308 Produce Degradation: Reaction Pathways and their Prevention
damage in fruit and vegetable tissues. Moisture losses are undesirable in various
products such as salads and packed fresh-cut produce but they can be considered as
positive in some end-product applications such as drying and canning.
Relative humidity is also an important factor in the case of MAP of respiring
products. The majority of plastic films are impermeable to water vapor. Due to
respiration, the internal humidity inside the package increases and vapor condenses
on the film and also on the surface of the packaged produce. The condensed water
increases the risk of microbial decay, worsens the appearance of packaged products,
and also affects the barrier properties of the plastic films. The conditions of the MAF
when a passive MAF is used are also affected. The increase of humidity inside the
package and subsequent condensation of water vapor decreases the permeability of
permanent gases, but the more polar plastic films can soften in contact with water
and increase the permeability for gases. The humidity inside the package can be
controlled by the use of water absorbers and the condensation of water vapor (but
not the increase of humidity inside the package) can be avoided by the use of
hydrophobic plastic films (Vermeiren et al., 1999).
10.2.1.4 Light
Light is one of the factors that could affect the shelf life of fresh fruits and vegetables.
It is required for anthocyanin synthesis in apples. Access to light enhances the color
development of tomatoes, when chlorophyll degradation is accelerated by the red
component of the spectrum and the blue component increases the biosynthesis of
carotenoids (Jen, 1974). Light (including artificial lighting) can initiate chlorophyll
formation in potato tubers, which is accompanied by the production of toxic steroidal
glycoalakaloids (chaconine and solanine) (Salunkhe et al., 1974). Light should also
be taken in consideration in the case of storage of packaged vegetables. Light may
affect the gas composition in the packaging by inducing photosynthesis in green
vegetables (Gorris, 1996). Access to light, especially UV light, is important for
processed fruits and vegetables. It can trigger the degradation of sensitive compounds
such as vitamins and pigments and initiate lipid oxidation. It is recommended that
fruits and vegetables be stored in darkness (Ahvenainen, 2000).
10.2.1.5 Mechanical Wounding
Mechanical damage of plant tissues considerably affects physiological and enzy-
matic changes. Wounding stress results in metabolic activation, which is apparent
with increased respiration rate and, in some cases, ethylene production (Varoquaux
and Wiley, 1997). The wound stress sensitivity of fruits is also highly influenced by
their state of maturity. Ripeness usually increases susceptibility of fruit to wounding
during handling and processing (Gorny et al., 2000). The optimal stage of processing
to minimize cutting damage also varies greatly depending on the species, cultivar,
cultivation, and harvest and postharvest conditions (Solomos, 1997). Damaged parts
must be removed before subsequent processing. However, a “mechanical damage”
is a usual processing step in the production of minimally processed fruits and
vegetables, which need peeling, cutting, or shredding. Peeling can done on an
industrial scale in several ways (Fellows, 2000b):