Packaging and Produce Degradation 145
some phenomena can interfere with the product respiration rate. Fruits and vegeta-
bles are contaminated by a wide range of microorganisms, the growth of which is
more or less affected by the storage atmosphere. Their growth will depend mostly
on temperature, relative humidity, and host-parasite resistance, which is extremely
complex to model. No model to date has integrated the effect of microorganisms on
respiration rate. Therefore, the prediction of atmosphere changes in the MAP system
and the optimization of packaging conditions is only possible when microorganisms
do not interfere with plant catabolism. To overcome this limitation it is recommended
that the shelf-life duration (sell-by date) be matched to the lag period of microor-
ganism growth at the distribution temperature. This difficulty underlines the utter-
most importance of either efficient disinfecting of packed commodities ([154] and
in this book Chapter 16) or active films with antimicrobial activity [155]. The
dynamic response of stored fruits and vegetables to their environmental conditions
(e.g., exposure to endogenous ethylene or changes in CO 2 and hypoxia sensitivities
due to natural senescence) is not taken into account in any proposed mathematical
simulations of MAP [150]. These considerations emphasize the role of new active
packaging material with special functions, such as ethylene scavenging and new
pretreatment of climacteric plant organs with 1-methylcyclopropene (1-MCP) [156]
or any other process reducing ethylene production or effects.
Moreover, some atmosphere compositions that are easy to generate and maintain
in CA are almost out of reach in MAP. For example, low O 2 –low CO 2 atmospheres
require diffusion membranes with very high permselectivity such as hydrophilic
films, either biofilm or polymeric membranes, which are not available on the market
or do not meet the minimal conditions for practical use.
In the case of temperature abuse, the Q 10 of the respiration rate (from 2 to 3)
does not match the Q 10 of the gas diffusion rate through the diffusion membrane
(from 1 to 2), thus resulting in a detrimental deviation of the atmosphere. Hydrophilic
film may match this difference in Q 10 for CO 2 but not for O 2 diffusion rate.
The choice of a packaging film depends on many other parameters not reviewed
in this chapter, including the following.
5.4.1 WATER TRANSMISSION RATE
This characteristic of the film will govern the dehydration rate of the commodity. A
moderate dehydration may be beneficial to some vegetables such as onion bulbs
[157] and mushrooms [158,159] but detrimental to most other fruits and vegetables
[160,161]. Conversely, excessive relative humidity favors fungal and bacterial decay
[162].
5.4.2 CONSUMER ACCEPTANCE OF APPEARANCE AND HANDLING
The choice of a film to pack fresh-cut commodities depends more often on the
physical appearance of the film, which must be glossy, transparent, and crispy, than
on the film’s permeability to gases. To match the commercial quality of polypropy-
lene to high-respiring commodities the membrane is microperforated.