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|標題:||Studies on drying characteristics and thin-layer drying models of freeze-drying with far-infrared radiation
|關鍵字:||遠紅外線;Far-infrared radiation;冷凍乾燥;薄層乾燥模型;反應曲面法;間歇性加熱;乾燥時間;復水率;硬度;總色差;掃描式電子顯微鏡;Tylose;甘藷;Freeze-drying;Thin-layer drying model;Response surface methodology;Intermittent heating;Drying time;Rehydration ratio;Hardness;Total color difference;Scanning electron microscope;Tylose;Sweet potato||出版社:||食品暨應用生物科技學系||摘要:||
本研究研發以實驗型遠紅外線冷凍乾燥裝置進行甘藷（Ipomoea batatas）立方塊樣品之乾燥特性的探討，探討中比較熱風乾燥、冷凍乾燥、遠紅外線冷凍乾燥等三種不同乾燥方式所需之乾燥時間。結果顯示運用遠紅外線冷凍乾燥方法可以縮短乾燥所需時間，在乾燥過程中亦可觀察到恆率及減率乾燥階段，並獲致一經驗方程式藉以描述減率乾燥階段之乾燥特性。另一方面利用四種數學模型模擬甘藷遠紅外線冷凍乾燥之乾燥特性，其中指數、Page及近似滲透模型的決定係數（R2）均高達0.98以上，而理論性的滲透模型的相關性雖然低於其他三種經驗型的數學模型，但決定係數亦達到0.92，模型適合性的順序依序為Page，近似滲透模型，指數，滲透模型。由於Page模型具有最低的殘差值及均方根誤差，因此能夠最適切預測甘藷應用遠紅外線輔助的冷凍乾燥程序為Page模型。此外並比較不同乾燥方法對於乾燥產品之顏色、復水性、微細結構等的影響。在絕對壓力0.1 mmHg操作條件之冷凍乾燥方式處理下，相較於其他處理方式可得到亮度（Hunter-L值）較高，紅色度（Hunter-a值）、黃色度（Hunter-b值）較低的產品，而且與其他乾燥處理方式得到的結果均具有顯著差異。在紅色度方面，以遠紅外線冷凍乾燥處理與控制組及熱風乾燥處理的結果相比較，顯示並無顯著的差異。黃色度方面，遠紅外線冷凍乾燥處理的樣品與熱風乾燥方式處理者並無顯著差異，而與0.1 mmHg壓力操作的冷凍乾燥樣品具有顯著差異。電子顯微鏡觀察結果顯示，以遠紅外線冷凍乾燥方式處理之樣品具多孔性結構，且其多孔性優於熱風乾燥方法處理之樣品。
甘藷的遠紅外線冷凍乾燥最適化加工程序研究採用三階層、三因子的反應曲面法實驗設計，探討遠紅外線加熱器設定溫度、遠紅外線加熱器至樣品表面之距離及樣品之厚度等獨立變數，對於乾燥時間、復水率、硬度及總色差等反應變數的影響，結果顯示乾燥時間與復水率以樣品之厚度與遠紅外線加熱器設定溫度等兩項變因影響較為顯著（5％顯著水準），另一方面總色差顯著地受到遠紅外線加熱器至樣品表面之距離變因的影響（5％顯著水準），硬度則是受到樣品之厚度變因的顯著影響（5％顯著水準）。最適條件的決定是基於控制組與遠紅外線處理的比較，所得之最適乾燥條件為控制遠紅外線加熱溫度33.5∼37℃，樣品厚度6∼7.5 mm，遠紅外線加熱器至樣品表面之距離為20 ㎜，經由驗證實驗所得，結果顯示反應曲面方程式可被接受，並且可以獲致最適之乾燥條件。
以遠紅外線間歇加熱方式應用於甘藷之冷凍乾燥研究，採用循環時間30分鐘，間歇率分別為0，0.25，0.5，0.75及1的實驗條件，探討間歇加熱乾燥的效果，結果顯示最適間歇率為0.5，最適之乾燥操作為遠紅外線加熱器至樣品表面之距離為20 ㎜，遠紅外線加熱溫度設定為34∼37℃，並將樣品厚度控制在6∼7 mm，比較連續性與間歇性遠紅外線冷凍乾燥的乾燥效果，乾燥產品具有相類似之組織結構，而且在品質方面復水率及硬度之差異不大，間歇性乾燥處理對於降低顏色差異的效果優於連續性乾燥處理。因此，利用間歇性乾燥處理方法可用以取代連續性遠紅外線冷凍乾燥藉以達到節省能源的目的。
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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