DEVELOPMENT AND EVALUATION OF AN INDIRECT SOLAR DRIER FOR TOMATO

A prototype of an indirect forced convection solar drier was designed for drying of tomato. The drier consists of a flat-plate concentrating collector, a heat storage cum auxiliary heating unit and a drying unit. The drier is a multi stack type with five trays having a loading capacity of 18 kg of fresh half cut tomato. The drier was tested in different weather and operating conditions such as day time drying using solar radiation and night time drying with ambient air (without external heating), night time heating with hot water flow from the storage tank without water heater, night time heating with hot water flow using water heater and in adverse weather (cloudy, foggy or rainy) day-night heating using water heater. Day time average temperature at the outlet of the collector was about 30°C over ambient air temperature and collector efficiency varied from 25 to 60%. Colour, ascorbic acid, lycopene and total flavonoids of fresh and dried tomato at different final moisture contents were determined. Tomato halves were pre-treated with UV radiation, acetic acid, citric acid, ascorbic acid, sodium metabisulphite and sodium chloride at different doses. Among them, 8 gl^-1 sodium metabisulphite was found effective to prevent the microbial growth at lower temperature (<45°C).