Determination of equilibrium moisture content characteristics of Manitoba grown soybeans as affected by post-harvest conditions

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Diaz Contreras, Lina Marcela
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The relationship between Equilibrium Moisture Content (EMC) and Equilibrium Relative Humidity (ERH) was measured in three varieties of soybeans grown in Manitoba (Akras R2, Lono R2 and Podaga R2) with the objective of modelling mathematically the adsorption and desorption isotherms. The effect of three post-harvest conditions: (a) freshly harvested soybeans (b) soybeans subjected to three drying and wetting cycles, and (c) soybeans subjected to three freezing and thawing cycles was investigated on the sorption isotherms of the soybean varieties. The adsorption and desorption experiments were conducted at 5, 10, 15, 20, 25 and 30°C using a dynamic ERH apparatus. Significant differences were found in all pairs of adsorption-desorption characteristics indicating the presence of hysteresis in the soybean samples at all the temperatures tested. The hysteresis magnitude ranged between 0.23 to 4.5% moisture content (MC) measured on dry basis. The modified Halsey equation and the modified Oswin equation were similar in predicting the sorption isotherms of the experimental data, with the former being superior in predicting the desorption data at low RH. The parameters calculated for the modified Halsey equation are applicable for temperatures above 10°C in the range of 10 to 80% RH for desorption and 30 to 80% RH for adsorption. Freshly harvested samples of the three soybean varieties produced similar isotherms, without significant differences among them. However, soybeans varieties responded differently to the effect of pre-treatments (i.e. drying/wetting and freezing/thawing cycles). Significant differences at 10, 15, 20, 25 and 30°C were found in the adsorption isotherms of Akras and Lono soybeans subjected to drying and wetting cycles. Adsorption isotherms of Akras and Podaga soybeans subjected to freezing and thawing cycles were significantly different from the fresh samples of iii these varieties. The effect of drying and wetting cycles on the desorption isotherms was only found in Akras soybeans at 10°C below 63% RH and at 15°C below 71% RH. The effect of drying and wetting cycles was found in Lono soybeans at 25 and 30°C above 69% RH at both temperatures. In general, the effect of both pre-treatments on the sorption isotherms of soybeans was reflected in a reduction in the EMC when compared to fresh samples.
Sorption isotherms, Soybeans, Post-harvest conditions