Stress relaxation phenomena in vegetable tissue

Abstract

The stress relaxation in some compressed vegetable fleshes, i.e. potato and kohlrabi tubers, and carrot and parsley roots, has been investigated. The relaxation curves obtained with different prestrain parts (different stress level and different prestrain rate) are approximated in the second part by simple equations, i.e. logarithmic and power equations. The similarity of these equations is demonstrated using similarly defined parameters: the initial slope of the relaxation curve and the time factor. The influence of prestrain on the strain relaxation is shown by the probability density of the decay processes. The activation volume was determined by two different methods: analysis of relaxation curves or/and by push change of the deformation rate. The second method gives systematically higher values than the first one. The activation volume strongly decreases with increasing stress, but only small differences in activation volumes were observed for different vegetable 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 obtained agree with the hypothesis of the controlling role of squeezing out of the cellular sap for stress relaxation in vegetable flesh.