Abstract
Changes in mechanical properties of potatoes are known to affect the quality of the products processed from the tubers. This study set out to ascertain the extent to which the changes in mechanical properties were attributable to turgor changes in the course of a storage season versus physiological changes that are expected to occur due to tissue ageing. Specimens taken from two potato cultivars ( Russet Burbank and Shepody) of two different solids contents were subjected to small strain oscillatory shear tests and wedge-penetration fracture tests. Prior to mechanical testing, specimens prepared from control (8°C) and pre-heated (33, 47 or 60°C) potato tubers were osmotically adjusted in a mannitol solution of 3, 5 or 7%. The tests were repeated every month from November to June. Turgor pressure and specific gravity were also measured during storage: turgor pressure slightly decreased, while specific gravity increased slightly throughout the storage period. Control and pre-heating temperatures of 33 and 47°C had little or no influence on shear modulus or fracture toughness; at a given mannitol concentration, pre-heating at 60°C caused both parameters to decrease. As mannitol concentration increased, the mechanical properties investigated decreased for the control, as well as for 33 and 47°C pre-heating treatments. Mannitol concentration had no effect on shear modulus and toughness for tubers that had been pre-heated at 60°C. Storage time had only a slight effect on the parameters of interest. Solids content and cultivar had no significant effect on either shear modulus or fracture toughness. To conclude, mechanical properties of tubers did change in storage, but the changes were much less pronounced than the changes caused by adjustment of turgor pressure. Thus, cell moisture content and its effect on turgor pressure affects tissue mechanical properties more dramatically than changes associated with ageing.
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