Solar energy is also used to dry fruits and grains, indirectly. In this method a solar collector captures solar energy and raises the air temperature. The warmed air is directed to a conventional batch or continuous dryer. In a flat-plate air-type solar collector, the temperature rise is usually about 5°C over a 24-hr period for airflows used in the near-ambient temperature drying of granular products. The air may be heated further by conventional fuels if high-temperature drying is desired. Overdrying and susceptibility to spoilage are major problems in solar drying of deep beds of cereal and oil seeds.