الفهرس 
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Abstract
Sweet potato (Batatas Ipomoea L.) is an important tuber crop, rich in beta-carotene (vitamin A), vitamins B, C and E, in addition to sugars, potassium, copper, magnesium and iron. It is also low in fat and cholesterol, with anti-cancer and prevention of cardiovascular diseases. The total cultivated area and production of sweet potato in Egypt are 42440.37 Fadden and 511682 tones, respectively, (FAO, 2021). Sweet potatoes are used in the food industry in the form of mashed potatoes, flour and starch. Sweet potatoes can be made into food products such as baby food, ice cream, baked goods, breakfast cereals, snacks, and various desserts. Sweet potatoes contain a high percentage of moisture, and to extend the shelf life of the product, they are dried and converted into dried slices or flour, and they can replace wheat flour. Infrared radiation is one of the most suitable methods for producing high-quality dried food at a low cost to reduce drying time, as well as suitable for drying many slices of vegetables and fruits for samples. Infrared radiation is one of the most suitable methods for producing high-quality dried food at a low cost to reduce drying time, as well as suitable for drying many slices of vegetables and fruits for samples. The main aim of the present work was to study and evaluate the use of infrared radiation combined with hot air as a heat energy source for drying sweet potatoes, decreasing the drying time and drying energy requirements, increase drying efficiency and asses the quality characteristics of the dried slices. So, specific objectives of this research are 1. To study and compare two different drying methods (combined infrared – hot air-drying and hot air-drying). 2. To study the effect of radiation intensity, air temperature and slices thickness on drying characteristics of sweet potatoes slices using the two studied methods. 3. To test and examine three different drying models (Lewis’s and Henderson and Pabis’s and the logarithmic) in describing the drying behavior and predict the changes of moisture content of sweet potato slices during the two studied drying methods. 4. To test and evaluate the effect of the studied drying method on thermal performance and final quality of the dried sweet potatoes chips. The experimental work was carried out through the period of 2020-2021 at the Agricultural Engineering Department, Faculty of Agriculture Tanta University for drying sweet potato. Sweet potato slices were pre-treated by dipping them into a solution of 0.5 % sodium meta- 5. bisulfite and 1% citric acid for 30 min. Two methods of drying (hot air drying, infrared drying and hot air) were used under four levels of radiation intensity (0.861, 0.973, 1.093 and 1.161 kW. m-2), three levels of drying air temperature (45, 55 and 65 ºC) and three levels of slices thicknesses (1, 3 and 5mm) at a constant air velocity of 1.2 m. sec-1. Experimental indicators included: 1- Moisture content of sweet potatoes slices 2- Moisture ratio of sweet potatoes slices 3- Drying rate of sweet potatoes slices 4- Mass changes of sweet potatoes slices 5- Quality evaluation of the dried sweet potatoes: • Color determination • Hardness and shear force determination • Shrinkage percentage • Total soluble sugar determination • Total carbohydrate determination • Beta-carotene determination • Rehydration ratio Mathematical Analysis The obtained data of the laboratory experiments was employed to examine the applicability of three different thin layer drying models (Lewis’s, Henderson and Pabis’s and Logarithmic model) on describing and simulating the drying data. The results and conclusion are summarized as follows a- Hot air-drying method Drying characterization -The moisture ratio decreased with the increase of drying time. The reduction rate of moisture ratio of sweet potato slices increased with the increasing of drying air temperature. -The drying time decreased with the increasing of drying air temperature. Changing the air temperature from 45 to 65°C at 5 mm thickness, the drying time decreased from 900 to 360 min. While, at 1mm thickness, the drying time decreased from 210 to 135 min. -The drying rate and the drying time of sweet potatoes decreased with the increasing of drying air temperature. Thin layer models 5. -The drying constant (kL), (kH) and (kLog) increased with the increase of drying air temperature. However, it was decreased with the increase of slices thickness -The examined drying models (Lewis’s model, Henderson and Pabis’s model and Logarithmic model) could describe the drying behavior of sweet potato slices satisfactory. -Logarithmic model could be considered more proper for describing the drying behavior of sweet potato slices and predicting the change in moisture content during the laboratory drying process due to simplicity of calculations. Color determination -For color the maximum lightness (L*), redness (a*) yellowness (b*), chrome (C*), hue (H*), Browning index (BI) and Whitening index (WI) were 75.78, 9.24, 35.93, 36.04, 85.57, 70.96 and 56.87. -The color change of the dried sweet potatoes slices should a significant decrease in ΔE values as the drying air temperature increased. Quality evaluation -The hardness of dried sweet potato slices at 45oC was significantly higher than that of samples dried at 55ºϹ and 65oC. Meanwhile, the shear force of samples dried at 65oC was significantly higher than that of samples dried at 45 and 55oC. -The diameter shrinkage percentage was decreased with the increase of drying air temperature at hot air method. The thickness shrinkage percentage was decreased with the increase of drying air temperature at hot air method. -The dried sweet potato slices using hot air-drying method had the highest reduced significantly on the properties of sensory quality comparing to the infrared drying method. -The total soluble sugar content for hot air method ranged from (0.83.25 to 0.9421mg/ml). -The total carbohydrate content was increased with the increase of drying air temperature, while it was decreased with increase of slices thickness. The total carbohydrate content varied from 0.1056 to 0.6187 mg/ml. -The degradation rate of beta-carotene increased with the increase of drying air temperature and slices thickness. Beta-carotene content varied from 0.28 to 0.59 mg/100 g. -The rehydration ratio increased with the increase of drying air temperature and slices thickness, but the hot air-drying method recorded lower value of the rehydration ratio at the sweet potatoes. b- Combined infrared and hot air-drying method Drying characterization 5. - The intensity of infrared radiation and drying air temperature had a significant effect on the moisture ratio of sweet potato slices. The moisture ratio decreased when the radiation intensity and the drying air temperature increased. -Changing the radiation intensity from 0.861 to 1. 161kW.m-2, the drying time decreased from 165 to 90 min at 1mm slice thickness. While, at 5 mm thickness, the drying time decreased from 630 to 330 min at the minimum air temperature of 45°c. Meanwhile, changing the radiation intensity from 0.861 to 1. 161kW.m-2, the drying time decreased from 105 to 60 min at 1mm thickness. While, at 5 mm thickness, the drying time decreased from 450 to 240 min at the maximum air temperature of 65°Ϲ. Thin layer models -The drying constant (kL), (kH) and (kLog) increased with the increase of drying air temperature and radiation intensity. However, it was decreased with the increase of slices thicknesses -An exponential relationship was found between the drying constant (kL), (kH) and (kLog) and the radiation intensity at all levels of air temperature and slices thicknesses. -The examined drying models (Lewis’s model, Henderson and Pabis’s model and Logarithmic model) could describe the drying behavior of sweet potato slices satisfactory. -Logarithmic model could be considered more proper for describing the drying behavior of sweet potato slices and predicting the change in moisture content during the laboratory drying process due to simplicity of calculations. -The final moisture content of the dried sweet potato slices under the studied conditions almost reached the recommended range of the dried sweet potato 4-8% (d. b). Thermal performance -The total energy consumed in drying the sweet potato slices increased 2.29 kW. h to 20.56 kW. h as the slice thickness increased, for Infrared drying method. For hot air-drying method, the maximum and minimum value of total energy 26.07 and 3.91kW. h was obtained at drying air temperature of 45 and 65 ºϹ, respectively. -For Infrared drying method, the specific energy (Esp) decreased as the drying air temperature and infrared radiation intensity increased. The maximum and minimum value of specific energy 99.48 and 11.08 kW. h/kg was obtained at a radiation intensity of 0.861 and 1.161 kW.m-2, respectively, while at hot air-drying method, the maximum and minimum value of (Esp) 126.15 and 18.92 kWh/kg was obtained at an air-drying temperature of 45 and 65 ºϹ and slice thickness of 5 and 1mm, respectively. -For Infrared drying method, the maximum and minimum value of specific energy (SEC) 119.98 and 13.14 kW. h/ kg was obtained at a radiation intensity of 0.861 and 1.161 kW.m-2, 5. respectively. For Hot air-drying method, the maximum and minimum value of specific energy 147.99 and 22.11 kW. h/kg was obtained at an air-drying temperature of 45 and 65 ºϹ and slice thickness of 5 and 1mm, respectively. -The maximum value of drying efficiency 98.83 % was obtained at a radiation intensity of 1.161 kW.m-2 and the minimum 6.60 % was obtained at a radiation intensity of 0.861 kW.m-2, respectively, for infrared drying method. The maximum value of drying efficiency 61.44 % at drying air temperature of 65 °C and the minimum value of drying efficiency 5.32 % was obtained at an air-drying temperature of 45 ºϹ, respectively, for hot air-drying method. -The energy efficiency increased as the thickness of sweet potato decreased. For IR-HA dryer, the highest value of energy efficiency 4.96 % was obtained at a radiation intensity of 1.161 kW.m-2 followed by hot air dryer 2.95 % was obtained at an air-drying temperature of 65ºϹ. The lowest energy efficiency values can be seen in IR-HA and hot air-drying with slices thickness of 5 mm (0.57 % and 0.45 %), respectively. -The highest thermal efficiency value 93.77 % was obtained at 65ºϹ and slices thickness 1mm, while the lowest value of this efficiency 6.03 % was achieved at 45ºϹ and slices thickness 5 mm. The thermal efficiency for a hot air dryer between 4.87 % and 58.49 % at 45and 65ºϹ, respectively Color determination -For color the maximum lightness (L*), redness (a*) yellowness (b*), chrome (C*), hue (H*), Browning index (BI) and Whitening index (WI) were 74.62,15.7, 54.39, 51.72, 88.54, 208.29 and 47.14. -The color change of the dried sweet potatoes slices showed a significant decrease in ΔE values as the radiation intensity and air temperature increased. Quality evaluation -The hardness of dried sweet potatoes slices at 45oC was significantly higher than that of samples dried at 55ºϹ and 65oC. Meanwhile, the shear force of samples dried at 65oC was significantly higher than that of samples dried at 45 and 55oC. -The diameter shrinkage percentage was decreased with the increase of drying air temperature, while it was increased with increase of infrared radiation. Meanwhile, the thickness shrinkage percentage was decreased with the increase of infrared radiation and air temperature. -The total soluble sugar content was increased with increase of air-drying temperature and intensity of infrared radiation, while it was decreased with increase of slices thickness. The total soluble sugar content varied from 0.556 to 0.833 mg/ml. -The total carbohydrate content was increased with increase of air-drying temperature and intensity of infrared radiation, while it was decreased with increase of slices thicknesses. The total carbohydrate content varied from 0.207 to 0.494 mg/ml. -The degradation rate of beta-carotene increased with the increase of air temperature, slices thicknesses and infrared radiation intensity. Beta-carotene content varied from 0.28 to 0.59 mg/100 g. -The rehydration ratio increased with the increase of drying air temperature and radiation intensity, while it was decreased with the increase of slice thickness. The rehydration ratio ranged from 2.55 to 4.26 for the sweet potato slices. CONCLUSION 1. The dried sweet potato slices using hot air-drying method had the highest reduced on the properties of quality comparing to the infrared-hot air-drying method. 2. The treatment of sweet potato slices reduced the time required to reach the safe moisture content by 12.5 % in comparison with the untreated samples and showed better quality of the dried slices. 3. The infrared-hot air-drying method reduced the energy consumption by 45.16 % and increased the drying efficiency by 66.58 % in comparison with the hot air only method. 4. The results show that, the coefficient of determination R2 and Standard error SE for the Logarithmic model recorded R2= 0.99 and SE= 0.122 were considered more proper for describing the drying behavior and predicting the changes in moisture content of sweet potato more than Henderson and Pabis’s were model recorded R2 = 0.94 and SE = 0.51 and Lewis’s model were recorded R2 = 0.96 and SE = 0.241.