Technology
The Science of Solar Crop Drying: Principles and Practices
Food holds a key position in development. High moisture crops are prone to fungus infection and spoilage during storage. Drying reduces the moisture content of a product to a level below which deterioration does not occur, facilitating early harvest, long-term storage, and better returns for farmers.
01
Advantages of Crop Drying
Drying offers numerous benefits beyond simply preserving food. It facilitates early harvest, permits planning the harvest season, helps in long term storage, and reduces the requirement of storage space. Furthermore, it helps farmers fetch better returns and sell a better quality product, while also aiding in the handling, transport, and distribution of crops. For seed crops, it permits maintaining the viability of seeds.
02
Working Principles of Solar Drying
There are three main modes of drying using solar energy: (i) Open Sun Drying (OSD), (ii) Direct Solar Drying (DSD), and (iii) Indirect Solar Drying (ISD). In OSD, short wavelength solar energy falls on the crop, is absorbed, and converted into thermal energy, which evaporates moisture. However, it is prone to unexpected rain, dust, and insects. In DSD (Cabinet Dryers), solar radiation transmits through a glass cover and is absorbed by the crop. The glass traps long-wavelength radiation, increasing the temperature significantly more than OSD and protecting against convective losses. ISD systems prevent direct exposure to solar radiation, minimizing discolouration and cracking.
03
Moisture Content and Safe Temperatures
Different crops have varying initial moisture contents and require specific final moisture levels for safe storage. Additionally, each crop has a maximum allowable drying temperature to prevent degradation. For instance, Paddy starts at 22-24% moisture and is dried to 11% at a max of 50°C. Vegetables like Onions and Garlic start around 80% moisture and must be dried to 4% at 55°C. Fruits like Apples and Bananas, starting at 80% moisture, are typically dried to 15-24% at temperatures up to 70°C.
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