Water boils at 100° C at sea level. The atmospheric pressure which acts
on water is 14.7 psia (pounds per square inch – absolute). This atmospheric pressure varies with the altitude. So the boiling point of water is influenced with respect to the atmospheric pressure. Perfect vacuum is ‘O’ psia. Any pressure between 14.7 psia and o-psia is ‘negtive pressure’, that is, partial vacuum. Vacuum drying takes advantage of the decrease in the boiling point of water that occurs as the pressure is lowered. This type of drying keeps the products with very low moisture content. If salt water is subjected to this negative pressure, the water will boil by taking the heat from the ambient. In case of reducing atmospheric pressure, i.e. 14.7 psia to 0.15 psia, water will start boiling at 7°C itself.
Freeze drying, convection drying, microwave drying, dielectric drying, conduction drying, U-V radiation drying, etc, are some other drying methods applied in various industries such as textile, plywood manufacturing, paper products, production of dried milk, soaps, detergents, pharmaceuticals, serums, bacterial and viral cultures, vaccines, fruit juices, vegetables, coffee and tea extracts, sea foods, meats, cereals etc.