Optimizing Open-Sun Drying of Paddy Rice: Effects of Drying Floor Type and Layer Thickness on Milling Quality

Abstract

Rice production in sub-Saharan Africa (SSA) is growing, but substantial postharvest losses, particularly due to inadequate drying practices, hinder its potential to meet high milling quality standards. This study investigated the effects of drying floor type (cemented floor with canvas vs. raised bed wire mesh screen) and layer thickness (1–4 cm) on the drying kinetics and milling quality of paddy rice under open-sun conditions in northwestern Ethiopia. A factorial experimental design was used to evaluate these factors. Results showed that the raised bed with wire mesh screen generally provided superior drying conditions, including faster drying rates and reduced grain fissuring, compared to the cemented floor with canvas. The traditional field drying method (i.e., delayed harvesting followed by field drying and heap stacking) resulted in the highest percentage of fissured grains (39.67%) and lowest HRY (52.20%). In contrast, a raised bed with a 3 cm layer thickness resulted in an HRY of 68.10% and a significantly lower percentage of broken rice compared to traditional drying methods. The Midilli model best described the drying kinetics, with drying rate constants influenced by layer thickness and drying floor type. The study underscores the potential of the raised bed and thinner layers to enhance milling quality and reduce postharvest losses, highlighting the need to shift to improved drying practices. These findings offer insights for optimizing open-sun drying techniques for smallholder farmers in SSA, while future research should explore cost-effective designs for the raised bed method and optimize operational parameters like stirring frequency to promote widespread adoption.