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This study attempts to compare the dehydrating characteristics of pretreated seedless grapes using two different modes of dehydrating (Modified-Quonset dehydrator and Natural-sun-drying) under climatic conditions of Mansoura University (31.043°N and 31.352°E). Solar dehydrating systems for drying agricultural products have clarified to be practical, saving energy and time, economical, and the responsible approach environmentally. Different measurements of macroclimatic parameters (incident solar radiation, air temperature, relative humidity, and wind speed) and microclimatic parameters (incident solar radiation, air temperature, bulk temperature of grape berries, and air relative humidity) had monitored, measured and recorded during the experimental work. The thermal performance analysis for the solar dehydrator, drying characteristics of thin layer of pretreated seedless grapes, and quality of raisins produced from two different modes of dehydrating were tested and evaluated. During the experimental work, the hourly average incident solar radiation outside was 673.8 W/m2 (± 214.2), while that amount inside the solar dryer was 588.9 (± 220.9), which achieved an hourly average effective transmittance of polycarbonate cover of 87.40% (± 6.9). The hourly average air temperatures outside and inside solar dehydrator during was 31.0 (± 2.0), and 49.9 (± 10.1). Accordingly, the increasing percentage in dehydrating-air-temperature was 61.25%. The hourly average air-relative-humidity inside the solar dehydrator was 26.4% (± 1.40), whilst, the outside air-relative humidity was 63.7% (± 3.6). Consequently, solar dehydrator reduced the air-relative-humidity under the outside by 37.3%. The overall thermal efficiency of the solar dehydrator was 72.21%, while, the drying efficiency of the solar dehydrator was 56.48%. As a result, about 27.79% of the total heating power was lost with the exhausted air. The initial moisture content of pretreated seedless grapes prior to the drying process was 77.19% w.b. (± 0.65 w.b.). During the drying process, the drying time, drying rate, and quantity of produced raisins for the Modified-Quonset and Natural-sun-dehydrating, respectively, was 49 and 66 hours, 574.7 and 434.1 g/hr., and 6.840 and 6.785 kg of raisins. Accordingly, each one kilogram of raisins requires 4.902 and 5.158 kg of fresh grapes, respectively. The solar dehydrator increased the rehydration ratio by 12.28% as compared with the Natural-sun-drying system. The total carbohydrates contained in the fresh seedless grapes (462.4 mg/g) decreased into 393.71 and 339.70 mg/g due to drying process, respectively. The concentration of sugar contained in fresh grapes (409.8 mg/g reduced in raisins and reached into 341.32 and 329.68 mg/g), respectively. Main contained of Vitamin C in raisins had also affected by the drying process with a 24.61% and 35.28% reduction for the two different modes of dehydrating, respectively.

Comparison of Drying Characteristics of Seedless Grapes (Vitis vinifera L.) Using Solar Dehydrator and Natural-Sun-Dehydration