Creep behaviour of tomato pericarp tissue as influenced by ambient temperature ripening and chilled storage

The influence of normal ripening and chilling stress on viscoelastic properties of tomato pericarp tissue were investigated by measuring creep behaviour of tissue from fruit stored at 22C (nonchilled) or 5C (chilled) for 28 days, or at 5C for 16 days prior to transfer to 22C for an additional 12 days (prechilled). Creep compliance of tissue from all treatments subjected to a constant shear stress of 150 Pa for 5 min was best represented by a 6-element Burgers model containing two discrete Voigt-Kelvin units characterizing fast and slow rate viscoelastic properties. The magnitude of instantaneous elastic, viscoelastic and steady-state viscous compliances each increased steadily and contributed to the overall softening of nonchilled and prechilled tissues during ripening, but remained unchanged during chilling of tomato fruit. Increased fluidity of ripening tissues occurred at the expense of elasticity, consistent with a decrease in molecular weight-size distribution of structural elements contributing to respective viscoelastic properties. The physico-mechanical changes in prechilled tissue preceded those in nonchilled tissue by several days, and occurred at a faster rate. The 6-element Burgers model defining the creep behaviour of tomato pericarp tissue was interpreted with respect to general plant cell wall structure and biochemical changes known to occur during ripening of tomato fruits. Multiple mechanisms of softening were thereby consolidated into a single physico-mechanical model.