Abstract
Fresh soybeans were stored at 30°C and 79%RH to induce the hard-to-cook (HTC) defect. Viscoelastic characterization of stored cotyledons was done using stress relaxation and by Alfrey's approximation for the distribution of relaxation times. The relaxation behavior of raw or soaked soybean cotyledons conformed to three-element generalized Maxwell models, except for the soaked/control sample for which a single element was required. Relaxation times associated with each Maxwell element decreased progressively with storage time for non-soaked samples but increased for soaked ones, implying different operating relaxation mechanisms. A superposition of three normally distributed relaxation spectra was obtained for all but the soaked/control sample using Alfrey's approximation. For non-soaked samples, stress relaxation occurred mainly via the mechanism characterized by the relaxation time τ1, the spread of the distribution of which progressively decreased and shifted to the left along the time axis with storage time. Relaxation in soaked samples was principally by τ2 whose spread of the distribution decreased with storage time and shifted to the right along the time axis. The relaxation mechanism characterized by τ1 in soaked cotyledons increased in prominence as storage progressed since its distribution extended beyond the experimental time in the case of the sample stored for five months. This may indicate storage-induced proliferation of the structural species responsible for relaxation via this mechanism. The influence of HTC on the viscoelastic parameters of soybean cotyledons indicates that the development of the defect is associated with structural changes in the cotyledon cell walls, and that stress relaxation can be used to study the phenomenon in the raw state.
