Firmness-temperature coefficient of kiwifruit

Abstract

Changes in flesh firmness of kiwifruit (Actinidia deliciosa) in response to fruit temperature were characterised for fruit removed from storage at 0°C at a range of levels of firmness generated by the normal softening associated with fruit ripening. Firmness was less after 24 h equilibration to 20°C (f2) than when measured immediately after removal from coolstorage (f1). Fruit regained firmness upon recooling to 0°C (f3) which, as a proportion of f1, occurred to greater extent in soft fruit than firmer fruit which were in the rapid phase of fruit softening. Thus, the firmness temperature coefficient (kft, %/°C, a measure of the percentage change in firmness caused by a given temperature change) of kiwifruit was itself shown to be a function of flesh firmness. It is suggested this resulted from contributions from two processes: a direct, reversible, physical effect that was a linear function of temperature; and in the phase of rapid fruit softening, an irreversible, physiological loss of firmness associated with stimulated fruit ripening at elevated temperature. Fitted functions relating kft-tof1 and f2 were used to develop expressions which permit approximate calculation of f2 given f1 and vice versa. These expressions could be used to link more effectively research data obtained at 20°C with commercial assessments of fruit firmness made on fruit at 0°C e.g., by correcting estimates of time required for fruit to soften to commercially important firmness thresholds. © 1994 The Royal Society of New Zealand.