Dynamic measurement of tissue rigidity during freezing and cooking of vegetables

Dynamic oscillatory shear measurements on vegetable tissue were carried out using a modified Weissenberg rheogoniometer. The modified bottom platen and its heating system provided rapid heat transfer and reproducible heating rates. Small discs of tissue (1-9 mm, 20-6 mm dia) were attached to the platen and rheological parameters were measured at 30°C. Some tissues gave measurements that were within the linear viscoelastic region; others demonstrated nonlinear behaviour. The applied stress was in phase with the strain, and shear modulus (G′) was therefore the important rheological parameter. When discs of vegetable tissue were heated in the range 20-80°C the G′ value rose slightly then declined rapidly. The initial G′ value depended on the type of vegetable used, and there was also sample-to-sample variation. Similarly, the temperature at which G′ declined was vegetable dependent. Samples were frozen on the rheogoniometer and then heated from — 18 to 80°C. The change in G′ as the tissue thawed was measured. The technique has limitations if absolute values of G′ are required. However, for comparing the behaviour of different vegetables during heating and cooling, the technique allows dynamic measurements to be made