Thin-layer hot air drying characteristics and moisture diffusivity of lotus seeds

Lotus seeds are the seeds of plants in the genus Nelumbo, particularly the species Nelumbo nucifera in Asia and Africa. Lotus seeds are of great importance to East Asian cuisine and used extensively in traditional Chinese medicine and Chinese desserts. Drying process is one of the most important processes for lotus seeds, which can suppress activities of microorganisms, enzymes or ferments and maintain its nutrition content. Moisture transfer in lotus seeds during the drying is a complex process, and the safe moisture content is fundamentally important for industrial processes, because quality and energy consumption are related to moisture content. A better understanding of the mechanism of moisture transfer should help improve the product quality and efficiency of drying process. Thin-layer hot air drying characteristics of lotus seeds were investigated in the temperature range of 50-90 ℃ at a constant air velocity, and water mobility and distribution in lotus seed samples were estimated based on the nuclear magnetic resonance (NMR) methods (relaxation time and imaging) in this study. The results indicated that the drying process involved a deceleration phase, and had no constant-rate phase. The strong influences of drying temperature on drying rate and drying curve were evident. Four empirical drying models, i.e. Lewis, Herderson-Pabis, Page and Midilli model, given in literature for describing time dependence of moisture ratio change, were used to fit experimental data, and their coefficient of regression (R²) and root mean square errors (RMSE) were predicted and compared by non-linear regression analysis using the Matlab R²012b software. Relative percent error (E%) was used to determine the goodness of the moisture prediction during the drying. It was found that the Midilli model could predict drying curves compared with experimental data point for the drying of lotus seeds in temperature range of 50-90 ℃ (R²>0.998, E<10%). The effective diffusivity was obtained using the Fick's diffusion model, which varied from 6.0567×10^-10 to 1.6603×10^-9 m2/s and increased with the air temperature. The effective diffusivity dependence on temperature was determined by an Arrhenius type relationship. The activation energy was found to be 24.2685 kJ/mol. Transverse relaxation time test results showed that there were 3 different states of water in fresh lotus seeds named bound water of 12.07%, semi-bound water of 85.59% and free water of 2.33%, separately. Moisture states and moisture distribution were changed during hot air drying. Mobility of water at different states decreased visibly during the drying process. Transverse relaxation time of semi-bound water showed a tendency of dropping with the reduced moisture content. The nuclear magnetic resonance imaging (MRI) results directly showed that the moisture of outer layer declined faster than that of inner layer in lotus seed in the process of hot air drying, and the germ of lotus seed was still kept at very high amount in the final drying stage. This research can provide theoretical basis for understanding water transport characteristics in lotus seeds and assist in optimization of lotus seed drying process.