Effect of moisture sorption hysteresis on thermodynamic properties of two millet varieties

Abstract

Application of reversible thermodynamic principles to gain fundamental understanding of food–water interactions in foods has met with limited success due to the presence of moisture sorption hysteresis which is a manifestation of irreversibility. This study was aimed at understanding the nature and extent of influence of hysteresis on thermodynamic properties of two millet varieties namely EX-BORNO and SOSAT C88. Moisture sorption data (adsorption and desorption) in the water activity and temperature ranges of 0.07 – 0.98 and 30°C – 70°C, respectively were used. Ratio of latent heat of moisture sorption to the latent heat of pure water was determined using Clausius-Clapeyron equation. Effect of moisture content on ratio of latent heat of sorption to latent heat of pure water was determined using Gallaher model. Net integral enthalpy was determined at constant spreading pressure with monolayer moisture contents calculated using Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-de Boer (GAB) models. Spreading pressure was determined using an analytical procedure. Gibbs equation was used to evaluate net integral entropy. Net isosteric heat of sorption and differential entropy were evaluated and used to investigate the enthalpy‐entropy compensation theory. Latent heat of moisture sorption in the two millet varieties decreased with increase in moisture content and approached latent heat of pure water at a ‘free water’ point of between 32% and 42% moisture content (d.b.) in adsorption and desorption, respectively. Desorption latent heat of moisture was higher than the adsorption values and the difference decreased with increase in moisture content. Monolayer moisture content decreased with increase in temperature, with the GAB being higher than the BET values. Desorption monolayer moisture was higher than that of adsorption. Spreading pressure increased with increase in water activity, with adsorption isotherm being higher than that of desorption and temperature having no significant effect. Net integral enthalpy decreased with increase in moisture content with effect of hysteresis being more marked in EX-BORNO than in SOSAT C88. Net integral entropy decreased with increase in moisture content to minimum values and thereafter, maintained sinusoidal trend with adsorption curve lagging behind desorption and varnishing at 18% and 16% (db) moisture content in EX-BORNO and SOSAT C88 respectively. Net isosteric heat of sorption and differential entropy decreased with increase in moisture content with effect of hysteresis being more pronounced in EX-BORNO. Moisture sorption process in the grains was found to be enthalpy driven with differential enthalpy varying linearly with differential entropy. Inequality in isokinetic and harmonic mean temperatures confirmed the enthalpy‐entropy compensation theory. Hysteresis was found to have effect on the isokinetic temperature, causing the values of desorption isokinetic temperature to be higher than the adsorption values.