The different ways of preserving the food by using low temperatures are shown in the .
Evaporative cooling: Evaporative cooling can be practised in conditions where no electricity is available for cooling and storing the harvest. It is based cooling by evaporation of water from the system. Water takes energy from its surroundings for evaporation. Coolers based on this principle are called as evaporative coolers. These work best in hot and dry climates. When hot and dry passes over a wet surface; the rate of evaporation is faster resulting in greater cooling. The efficiency of an evaporative cooler depends on the humidity, temperature and movement of the surrounding air and surface area of food product. Examples of evaporative coolers are Janta cooler, pot in pot cooler, which are movable in nature and static cooler named zero energy cooling chamber.
Janta cooler have been developed by Food and Nutrition Board (FNB) of India. It has smaller storage pot placed in a bigger earthen pot filled with water. The bigger earthen pot is kept on wet sand. The inner pot stores the fresh food commodities, which is covered by a damp cloth that is dipped in the water pool. Cloth draws water from the pot and gets wet. Water evaporates from the wet cloth and keeps the storage pot cool.
Pot in pot cooler Pot in pot cooler was developed by a Nigerian teacher Mohammed Bah Abba. It is also known as zeer storage. In this cooler, two pots of different sizes, smaller one lying inside the bigger pot separated by the wet sand . The inner pot is used for storage of fresh produce. Use of this system for cooling increases the shelf life of vegetables from 2-4 days to 17-20 days.
Zero energy cool chamber (ZECC)
Zero energy cool chamber is a immovable cooling chamber developed by Indian Agricultural Research Institute (IARI), New Delhi, for short duration storage of fruits and vegetables on the farm . It is a double walled structure and the gap of about 75 mm (3”) between the two walls is filled with sand. It is covered by a cover made of cane or sack. The sand is saturated with water to keep it moist. As the water evaporates, it removes the heat from within the chamber through the process of evaporative cooling. The chamber can keep the temperature 10-15°C cooler than the outside temperature and maintain about 90 per cent relative humidity. It is suitable for almost all fruits and vegetables except onion, garlic, ginger, potato etc., as these crops require lower relative humidity (65-80 per cent).
Walk-in Evaporative cool chamber These are enlarged versions of ZECC and require electricity to run exhaust fans for better air circulation and cooling. Big walk-in chambers of 2 meters height are made wherein the fruits and vegetables are kept in plastic crates.