PERFORMANCE ANALYSIS AND TECHNOLOGY OPTIMIZATION OF INFRARED DRYING OF SWEET POTATO SLICE

Sweet potato (Ipomoea batatas L.) is an important tuber crop for the daily consumption. Efficient processing must be taken to reduce wastage, and to improve quality and extend shelf period of sweet potato products. Infrared (IR) drying has advantages of high drying rate, good uniformity, and high production efficiency. A laboratory infrared (IR) dryer was developed to study the drying performance of sweet potato slice and its technology optimization in this paper. Single-factor, orthogonal, and temperature-varying experiments of IR drying of sweet potato slice were conducted sequentially. Temperature, slice thickness and steaming time were defined as control factors, and effective moisture diffusivity (EMD), total color change (TCC), specific energy consumption (SEC) and drying time were defined as evaluation indexes. Same weights were applied to the synthetic evaluation index (SEI). Experiment results and statistical analysis showed that: temperature-varying IR drying technology of temperature-decrease mode, under drying conditions of 70ºC (75min) - 65ºC (to end), showed the best drying performance; the optimal combinations for temperature-constant were slice thickness 3 mm, temperature 70ºC, and steaming time 6 min; Midilli et al. model gave the best approximation to experimental data of moisture ratio, with coefficient of determination 0.99933, reduced Chi-square 0.00007, and root mean square error (RMSE) 0.00838; high temperature (75ºC) and large slice thickness (9 mm) were not suitable for IR drying of sweet potato slice. The results of this study can provide references for research on IR drying technology and design of IR dryer for sweet potato slice.

Potato thin layer convective dehydration model and energy efficiency estimation

The dehydration parameters (temperature, thickness, and mass load) statistically significantly (p<0.05) affect the thin-layer convective dehydration of potato slices. The slices with thicknesses of 3, 5, and 8 mm were dehydrated as monolayers at different temperatures (30, 50, and 70 °C) and mass load (1.00, 0.63, and 0.38 kg m-2). The results showed that the shortest dehydration time (183 minutes), the smallest energy consumption (0.176 kWh), and the smallest emission of carbon dioxide (0.17 kg) had the dehydration model of potato slices with a 3 mm thickness, 0.38 kg m-2 mass load, dehydrated on the temperature of 70 °C. Dehydration of potato slices of 8 mm slice thickness dehydrated at 70 °C, with 0.38 kg m-2 mass load, showed the highest resistance to mass transfer (the maximum effective moisture diffusivity 2.3761 × 10-7 ± 4.45646 × 10-9 m2 s−1) and the minimum activation energy (27.02 kJ mol-1). Data obtained from these mathematical models could predict and optimize the thin layer dehydration of potato slices, with a dominant influence of temperature and potato slice thickness parameters as variables.

Deformation of Potato during Convective Drying

Deformation of potato is estimated by experimentally during convective drying. Size of the potato slice is 4cm x 2cm x 2cm. The percentage changes in length, breadth and width of potato are estimated during drying. Shrinkage of the object during drying is estimated. Air velocity chosen for this present analysis is 2 m/s and the range of air temperature is selected as 40 to 70 °C. The product experiences the maximum dimension changes upto 30% in length and 47.5 % in both breadth and width wise. The parameters are non dimensionalised to get generic solution.

Modeling of mass transfer performance of hot-air drying of sweet potato (Ipomoea Batatas L.) slices

In order to investigate the transfer characteristics of the sweet potato drying process, a laboratory convective hot air dryer was applied to study the influences of drying temperature, hot air velocity and thickness of sweet potato slice on the drying process. The experimental data of moisture ratio of sweet potato slices were used to fit the mathematical models, and the effective diffusion coefficients were calculated. The result showed that temperature, velocity and thickness influenced the drying process significantly. The Logarithmic model showed the best fit to experimental drying data for temperature and the Wang and Singh model were found to be the most satisfactory for velocity and thickness. It was also found that, with the increase of temperature from 60 to 80?C, the effective moisture diffusion coefficient varied from 2.962?10-10 to 4.694?10-10 m2?s-1, and it fitted the Arrhenius equation, the activation energy was 23.29 kJ?mol-1; with the increase of hot air velocity from 0.423 to 1.120 m?s-1, the values of effective moisture diffusion coefficient varied from 2.877?10-10 to 3.760?10-10 m2?s-1; with the increase of thickness of sweet potato slice from 0.002 m to 0.004 m, the values of effective moisture diffusion coefficient varied from 3.887?10-10 to 1.225?10-9 m2?s-1.