Characterising ripening behaviour of coated pears in relation to fruit internal atmosphere

Pear (Pyres communis L.) cultivars 'Bartlett', 'Beurre Bosc', 'Doyenne du Comice', and 'Packham's Triumph' were treated at harvest or after cold storage with concentrations of 0, 5, 10, 20, 40, and 100% (v/v) of a commercial carnauba-based wax emulsion. Fruit were assessed for ripening and internal atmosphere composition at 0 degreesC/90-95% RH (only for 'Cornice') and at 20 degreesC/60-70% RH (for all cultivars, after they achieved the chilling requirement to ripen). For all cultivars coated at harvest and ripening at 20 degreesC, respiration rates, softening and colour change followed a Michaelis-Menten model when plotted against internal partial pressure of O-2 (p(i)o(2)), while internal partial pressure of CO. (p(i)co(2)) had virtually no explanatory power for these variables. This supports the notion that modification of p(i)o(2). rather than p(i)co(2) is the principal means by which coating achieves its effects on ripening behaviour of pears during shelf life. Michaelis-Menten constant values (Km) for respiration and softening were lower than those for colour change for all cultivars at 20 degreesC. This resulted in colour change being retarded by any level of depression in p(i)o(2) created by coating, while firmness was substantially reduced only at much lower pot during shelf life. Plots of P(i)co(2) and respiration versus p(i)o(2) for fruit coated either pre-climacteric (at harvest) or entering the climacteric (after cold storage), indicated that 'Comice' and 'Packham's' were tolerant to hypoxia whereas, with 'Bartlett' and 'Bosc', tolerance reduced with advancing ripening, and the latter cultivar was the least tolerant of all four cultivars. During storage at 0 degreesC of 'Cornice', coatings had a more substantial effect in delaying softening than colour change; firmness retention seemed to be related to increase of p(i)co(2), while the limited suppression of colour change seemed to be related to small reductions of p(i)o(2). Therefore, optimisation of surface coatings for pears must account for differences between cultivars and ripening stage at which fruit are coated, as well as the temperature at which the fruit are going to be held. (C) 2001 Elsevier Science B.V. All rights reserved.