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Fundamental Studies on Yam Slices Mechanical Drying
|關鍵字:||Yam slices;山藥片;Drying model;Static Equilibrium Moisture Content;Dynamic Equilibrium Moisture Content;Drying constant;乾燥模式;靜態平衡含水率;動態平衡含水率;乾燥常數||出版社:||農業機械工程學系||摘要:||
This research began with the study of an appropriate layer number of yam slices in drying operations. Using the hot-air temperature 50℃ with the optimum drying thickness obtained, the best pre-processing conditions of two pre-processing methods, physical and chemical respectively were then explored based on the quality indexes of drying products such as the hue, the cubic and the weight rehydration rates, and the viscosity of the dried yam slices. Under the best condition of each pre-processing method, five hot-air drying temperatures 40, 50, 60, 70, and 80℃and five desiccant dehumidification drying temperatures 20, 25, 30, 35, and 40℃ were followed to process the single temperature drying tests to find the optimum drying temperature for the hot-air and the desiccant dehumidification drying, respectively. The experimental results show that the optimum drying thickness is 1.5 cm with three yam slices. The best pre-processing condition of the physical method is blanching one minute with the water solution of temperature 70℃and then cooling with the indoor air sixteen minutes, whereas the chemical method is soaking one minute under the citric acid solution of concentration 0.5%. The optimum drying temperatures are 40℃ for the hot-air drying and 30℃for the desiccant dehumidification drying. Both can obtain the optimum quality of the drying products for those two pre-processing methods. However, the quality of the desiccant dehumidification drying is better than that by the operation of hot-air.
This study uses the Equilibrium Relative Humidity (ERH) scheme to measure the static Equilibrium Moisture Content (EMC) Me of yam slices during the dehydration. Four different ERH/EMC models including the Henderson, Chung-Pfost, Halsey and Oswin equations were used to investigate the fitting agreement of the measured data. Through the comparison of the residual distributions, the coefficient of determination R2, the residual sum of squares SSE, and the mean relative percentage deviation P obtained from the regression analysis, the Oswin model with two parameters and the modified Oswin model with three parameters are the best predicting models for the desorption isotherms of yam slices in both of the physical and chemical pre-processing methods. Applying the analysis of variance test for a specified pre-processing method, the difference of two individual sets of the residual absolute values in each temperature is not significant and both models with two and three parameters obtain similar predicting abilities. Hence, in each pre-processing method the model with three parameters can be used for the prediction of ERH/EMC values. Further comparing the predicted values of ERH calculated from each established equation with three parameters for both pre-processing methods within the valid range at a given temperature, the results of the analysis of variance test indicate that no significant difference between these two predicted values is obtained in each temperature. Both equations with three parameters have the equivalent representative to the prediction of ERH/EMC values for the samples of yam slices.
Newton's, modified Newton's, and Page's models were used to study the fitting accuracy of the moisture content change of yam slices during the drying process. Through the comparison of two statistical values, the coefficient of determination R2 and the standard error of estimate S.E. obtained from the regression analysis, and the curves of the measured data and the predicted values, the Page model is the optimum drying model by applying the measured dynamic Equilibrium Moisture Content Med and the calculated static Equilibrium Moisture Content Me into each model for both pre-processing methods in each drying temperature. The regression analysis using the dynamic Equilibrium Moisture Content Med into the above drying models for the cases of 40℃ of the optimum hot-air drying and 30℃ of the optimum desiccant dehumidification drying in both pre-processing methods, only the drying constant K obtained by Page's model is reduced as the drying time increased. Its correlative equation then established for the drying constant K is in the form of a polynomial equation with third order in temperature (℃) and first order in relative humidity RH(%). While utilizing the static Equilibrium Moisture Content Me, the drying constant K, exclusive of the case using 40℃ hot-air drying with the chemical pre-processing method, obtained has the same characteristics of distribution as using Med during the initial and middle drying stages. In the final drying stage, only the drying constant K obtained with both pre-processing methods of hot-air drying 40℃ by Newton's and modified Newton's models retains the same performance as the value of K obtained by using Med, and the characteristics K of the others is different. After comparing the statistical values and the predicted curves for each drying model, the predicted values obtained by using Me is equivalent to those values obtained by using Med in the hot-air drying operations. While in the desiccant dehumidification drying, the predicted values obtained by using Me is worse than those obtained by using Med. Therefore, the utilization of the dynamic Equilibrium Moisture Content Med for the establishment of the drying model is more appropriate than the use of the static Equilibrium Moisture Content Me. It could also achieve better predicting accuracy.
|Appears in Collections:||生物產業機電工程學系|
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