118 Produce Degradation: Reaction Pathways and their Prevention
5.1 INTRODUCTION
Consumer demand for fresh fruit and vegetables has triggered the need for proce-
dures, including packaging, that maintain the freshness, safety, and quality attributes
of these commodities. Research aimed at improving the quality and extending the
shelf-life of these commodities has been conducted in the fields of physiology,
microbiology, and nutrition.
The first way of reducing postharvest spoilage is to lower the temperature. Low
temperature exponentially reduces the physiological activities of both plant tissues
and microorganisms. The effects of refrigeration are limited by the sensitivity of
some fruits and vegetables to chilling injury. The other technique, most often asso-
ciated with the first one, is modification of the storage atmosphere.
Controlled atmosphere (CA) is most often used for long-term storage of fruits
(a few months) or for transportation in mobile containers (a few weeks). Modified
atmosphere packaging (MAP) permits atmosphere modification in smaller contain-
ers. The internal atmosphere may be changed actively, flushing the packaging with
a gas mixture at sealing, and/or passively by natural equilibration between the
respiration of the plant tissue and diffusion through the permeable membrane of the
pack or tray. This technique is more difficult to handle than CA because the gas
transmission rate (permeability divided by the thickness of the film) of the films or
exchange membranes must be properly optimized; otherwise, MAP may be ineffi-
cient or even detrimental [1]. For example, a film with a permeability that is too
low for O 2 and for CO 2 leads to anoxia and fermentation of plant tissues [2]. The
same phenomenon may occur if the package is stored at a temperature higher than
that of the recommended cold storage [3].
In order to define the optimal permeances (or gas transmission rates) of the
packaging film for O 2 , CO 2 , and water vapor, the respiration rate of plant tissue must
first be measured as a function of temperature and atmosphere, and then the O 2 and
CO 2 concentrations that maximize shelf life must be determined. The best combi-
nation of films, product, and temperature is computed and verified experimentally.
The standardized unit for gas partial pressure in the atmosphere is the Pascal (Pa).
Since 1 kPa = 0.00987 atm, it is possible to approximate 1 kPa to 1% of the
considered gas in the storage atmosphere under atmospheric pressure.
In this chapter we review the effects of gases on respiration (aerobic and anaer-
obic catabolisms), on growth of spoilage microorganisms, and on quality attributes
of fruits and vegetables. A general procedure to optimize modified atmosphere
packaging is proposed.
5.2 MAP OPTIMIZATION
5.2.1 RESPIRATION RATES
Fresh fruits and vegetables are living tissues and must stay alive until their final
consumption by consumers. Respiration is a catabolic process that supplies the
energy (ATP and ADP) for the plant to thrive and, after harvest, to survive. This
energy is mostly provided by the oxidation of organic metabolites: sugars, lipids,