Dielectric heating and dehydration
Dielectric heating uses microwaves or radio waves to make water molecules spin and heat food from within. Dehydration uses controlled artificial heat to remove most of a food's water.
How dielectric heating works
Food is a dielectric: a poor conductor of electricity that contains polar water molecules and salts. In a rapidly alternating electromagnetic field, the water molecules keep flipping to line up with the field, millions to billions of times a second; the friction makes heat inside the food itself (volumetric heating), not just at the surface. Two kinds are used: radio-frequency (RF) heating at 13.56, 27.12 or 40.68 MHz, which penetrates deeper and suits large blocks, and microwave heating at 915 MHz (industry) or 2450 MHz (home ovens).
Uses of dielectric heating
- Thawing and tempering frozen blocks of fish, meat and butter.
- Post-baking drying of biscuits, which removes the last moisture and prevents cracking (checking).
- Pasteurising and sterilising packed ready meals.
- Killing insects in grains and pulses (RF disinfestation).
- Cooking, reheating, blanching and microwave or vacuum-microwave drying of fruits and vegetables.
Advantages and limitations of dielectric heating
Dehydration
Dehydration is removing most of the water from food by artificial heat under controlled temperature, humidity and air flow, in dryers such as tray, tunnel, drum and spray dryers (Unit IV). It differs from sun drying, which depends on the weather. Heat supplies the energy to evaporate water and moving air carries the vapour away. Lower water activity stops microbes and slows enzymes. Textbook guides: dried fruits keep at about 15-20% moisture, because their sugar holds water safely, while vegetables must be dried much lower, to about 5-8%. Yields are given as the dehydration ratio and the rehydration ratio.
Worked example: dehydration and rehydration ratios
10 kg of fresh carrots lose 2 kg in trimming and peeling. The prepared carrots dry to 1 kg. 20 g of the dried carrot weighs 110 g after soaking. Find the dehydration ratio, rehydration ratio and yield.
- Prepared weight = 10 − 2 = 8 kg.
- Dehydration ratio = prepared weight ÷ dried weight = 8 ÷ 1 = 8 : 1.
- Some books divide the raw weight instead: 10 ÷ 1 = 10 : 1. Always say which one you used.
- Rehydration ratio = soaked weight ÷ dried weight = 110 ÷ 20 = 5.5 : 1.
- Yield of dried product from raw carrots = 1 ÷ 10 × 100 = 10%.
Dehydration ratio 8 : 1 (on prepared weight), rehydration ratio 5.5 : 1, yield 10% of the raw weight.
Microwave cooking and blanching are short and use little water, so they keep more vitamin C and B vitamins than boiling. Dehydrated vegetables and powders of amla, drumstick leaf and tomato carry vitamins and minerals into seasons and places where fresh produce is scarce.
2 marks: “What is dielectric heating?” Heating of food by high-frequency radio or microwave fields that make polar water molecules oscillate, producing heat inside the food.
2 marks: “Differentiate drying and dehydration.” Drying usually means sun drying under natural conditions; dehydration means drying by artificial heat under controlled temperature, humidity and air flow.
5 marks: “Write a note on dielectric heating and its uses in food processing.” Principle (dipole rotation); RF and microwave frequencies; uses (thawing, drying, pasteurising, disinfestation, cooking); advantages and limitations.
Microwave: 2450 at home, 915 in industry. Dehydration = drying with a thermostat.