Gel Drying by Refractance Window: Drying Kinetics and High-Quality Retention.

In most cases, conventional drying produces inferior quality products and requires higher drying times. A continuous pilot Refractance Window equipment was used to dry gel slabs of 5 and 10 mm thick at 60, 70, 80, and 90 °C, seeking a dry product with high-quality retention. Based on five empirical models, drying kinetics, diffusion coefficient, and activation energy were analyzed. Midilli-Kuck was the best predicting model. Short drying times (55-270 min) were needed to reach 0.10 g water/g solid. In addition, the technique yielded samples with high rehydration capacity (24-29 g water/g solid); high retention of color (∆, 3.74-4.39); relatively low losses of vitamin C (37-59%) and vitamin E (28-37%). Regardless of the condition of temperature and sample thickness, a high-quality dried gel could be obtained. Compared with other methods, Refractance Window drying of achieved shorter drying times with higher quality retention in terms of color, vitamins C and E, and rehydration. Finally, the dried gel could be reconstituted to a gel close to its fresh state by rehydration.