11.6.4 Physiology of controlled atmosphere storage

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How do these altered atmospheres delay ripening and retard senescence? Several routes are possible. One common obser-vation is that fruit respiration drops in response to the changed atmosphere (Knee 1991). This could occur via acidification of the cytosol, resulting from an elevated CO2 concentration redirecting metabolism towards alcohol or lactate/succinate or malate production rather than CO2 production. Another alternative is a direct effect of ultra-low O2 concentrations (<2%) on cytochrome c oxidase in the mitochondrial electron transfer pathway, preventing that enzyme from functioning properly. When both high CO2 and low O2 concentrations are combined then the beneficial effects may be additive.

Fruit differ with respect to critical values for tolerance to O2 or CO2 concentrations, and ideally we might make a model for predicting the tolerance limits for a new cultivar or fruit from our specific background information on its physiological behaviour. However, there is a key problem in manipulating atmospheres by static modelling approaches. The critical gas composition exists within the flesh of a fruit, not in the environment around it, while differences in genetic background cause each cultivar to behave differently with respect to metabolism and thus internal gas composition.

Conditions during storage are especially critical because optimum levels of CO2 and O2 teeter on a threshold between aerobic respiration (desirable) and anaerobic respiration (undesirable). Fruit differ in their sensitivities to anaerobic respiration, but are normally intolerant of prolonged periods (>3 d). Disorders and off-flavours then appear. Species vary in their response to the altered atmospheres of CA, and can even differ according to cultivar and harvest. This variation is seen in both the final concentration of CO2 and O2 within stored fruit, and in the time taken to equilibrate. Understandably, commercial enterprises operate under safe conditions, that is, close to an ideal atmosphere but with a margin of safety to avoid anaerobiosis. This in turn means that fruit storage is almost never completely optimal for that fruit. Normally, an internal 0.5% (0.5 kPa) partial pressure is the minimum O2 level tolerable, and 10% or 10 kPa is the maximum for CO2.

Kiwifruit tolerate a storage atmosphere of 8% CO2 and 1% O2 without detriment (see McDonald 1990). Atmospheres of 5% CO2 and 2% O2 are considered optimal for long storage in CA. These conditions delay fruit softening markedly, especially when combined with low temperature. When fruit are held in too high a concentration of CO2 (10–14%) there is a differential effect on core tissue and when that fruit is removed to air, cores fail to soften even though the green pericarp tissues ripen normally. If fruit remain for extended periods at 10–14% CO2, pericarp tissues also break down and off-flavours develop upon transfer to air.