Concentrated solar drying of tomatoes

Abstract:  Fruits and vegetables are an integral part of the human diet. Many developing countries such as Tanzania experience post-harvest losses of 40%, and there is little ability to preserve and store foods for off-season consumption due to expensive or unreliable energy and a lack of access to refrigeration. Alternatively, fruits and vegetables can be dehydrated using solar crop dryers. Because many developing countries are in tropical regions, properly dehydrating fruits and vegetables to moisture levels appropriate for storage and off-season consumption can be difficult. In an attempt to overcome the challenges of the high humidity, intermittent clouds, and haze often present in tropical climates, this paper investigates the effectiveness of adding a concave solar concentrator built from low-cost, locally available materials to a typical Tanzanian solar crop dryer. Two identical solar crop dryers were constructed, with one serving as the control and the other for testing the solar concentrator. Drying trials using Roma tomatoes with initial moisture content of approximately 90% were conducted in Davis, California (38° 32′ 42′′ N/121° 44′ 21′′ W) in various climatic conditions throughout the summer and fall. Tomatoes were considered dried at 10% moisture content. Temperature, relative humidity, and solar radiation were measured outside as well as within each of the dryers to determine how the addition of a solar concentrator can affect the drying rate of tomatoes in solar crop dryers. The concentrator proved to be effective, reducing drying time by 21% in addition to increasing internal dryer temperature and reducing relative humidity. An additional study on the quality of the fresh and dried tomatoes found that the pH, titratable acidity, color, Brix, lycopene, and vitamin C determined there was no significant difference in quality between tomatoes dried with and without the concentrator.