Activation volume from stress relaxation curves in raw and cooked potato

Stress relaxation in two varieties (Nicola and Panda) of potato tubers (raw and cooked) was studied with the aim to specify fine structure of the activation volume obtained during the test. It is shown that activation volume decreases during the most part of stress relaxation test. The whole test was divided into five parts with quasi-linear plots of activation volume vs. the actual stress. Different strain rates used in the pre-stress (PS) part influenced mainly two first parts of the plot. Part 3 plays crucial role in the whole stress-relaxation process. In raw state, this part is relatively insensitive to the variety and its slope decreases with increasing PS (from about 3 to about 2). In cooked state this part depends on the tested variety. The slope of the Relative apparent activation volume (RAAV)-Relative actual stress (RAS) plot in part 3 is changed dramatically with increasing pre-stress in cooked Nicola. This is the main source of big difference in the slope and in this manner also in relaxation rate between cooked Nicola and cooked Panda.