STRESS-RELAXATION PHENOMENA IN VEGETABLE TISSUE .1. EXPERIMENTAL RESULTS

The stress relaxation in some compressed vegetable fleshes, i.e, potat o and kohlrabi tubers, and carrot and parsley roots, has been investig ated. The relaxation curves obtained with different prestrain parts (d ifferent stress level and different prestrain rate) are approximated i n the second part by simple equations, i.e. logarithmic and power equa tions. The similarity of these equations is demonstrated using similar ly defined parameters: the initial slope of the relaxation curve and t he time factor. The influence of prestrain on the strain relaxation is shown by the probability density of the decay processes. The activati on volume was determined by two different methods: analysis of relaxat ion curves or/and by push change of the deformation rate. The second m ethod gives systematically higher values than the first one. The activ ation volume strongly decreases with increasing stress, but only small differences in activation volumes were observed for different vegetab le fleshes. The activation volumes between 50 and 110 nm(-3) are in a good agreement with the pore volume in the cell walls. The results obt ained agree with the hypothesis of the controlling role of squeezing o ut of the cellular sap for stress relaxation in vegetable flesh.