Mechanisms of Food Additives, Treatments, and Preservation Technology 307
oriented polypropylene. The shelf life of shredded cabbage and grated carrots pack-
aged in these materials is 2 to 3 days longer (7 to 8 d at 5°C) than in those packaged
in oriented polypropylene, which is generally used for this purpose (Ahvenainen,
2000). The application of these “new” types of packaging films (microperforated or
perforated films) can be unsuitable for produce that is sensitive to CO 2. The rate of
gas movement through a perforated film is a sum of gas diffusion through the
perforation and gas permeation through the polymeric film. The flow rate of CO 2
and O 2 through perforations is similar and the concentration of CO 2 inside the
package can reach the critical limit (Mir and Beaudry, 1999). New developments
also include films that change permeability to moisture and gases under specified
temperatures designed to match the respiration rate of fresh produce. An example
is the recent development of adjustable “temperature switch point” films that contain
long-chain fatty alcohol-based polymeric chains. At a predetermined temperature-
switch point, these chains are in a crystalline state, providing a gas barrier. When
this point is reached, the side chains melt to a gas–permeable, amorphous state.
Another system consists of film made from two different thicknesses of the same
material, both layers containing minute cuts. When temperature rises or falls, the
layers expand at different rates. As the temperature rises, the film at the cut edge
retracts and curls upward, the holes enlarge, and permeability significantly increases
(Ahvenainen, 2000). In addition to these “smart” packaging films other active pack-
aging systems can be used for MAP of fresh produce (Vermeiren et al., 1999).
Oxygen scavengers that are important in packaging of nonrespiring foods are not
effective because of the usual lack of oxygen inside the package, but ethylene
scavengers can be used. For MAP purposes, an absorbing system similar to that
used for controlled storage can be employed, especially C 2 H 4 scavengers based on
potassium permanganate (KMnO 4 ), which oxidizes ethylene to acetate and ethanol
(Zagory, 1995). Another scavenging system is based on the adsorption and subse-
quent breakdown of ethylene on activated carbon, charcoal containing PdCl as a
metal catalyst. This is effective at 20°C for some types of fresh produce (kiwifruits,
bananas, and spinach) (Abe and Watada, 1991). The system based on the incorpo-
ration of finely dispersed minerals such as zeolites, clays, or Japanese oya into the
packaging films incorporates minerals that can absorb ethylene and affect the per-
meability of the films (Suslow, 1997). Ethylene scavengers are not yet very successful
in MAP, probably because of insufficient adsorbing capacity. They are more fre-
quently used in controlled atmosphere storage and in the case of MAP will become
more important after the principal problems with internal atmosphere composition
have been solved. Scavenging systems for CO 2 could be useful for reduction of
internal CO 2 in the package. Various systems based on the absorption of CO 2 by the
reaction with Ca(OH) 2 , such as those used for packaging roasted coffee, have not
yet been applied to fresh produce (Vermeiren et al., 1999).
10.2.1.3 Humidity
Relative humidity is another very important environmental factor in the shelf life of
fresh produce. A low-humidity environment reduces growth of decay microorgan-
isms, but it can increase moisture loss. Moisture loss is associated with structural