Drying characteristics and quality attributes of quick-cooking Job's tears as affected by rotary drying combined with microwave heating

A combined microwave heating and rotary drying process was developed to prepare the quick -cooking Job's tears. Using operating parameters of hot air temperature (70-90 degrees C), microwave power (150-450 W) and rotational drum speed (0.17-0.50 s(-1)), drying characteristics were described using the Page drying model. The model parameters (drying rate constant k and power constant n) considerably correlated with drying factors using the quadratic equation. Drying time (DT) predicted by the Page model and effective diffusivity coefficient (D-e) ranged from 42 min to 116 min and from 487.69 x 10-8 m(2)center dot s(-1) to 1 249.96 x 10(-8) m(2)center dot s(-1), respectively. The proposed drying technique resulted in lower DT and higher D-e, while utilizing less energy than hot air drying. Response surface methodology showed the influence of all operating factors on drying rate constant (k), DT, D-e, and specific energy consumption (SEC). Optimal drying conditions were determined by maximizing k and De while minimizing DT and SEC as air temperature of 76 degrees C with 450 W microwave heating and 0.17 s(-1) rotational drum speed. Quality attributes of quick -cooking Job's tears prepared by drying under such optimal conditions were comparable to those prepared by conventional cooking and freeze-drying. Findings of this study can be used for practical application of Job's tears grains.