الفهرس 
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Abstract
Introduction : Onion (Allium cepa) is one of the most popular vegetables and the oldest herb cultivated in many countries. Onion is providing many health benefits, which makes it very important among the consumers day by day. Onion serves as a good medicinal compound for cataract, cardiovascular disease and cancer due to its hypocholesterolemic, thrombolitic and antioxidant effects. In Egypt, the total onion production is 2,958324 tons yield from planted area of 194,092 feddans yearly, FAO, (2018). Dried onions in the form of flakes or powder are in extensive demand in several parts of the world which used as flavour in the formulation of meat products, gravies, canned foods, sausage and season¬ings, soups, dry soup mixes, cheeses, crackers, other snacks and special food products as ketchup, sauces, soups, salad dressings, sausage Gouda, et al. (2014) . In Egypt natural sun drying for preserving food and agricultural crops has been practiced since ancient times. Conversely, this process has many disadvantages, i.e., products get spoiled due to rain, wind, moisture, and dust; loss of produce due to birds and animals; deterioration in the harvested crops due to putrefaction, insect attacks, and fungi. The modern trend of solar drying equipment such as hybrid solar dryers offers an exceptional method that can process the vegetables and fruits in clean, disinfected, and hygienic conditions to national and international standards with low costs of energy. The aim of this study is testing and evaluating hybrid solar dryer (with auxiliary heaters). The specific objectives of this study are : 1- To test hybrid solar dryer with auxiliary heaters for drying onion slices under Egyptian climatic conditions. 2- To test the effect of air temperature and slices thickness on drying characteristics of onion slices. 3- To examine two different drying models for describing the drying behavior and predicting the changes in onion slices moisture content. 4- To examine the effects of the hybrid solar drying method on quality changes of the dried onion slices. Materials and methods : Materials : The Beheri (Allium cepa L.) onion available in the local market was selected for experimental work. It was obtained from a local producing farm in Dakhlia governorate. The initial moisture content of the freshly harvested Onion ranged from (488.2– 614.3% (d. b.)). Experimental Set-Up : A hybrid solar dryer with auxiliary electrical heater was used for the experimental work. The dryer is a forced convection solar dryer with auxiliary electrical heaters. The structure of the dryer included the drying bed with plenum chamber covered by semicircular secondary collector and two suction fans exhaust of moist air out of the dryer, main solar collector with auxiliary heaters for heat supply to the drying air and temperature control system to get constant levels of air temperature during the drying process. Experimental Treatments : 1- pre- Treating onion slices with a solution containing 1% potassium meta-bisulphite and 1% citric acid for 5 min and untreated samples. 2- Three different temperatures for the drying air (50, 60, and 70°C). 3- Three different thickness of onion slices (2, 4, 6 mm) with a constant air velocity of 0.32 m/sec. Experimental Measurements and Instrumentation. 1. Onion moisture content. 2. Air velocity over the drying bed. 3. Onion mass measurement. 4. Air temperature and relative humidity. 5. Solar energy measurement. 6. Quality changes of the dried onions (Flavour in terms of thiosulphinate content (TC), The Colour in terms of (Anthocyanins content or non-enzymatic browning determination), Vitamen C or Ascorbic acid content and Rehydration Ratio). Experimental Procedure : Prior to each drying run, the dryer auxiliary heaters were adjusted for the required level of drying air temperature. After the dryer working at stable condition, the dryer bed was loaded with the onion slices which were distributed uniformity in a single layer. The dryer bed was divided into six equal parts to accommodate the samples of onion representing two onion conditions (treated onion and untreated onion) in three replicates for each condition. Air temperature and relative humidity were measured every half hour during the drying process at different points. Moisture content was measured before the drying process. The drying process was stopped when there was no noticeable change in the weight of the samples. For measuring, the electrical consumption of the auxiliary heaters in kWh. An electric counter was connected to the auxiliary heater and the total electricity consumption of each experimental run was recorded. Drying Models for Simulating the Drying Data : In this study Lewis’s equation and Henderson and Pabis’s equation were used to describe thin layer drying of onion slices. Results : 1. The hourly average available solar radiation during the period of the experimental work was 563.4 W/m2. 2. The solar collector of the dryer could increase the air temperature inside the plenum chamber by arange of 7.58 to 8.46 OC. 3. The reduction in onion slices moisture content was varied and increased with the increase of drying air temperature and the reduction of slices thickness. 4. The constant drying rate period was not detected for most studied levels of drying air temperature and slice thickness, while all the drying process occurred during the falling rate-drying period. 5. The drying constants (kL), ( kH) and (A) increased with the increase of drying air temperature but decreased with the increase of slice thickness. 6. Both Lewis’s model and Henderson and Pabis’s model described the drying behavior of onion slices satisfactorily as indicated by the high values of coefficient of determination and the low values of standard error (SE). 7. Lewis model showed the highest value of coefficient of correlation (r) and the lowest values of chi-square (χ2), mean bias error (MBE), root mean square error (RMSE). So, one may say that Lewis model was the most proper model in describing the drying behavior of onion slices under the examined conditions. 8. The absorption efficiency of the solar collector ranged from 69.6 to 72.8% and the heat transfer efficiency ranged from 79.5 to 82.6%. While the overall thermal efficiency of the solar collector ranged from 63.1 to 65.2 %. 9. The overall thermal efficiency of the examined solar dryer with the auxiliary heaters ranged from 22.9 to 34.7% and the solar collector provided 87.74, 78.83 and 67.46 % of the total heat energy consumption for air heating to the level of 50, 60 and 70 °C, respectively. 10. The final moisture content of the dried onion slices under the studied conditions almost reached the recommended range of the dried onions 6-7%.The treatment of onion slices reduces the time required to reach the safe moisture content by nearly 22% in comparison with the untreated samples. 11. The rehydration ratio ranged from 3.95 to 5.53 for the treated onion slices and between 4.37 and 5.6 for the untreated slices and it was well within the permissible levels of the dried onions (R.R < 2.5). 12. The increase in temperature resulted in a slight decrease in thiosulphinate content in onion but the increase in thickness facilitated significantly higher thiosulphinate content. The treated samples showed higher thiosulphinate content than the untreated samples. 13. The optical index (OI) values for anthocyanin values varied from 61.84 to 69.73 for treated onion slices and from 55.81 to 63.64 for untreated slices. Meanwhile, greater thickness increased the drying time thereby exposing the slices to heated air for a longer time compared to thin slices. 14. The increase in drying temperature and thickness resulted in a higher reduction in the ascorbic acid. However, the treated slices showed less reduction in the ascorbic acid due the reduction in the drying time. CONCLUSION : 1- The developed solar dryer with auxiliary heaters could dry the onion slices to the required level for safe handling, storage and keeping the quality of the dried slices without significant degradation. 2- The treatment of onion slices with solution containing 1% potassium metabisulphate, 1% citric acid for 5 minutes reduced the time required to reach the safe moisture content by 22 % in comparison with the untreated samples and showed better quality of the dried slices. 3- Both Lewis’s model and Henderson and Pabis’s model could describe the drying behavior of onion slices and predict the changes in moisture content However; Lewis’s model was more precise. 4- The onion slices dried at air temperature of 50oC and slices thickness of 2 mm for treated samples recorded the highest quality in terms of higher thiosulphinate, anthocyanin contents, ascorbic acid, rehydration ratio and less energy consumption.”