75–80% for dry onions. Stored in air with a lower RH, most vegetables
lose water with the rate of transpiration dependent on water vapor pres-
sure deficit and the resistance of the tissue to water loss. Storage in air
with an RH at 97–98% requires a proper temperature control. Even small
temperature changes will result in water condensation that may promote
microbial decay. Recommended humidities for individual products are
usually a compromise between these two conflicting requirements.
Air Composition
Metabolic processes in plant cells are mainly either an oxidation or a
reduction. The availability of oxygen at the reaction center will there-
fore determine the rate of many catabolic processes that influence qual-
ity. Respiration in plants is an oxidative degradation of sugars, organic
acids and lipids to produce carbon dioxide and water with release of en-
ergy. Modifying the atmosphere around the produce, i.e., lowering the
amount of O 2 with an increased amount of CO 2 , may lower the metab-
olism with decreasing CO 2 production and O 2 consumption. The effects
of low O 2 and high CO 2 are additive but the optimal concentrations of
the two gases in the storage atmosphere vary between vegetables and
even between cultivars of the same species (Saltveit, 1989).
Due to the high affinity for O 2 of the terminal oxidazes in the elec-
tron transport chain located in the mitochondria, the amount of O 2 in the
surrounding air must be reduced below 10%. On the other hand a change
to anaerobic respiration will take place if the O 2 concentration ap-
proaches 2%. The effects of an increased CO 2 concentration are more
difficult to explain since elevated CO 2 normally stimulates respiration
in stored roots and bulbs (Weichmann, 1977). High CO 2 levels are re-
ported to inhibit enzymes in the Krebs cycle (Nanos et al., 1994) and
decreasing pH of the cell sap.
The benefit of modified-atmosphere storage relies on the application
of an optimal storage temperature. Under these circumstances, modified
atmospheres may delay quality deterioration and prolong the storage pe-
riod. Long-term (6 months) controlled atmosphere (CA) storage is
mainly used for storage of climacteric fruit, onions, cabbage and Chi-
nese cabbage. The benefit of CA storage of these vegetables is a delayed
vernalization and sprouting, retention of color due to retarded break-
down of chlorophyll and reduced microbial decay (Isenberg et al., 1987).
Modified atmosphere (MA) has become a valuable tool for consumer
packaging of vegetables (MAP) and mainly minimal processed refrig-
Influences of the Environment 105