In this study, we analyzed the effect of the duration of relaxation experiments on the parameters of the model proposed by Peleg and Normand. The relaxation experiments were conducted for individual intact wheat kernels in compression. Single kernels were initially loaded with a force of 20 N, then maintained at a constant deformation over time of 300 s. For the following time intervals 0-10, 0-20, 0-30, 0-40, 0-50, 0-60, 0-120, 0-180, 0-240, and 0-300 s coefficients k1 , k2 and Y(t) were determined according to the Normand and Peleg equation. The effect of time was compared at variable wheat moisture ranging from 8 to 20%. For longer holding times, the relaxed force was higher and the decay parameter, Y(t), decreased. The coefficient k1 increased proportionally with increase in the time of relaxation from ~3.3 to 23.3 s. The values of coefficient k2 declined for longer experiments, and the decline was relatively quick for up to 60 s and then asymptotically diminished. The effect of time on the decay parameter Y(t) and coefficients k1 and k2 depended on the moisture of kernels. The accuracy of the models was significantly different for the studied holding times. The highest standard error of the estimate was evidenced for 10 s experiments, and its minimum was noted at times close to 120-180 s.
Keywords: Peleg and Normand; compression; holding time; relaxation; wheat.
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