Experimental investigation of shrinkage and microstructural properties of date fruits at three solar drying methods

The effect of solar drying methods on shrinkage and the corresponding influence on microstructural changes of dates at three ripening stages (khalal, rutab and tamr) were investigated and evaluated shrinkage models that are best describing the shrinkage behavior. Drying was accomplished by forced convective solar dryer, greenhouse tunnel dryer and open sun drying. It was observed that, the greenhouse tunnel dryer had 37, 38 and 38% lower drying time for khalal, rutab and tamr stage dates, respectively compared to open sun drying, whereas the drying time was lower by 15, 29 and 30%, respectively in forced convective solar dryer. The drying methods and ripening stages had significant effect on shrinkage attributes of dates. Dates dried in the forced convective solar dryer had the lowest percentage of shrinkage in length (7–9%), diameter (10–27%) and volume (25–51%) at the three ripening stages followed by greenhouse tunnel dryer (7–13%, 12–27% and 29–53%, respectively) and open sun drying (8–13%, 17–32% and 37–60%, respectively). The quadratic equation was proposed as the best model to describe the volumetric shrinkage of dates as a function of moisture content or moisture ratio at the three ripening stages for all the three drying methods. Microstructural analysis of date tissue revealed that forced convective solar dryer had the least structural deformation at the three ripening stages. Overall, forced convective solar dryer produced the best quality of dried dates.