Mass Transfer in Infrared Drying of Gel-Coated Seeds

Abstract

In order to contribute for a better understanding of mass transfer in drying of shrinking particles, in this study shrinkage and drying characteristics of sorghum seeds encapsulated into gel-based polymeric matrix were experimentally determined by using infrared (IR) radiation. IR drying of gel coated seeds was carried out at three different temperatures (65˚C, 80˚C and 93˚C). The shrinkage of the individual particles during drying was quantified by means of the volume and surface area changes evaluated from geometric measurements. The product quality was evaluated in terms of the changes of particle density and percent of cracks in gel coating incurred during drying. Surface area and volume of the gel-seed system decreased about 65% and 80% until the end of the process, respectively, stressing the need to take into account the surface area changes to calculate water flux density as function of moisture content and obtain an accurate interpretation of the drying mechanisms well as to include the volume shrinkage in mass transfer models to determine reliable values of moisture diffusivity. The IR drying behavior of gel-coated seeds was then characterized by the presence of three drying periods: heating up, constant moisture flux and falling moisture flux. Accelerated drying of gel coated seeds was obtained by applying higher IR radiation intensities. The effect of IR source temperature on the particle shrinkage was more pronounced at the constant moisture flux period and practically negligible at the decreasing moisture flux period. Neglecting shrinkage of individual coated-seeds during IR drying led to an erroneous absence of constant flux period and overestimation of the mass transfer by diffusion. Apparent density of the particles was greater at low-temperature IR drying than at high-temperature IR drying. Coated particles keep their original geometry, but a significant cracking of gel coating was observed at rapid drying rate conditions.