Produce Degradation: Reaction Pathways and their Prevention
of ethylene on quality center on its ability to alter or accelerate the same natural
processes of development, ripening, and senescence that are viewed as beneficial in
a different context [172].
Ethylene may be controlled by various strategies such as isolating ethylene-
sensitive produce away from commodities that produce high levels of ethylene (or by
storage of unripe fruit away from ripe ones), providing adequate ventilation, use of
ethylene scrubbers (catalytic or adsorption mechanisms), use of ethylene antagonists
such as CO 2 , or by destruction of ethylene by reaction with ozone [167].
The inhibition of ethylene biosynthesis or action will inhibit not only ripening
but also the production of characteristic aroma compounds. Intentional exposure of
ethylene is primarily used to ripen harvested climacteric fruit [172]. In general, the
effect of ethylene on taste and aroma is caused by stimulating the ripening of fruit
[173]. Fruits ripened in this way most often have inferior flavor profiles since
production of aroma compounds is diminished [174,175], although some exceptions
have been reported [73]. In the case where fruit have been stored for a prolonged
period in an atmosphere in which ethylene activity has been suppressed, it may take
some time after reintroduction to air in order for volatile production to return to
normal and reestablish the characteristic aroma profile [172].
6.4.3 CONTROLLED AND MODIFIED ATMOSPHERIC STORAGE
Fruits and vegetables are living materials, and rate of respiration is a critical factor
in determining shelf life. Thus, the greater the respiration, the shorter the shelf life.
Immature produce such as peas and beans have greater respiration rates and, there-
fore, shorter shelf lives, whereas the opposite is true for mature storage organs
(rhisomes) such as potatoes and onions. Excessive respiration in nonstorage tissues,
such as leafy vegetables or immature flower produce such as broccoli, leads to rapid
degradation due to rapid depletion of energy reserves. Climacteric fruit, such as
apples, bananas, and tomatoes, can be harvested in the unripe stage and then artifi-
cially ripened at a later stage. The respiration rate of these fruits must be carefully
controlled and monitored (e.g., temperature and ethylene levels), because the rate
is known to increase dramatically during a short time period, leading to overripening,
senescence, and generation of uncharacteristic volatile compounds or off-odors.
The basic principle behind the use of controlled (CA) and modified atmospheres
(MA) during storage is to reduce the rate of respiration, reduce microbial growth,
and retard enzymatic deterioration by manipulating the gaseous environment sur-
rounding the produce. Generally, this is achieved by reducing the concentration of
O 2 , which is required for respiration, or by adding an inhibitory gas such as CO 2.
CA storage generally refers to use of decreased O 2 and increased CO 2 with contin-
uous monitoring and adjustment of the gas composition. CA storage of produce has
been recently reviewed and optimal conditions have been determined for various
fruits and vegetables [167,176].
In contrast to CA, the gas composition used in MA packaging (MAP) is neither
monitored nor adjusted. Eventually an equilibrium gas composition will be reached
in MAP, which is a result of a balance between metabolic rates of the produce and
diffusion characteristics of the packaging materials [164]. In MAP, polymeric films