Effects of canola pretreatments on sorption hysteresis and rheological behavior of single kernels

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Bielewicz, Jerzy
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Equilibrium moisture content (EMC) and creep and recovery tests were conducted for canola seeds (Brassica napus L., cv. Westar). The effects of sample age, origin and pretreatment (rewetting/drying cycles, irradiation and long-term stress) on EMC properties and viscoelasto-plastic behaviour were investigated. Equilibrium moisture content hysteresis was studied by equilibrating rewetted and dried samples together in an air-tight environment at constant temperature of 25oC. The measured EMC values compared well with the published data. The viscoelastic properties of canola were evaluated using rheological models. A significant difference in viscoelasto-plastic properties, greater than that which can be explained by the EMB loop between adsorption and desorption isotherms was found. The actual difference in average ratio of elasticity between adsorption and desorption samples was approximately 6 times greater than the difference in ratio of elasticity corresponding to the moisture content loop (0.4% db) for the canola kernels equilibrated at 67% relative humidity (RH). Absorption and desorption isotherms obtained for three different samples of different age and origin differed substantially. The difference in EMC between two adsorption samples harvested at different locations was approximately 1.3% db at 23% RH. A discrepancy in instantaneous loading deformation of 13% was attributed to the difference in age and growing history of two canola samples at 8.5% db MC. The rewetting/drying pretreatment resulted in a significant mold development. The adsorption and desorption isotherms (EMC - ERH curves) shifted down as a result of the pretreatment. The hysteresis loop widened for low and intermediate moisture contents. The EMC hysteresis loop widened 2.5 times as a result of the pretreatment for samples equilibrated with air at approximately 25% RH. The pretreatment increased instantaneous loading deformation by 35% over the predicted value due to the EMC difference alone for samples equilibrated at 42% RH. The EMC behaviour was not affected by irradiation or long-term stress. Long-term stress, however, had significant effects on the viscoelastic properties of the canola. Instantaneous loading deformation increased approximately 22% as a result of this pretreatment for samples equilibrated at 58% RH. The viscoelasto-plastic behaviour of the canola kernels during creep and recovery tests was explained using a rheological model consisting of a plastic component in series with a viscoelastic component.