Drying: principles and sun drying
Drying preserves food by removing water until microbes cannot grow. Sun drying is the oldest, cheapest method, used for vadam, papad, chillies and dry fish.
How drying preserves
Drying removes water with heat and moving air: heat turns water into vapour and the air carries the vapour away. As moisture falls, water activity drops below what microbes need (below about 0.6 nothing grows), enzymes slow down, and the food becomes light and compact to store and carry. Sun drying uses the sun's heat in the open; dehydration uses artificial heat in controlled dryers. Common pre-treatments: washing, peeling and thin slicing; blanching vegetables; sulphuring or a KMS dip for light-coloured fruits; soaking in salt or buttermilk; and an alkaline oil dip for grapes to crack their waxy skin.
Draw it: the drying curve
Constant-rate drying is controlled by the air (temperature, humidity, speed); falling-rate drying by the food. Case hardening: if the air is too hot and dry, the surface hardens and traps water inside.
Factors that affect the rate of drying
- Air temperature: hotter air dries faster, but too hot scorches the food and causes case hardening.
- Air humidity: dry air takes up more water; humid air slows drying.
- Air speed: moving air sweeps away the moist layer around the food.
- Surface area and piece size: thin slices dry much faster than thick pieces.
- Food composition: sugar and fat hold water; a waxy skin (grapes) slows drying.
- Load on trays: a thin single layer dries evenly; heaped food dries unevenly.
- Pressure: under vacuum water evaporates at a lower temperature.
Sun drying in Indian homes
| Product | How it is made |
|---|---|
| Vadam / vathal (rice or sago) | Rice or sago (javvarisi) batter is cooked, squeezed or spread on cloth, sun-dried for two or three days and fried before eating |
| Papad (appalam) | Urad dal dough with salt and spices is rolled thin and sun-dried |
| Mor milagai and sundakkai vathal | Green chillies or turkey berries are soaked in salted buttermilk and sun-dried in turns over several days |
| Red chillies, turmeric, coriander | Spread on clean mats or floors; turmeric rhizomes are boiled (cured) before drying |
| Amchur (raw mango powder) | Peeled raw mango slices are sun-dried and powdered |
| Raisins | Grapes are dipped in an alkaline oil emulsion and dried, usually in shaded, airy sheds |
| Dry fish (karuvadu) | Fish is salted, then dried on racks or clean sand in the sun |
Advantages and disadvantages of sun drying
A solar dryer (a box with a clear cover that traps the sun's heat) is cleaner and faster and keeps more vitamins.
Sun drying is cheap, but sunlight destroys carotene and vitamin C, and dust and flies dirty the food. Shade or solar-dryer drying keeps more vitamin A in drumstick leaves, curry leaves and greens; such dried leaf powders are used to enrich supplementary feeding recipes.
2 marks: “What is case hardening?” A hard, dry, impervious skin formed when the surface of a food dries too fast in very hot, dry air, trapping moisture inside.
5 marks: “Explain the factors affecting the rate of drying.” Air temperature, humidity and speed; surface area and piece size; food composition; tray load; pressure; add the drying curve.
10 marks: “Discuss sun drying of foods with its advantages and disadvantages.” Principle; pre-treatments; Indian products and how they are made; advantages; disadvantages; solar dryer as an improvement; contrast with dehydration.
Faster drying = hot, dry, fast air on thin pieces; too hot gives case hardening.