An investigation of drying and absorption characteristics of grain sorghum

Abstract

Published data on sorption and drying characteristics of sorghum grains is inadequate and scattered over temperature and relative humidity ranges. There is need to generate more data, test it on the existing models, and to develop new models that can fit a wide range of temperature and relative humidity Sorption data for adsorption and desorption of water by sorghum grains were obtained at 2SnC, 40nC, 55°C and 70ne over a relative humidity range of 0.11-0.97, using a static gravimetric method. The experimentally obtained data for three sorghum varieties (White, Brown and Red) were used to evaluate the suitability of BET, Halsey, Henderson, Oswin, Smith and GAB sorption models. The models' using an iterative non-linear parameters computer were computed program. The experimental data was also fitted to an empirical model using multiple regression analysis and the results were compared to those from the above models. Mono layer moisture contents for the three varieties were determined from BET and GAB models. The equilibrium moisture content (EMC) of grain sorghum was found to be independent of variety but was dependent on temperature. Both desorption and adsorption isotherms were sigmoid in shape (BET type I I) and exhibited hysteresis phenomenon. Comparison between the empirical model and the other models indicated that no model was significantly superior to the empirical model. The GAB model fitted the sorption data over the entire relative humidity range better than all tested models. The ranking based on order of best fit was as follows: 1. GAB; 2. Henderson; 3. Empirical; 4. Oswin : 5.Smith; 6.Halsey; 7.BET. Drying characteristics of both naturally moist and rewetted grain sorghum were also studied. Drying air temperature ranged between 40 c and loo c. Two air flow rates were used. Thin layer drying equations based on Pabis and Henderson equation and Page model were developed. Drying air temperature was the major factor affecting drying rates of sorghum grains. The difference in sorghum varieties also influenced the drying rates. These differences were more pronounced at high air flow rates. The relationship between parameter k in Pabis and Henderson equation and drying air temperature was found to be of Arrhenius type. Parameter K in Page equation, for naturally moist grains, was expressed as an exponential function of temperature terms. But for rewetted grains, it was an exponential function of temperature terms, air flow rate, iv relative humidity and initial moisture content. Parameter N in Page equation was expressed as an exponential function of temperature. Page equation fitted drying data better than Pabis and Henderson equation.