Effects of thermodynamic properties of rice and ambient conditions on moisture migration during storage at naturally ventilated warehouses

Abstract

The current study dealt with characterizing the effect of external variables on the moisture migration phenomenon in two naturally ventilated rice warehouses. Secondly, the thermodynamic properties of rice during the rehydration cycle were illustrated as numerical models to predict their behavior. Thai rice was stored at Shiraz city and Abadeh town for a total of 9 months in two identical warehouses. The effect of outside temperature and relative humidity on ambient conditions inside the warehouses as well as rice moisture content was evaluated. The dehydration rate of rice stored at Shiraz facility was higher than those stored at Abadeh warehouse by an average of 166% resulting in lower rice moister content. The 60-day latency in reaching minimum rice bulk moisture content at Abadeh warehouse was due to its cooler climate and less intense boundary area temperature gradient. The type II sigmoid-shaped sorption isotherm (fitted with the GAB model) indicated moisture content elevation above 11% sharply increased with the water activity beyond 0.7. The isosteric heat of sorption was linearly correlated with the entropy of sorption indicating adsorption was governed by compensation theory, was enthalpy driven and non-spontaneous. The most suitable conditions to store rice were determined by relating the grain’s moisture content and its thermodynamic properties during the sorption process. Therefore, storage of rice for prolonged durations was possible by maintaining the ambient temperature and relative humidity between 20.0 °C and 28.5 °C and 15.0% to 25%, respectively.