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An experimental dryer was developed to determine the drying characteristics of sweet potato (Ipomoea batatas) cubes samples during freeze-drying with far-infrared (FIR) radiation. The drying time of sweet potato cubes dehydrated by three drying methods, i.e., air-drying, freeze-drying, and freeze-drying with FIR were compared, and freeze-drying with FIR was found to be able to reduce the drying time. Both constant and falling rate drying periods were observed, and empirical equations were developed to study the behavior of drying rate in the falling rate period. On the other hand, four mathematical models were used to describe the drying characteristics of sweet potato during freeze-drying with FIR. The coefficient of determination (R2) in the exponential, Page, and approximate diffusion models were all found to be above 0.98. The coefficient of determination (R2) of diffusion model was above 0.92. The rank of fitness of those models was Page, approximate diffusion, exponential and diffusion model. The Page model was chosen because of the lowest residuals as well as root mean square error (RMSE). The Page model described the FIR freeze-drying characteristics of sweet potato properly. Color, rehydration ratio, and scanning electron microscopic examination of else dehydrated sweet potato cubes were compared. Higher Hunter-L value as well as lower Hunter-a and Hunter-b values were observed by freeze-drying at 0.1 mmHg absolute pressure, and results obtained under this treatment were significantly different compared with other drying treatments (p<0.05). Comparing with control and air-drying treatments, Hunter-a value obtained by FIR freeze-drying was not significantly different (p<0.05). Similarly, Hunter-b value obtained by FIR freeze-drying method was not significantly different with air-drying. Porous structure was observed by scanning electron microscopic examination in freeze-drying with FIR, and the porosity of the product was found to be better than that of air-drying.
Four thin-layer drying models were investigated for the description of FIR-assisted freeze-drying characteristics. The models were exponential model, Page model, diffusion model, and approximation of the diffusion model. Tests were conducted using a three-level, three-factor (drying temperature, distance between sample and FIR heater, and sample thickness) design, and quadratic polynomial analyses were performed to relate the drying parameters of thin-layer drying model. The performance of these thin-layer drying models was evaluated by comparing the coefficient of correlation (R2), the sum of squares (lack of fit), and the RMSE amongst four models. Drying parameter K of the Page model gave the best results in describing freeze-drying with FIR of Tylose and sweet potato.
Optimisation of FIR-assisted freeze-drying of sweet potato was conducted using response surface methodology (RSM) and aimed to determine the optimum combinations of drying temperature (X1), distance between sample and FIR heater (X2), and sample thickness (X3) that led to the optimum results of drying time, rehydration ratio, hardness, and total color difference. The response variables of drying time and rehydration ratio were affected by drying temperature and sample thickness over the 5% significance level. On the other hand, total color difference was affected by distance (X2) at 95% level confidence and hardness was affected by thickness (X3) at a 5% level of significance. The determination of optimum operation was based on the comparison between control and FIR-assisted treatments. The optimum drying conditions could be established at 20 mm distance from FIR plate and by controlling the drying temperature in the range of 33.5∼37℃ and sample thickness of 6∼7.5 mm for freeze-drying with FIR heating of sweet potato slices. Verification tests indicated that those generated second order polynomial models were acceptable. Optimum drying conditions were recommended and validation proved the results to be adequate and acceptable.
An intermittent heating of FIR was applied to the freeze-drying of sweet potato, and drying tests were made. Intermittency levels of α=0, α=0.25, α=0.5, α=0.75, and α=1 were studied with the cycle time of 30 min. The effect of different FIR intermittencies were investigated, and it was found that the optimum intermittency level was α=0.5, and drying conditions could be established by controlling the drying temperature in the range of 34∼37℃ and sample thickness of 6∼7 mm for freeze-drying with FIR heating of sweet potato slices at 20 mm distance from FIR plate. Results indicated that the effect of continuous (α=1) and intermittent heating (α=0.5) were similar in the tissue structures and the qualities of dried product in rehydration ratio and hardness. Using intermittent FIR heating, the color degradation could be reduced significantly. Therefore, the continuous heating with FIR-assisted freeze-drying could be replaced by the intermittent heating for energy-saving.
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