The influence of tissue porosity on the material properties of model plant tissues

Plant cell walls are a key determinant of the material properties of vegetable tissue following thermal processing. This study focussed upon the effects of heat on the material properties of model vegetable tissues ( Chinese water chestnut and carrot), with particular emphasis on the rate of compression during mechanical testing. In a homogeneous system such as cooked Chinese water chestnut, low strain rate compression (0.2 min(-1)) produced buckling of the cell walls, steady fluid expression, low stiffness and fractures at high strains. However a high compressive strain rate (>50 min(-1)), produced a high stiffness and failure by a rapid, brittle fracture at low strain, which propagated by cell rupture causing limited, rapid fluid release. These differences in mechanical behaviour are ascribed to the ability of cellular fluid to flow through the tissue during the timescale of the compressive test. When carrot tissue was heated for short periods of time (<10 min at 100&DEG;C), the tissue stiffness again increased with strain rate. For longer heating times (>20 min at 100degreesC), weakening of the inter-cellular adhesion changed the failure mechanism from cell rupture to cell separation. These results show that the material properties of processed vegetable tissue depend upon, not only the previously known factor of inter-cellular adhesion and cell wall strength, but also the cell wall porosity and the ease of fluid flow. (C) 2004 Kluwer Academic Publishers.