SOLAR DRYING OF RAISING

The drying of Sultana seedless grapes was investigated under intermittent and continuous operating conditions in a laboratory solar installation involving a thermal storage bed and an auxiliary heater. The effect of pretreatment, and of the a i r velocity on the drying rate of the grapes at constant temperature was also studied in relation t o the quality of the dried product.

Solar drying of the grapes was accomplished in 30.5 to 60.5 h of intermittent operation, or 19 to 60 h of continuous drying, involving the thermal storage bed and the auxiliary heater. The shortest drying time (19 h ) and the highest quality dried product were obtained with grapes dipped in a hot (80 C ) solution of sodium hydroxide and ethyl oleate, which were dried continuously at 42°C and 2m/s, air temperature and velocity respectively.

The mean apparent diffusivity of moisture in raisins at 6o°C and air velocity 2m/s was estimated as 1.0.10-10 m2/s.