Thermodynamic properties and moisture equilibrium characteristics of rapeseed (Canola) during multiple adsorption and desorption cycles

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Yang, Weihua
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Thermodynamic properties, including the Gibb's free energy changes, isosteric heat of sorption, latent heat of vaporization, entropy of sorption, and monolayer coverage, and moisture equilibrium characteristics, including isotherm and hysteresis behaviour, hygroscopicity variations, and isotherm equations, of rapeseed (Canola) during multiple adsorption and desorption cycles have been scrutinized in this study. For ambient-air (25oC) dried canola, the Gibb's free energy, isosteric heat and entropy of sorption became, on the whole, less negative at moisture contents below 8.00 %db at the equilibrium temperatures of 3.5, 25.0 and 40.0oC with the increased number of adsorption and desorption cycles. At moisture contents above 8.00 %db, the effect of multiple adsorption and desorption cycles was not significant. For heated-air (50oC) dried canola, little effect of nultiple adsorption and desorption cycles was found on the Gibb's free energy changes. Postulation was made to explain why the effect of multiple adsorption and desorption cycles on the Gibb's free energy was little for heated-air-dried canola. Latent heat of vaporization of four rapeseed (canola) varieties: Westar, Tobin, Global, and Candle ranged from 44 to 152 kJ/mole at moisture contents between 4.0 and 22.0 %db, temperatures between 4 and 75oC, and multiple drying and rewetting cycles between 1 and 5. The magnitude of latent heat for these four rapeseed (canola) varieties was ranked as follows in ascensive order: Candle, Global, Tobin and Westar. The constant a in Eq.(11) was foudn to be the predominant factor to reflect the effect of multiple adsorption and desorption cycles on latent heat of vaporization of canola. The BET monolayer coverage values for canola ranged from 2.90 to 3.40 %db at the equilibration temperature of 25.0oC and on five consecutive drying and rewetting cycles. Increase in both the number of the successive moisture treatment cycles and the equilibration temperature were found to decrease the monolayer coverage of canola, as well as its hygroscopicity especially at the relative humidities below 75%. It was statistically shown that the Henderson and Chung-Pfost isotherm models were inferior to the Halsey equation. A new empirical equation was proposed based on the Halsey equation, and was proven to be better than the Halsey equation in terms of the goodness-of-fit critieria of 56 adsorption and desorption isotherms of rapeseed (canola) involving four varieties, equilibration temperatures from 4 to 75oC, relative humidity from 13 to 93%, and drying and rewetting cycles from 1 to 5. A new theory was developed in this thesis to account for the origin of sorption hysteresis. The characteristic hysteresis behaviour of canola during multiple adsorption and desorption cycles were explained in terms of the hydrophillic and hydrophobic interaction between water molecules and lipid and protein particles forming a lipid-protein complex typical of rapeseed (canola) and other oil seeds.