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N.A. Mir and R.M. Beaudry

O2 and C2H4 are biologically active molecules of importance in plant metabolism and their availability is manipulated to modify plant behavior during storage and the shelf-life period of harvested plant products. Respiratory curves describing the dependence of O2 uptake on O2 were obtained for slicing and `Roma'-type tomato, and `Jonathan' and `Empire' apple fruit at 20 °C for ripening fruit and for mature, non-ripening fruit. Mature, non-ripening fruit were maintained in that state by the application of 1-methylcyclopropene (1-MCP) or the use of a nonripening mutant in the case of tomato. The range of O2 atmospheres wherein the reduction of O2 relative to ambient yielded a significant (50%) reduction in respiration relative to the maximal rate of respiration, but was above the fermentation threshold, was termed the `safe working atmosphere' (SWA). For apple, there was no SWA for non-ripening apple fruits since a 50% reduction in respiration occurred at the fermentation threshold. During ripening, the respiratory curve shifted, revealing a marked increase in the apparent Km and maximal rate of respiration with no change in the fermentation threshold, resulting in the creation of a SWA of 6.5 kPa O2. A similar, less dramatic, shift in the respiratory curve for tomato fruit also occurred. In a flow-through system, low O2 reduced the rate of respiration of ethylene insensitive tomato fruit by ≈50% and resulted in an approximate doubling of the storability of the fruit. Insensitivity to ethylene yielded fruit with a respiratory rate approximately one-half that of ripening fruit, but storability was improved about 5-fold. The data collectively suggest that inhibition of ripening, rather than global metabolism via reduced respiration is key to preserving fruit quality.

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Randolph M. Beaudry

The application of low oxygen through modified atmosphere packaging (MAP) is a technique used successfully to preserve the visual quality of lettuce and some other commodities. The expansion of use of low O2 via MAP to preserve quality of most commodities is limited by technical difficulties achieving target O2 concentrations, adverse physiological responses to low O2, and lack of beneficial responses to low O2. Low O2 often is not used simply because the physiological responses governed by the gas are not limiting quality maintenance. For instance, shelf life may be governed by decay susceptibility, which is largely unaffected by low O2 and may actually be exacerbated by the conditions encountered in hermetically sealed packages. Physiological processes influenced by low O2 and limit storability are discussed. The interdependence of O2 concentration, O2 uptake by the product, and temperature are discussed relative to requirements for packaging films.

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M. Helena Gomes, Randolph M. Beaudry, and Domingos P.F. Almeida

oxygen and implications for modified atmosphere packaging. A positive difference between for O 2 uptake and the fermentation threshold—named “safe working atmosphere” by Beaudry (2000) —can be taken as an indicator that significant decreases in