FOOD PROCESSING AND PRESERVATION: HEATING METHODS
Diagram
Heat from current, or heat from waves
Ohmic heating
Electric current passes through the food itself. The food resists the current, and that resistance makes heat throughout the food.
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Microwave heating
Electromagnetic waves enter the food and make water molecules vibrate, which makes heat. The waves only reach so far in, so heating can be uneven.
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The key point
Both heat fast from the inside, but one uses current and the other uses waves.
Figure 1: Ohmic heating passes current through the food; microwave heating uses electromagnetic waves. Drawn in our own style from Rahman, Handbook of Food Preservation, chapter 28 on microwave pasteurisation and sterilisation and chapter 30 on food preservation aspects of ohmic heating, and Sun, Handbook of Food Safety Engineering, chapters on microwave processing and ohmic heating treatment.
Introduction
Ohmic heating and microwave heating both heat food fast from the inside, but they use different energy. Ohmic heating passes an electric current through the food itself, and the food resists the current, so heat is made throughout the food at once. Microwave heating sends electromagnetic waves into the food, and the waves make water molecules vibrate, which makes heat. The current route heats evenly wherever current flows, while the wave route is limited by how far the waves reach into the food.
Rahman gives a chapter to each method, and Sun gives a chapter to each in the food safety engineering handbook. This article follows those two books.
Quick comparison
Table 1: Ohmic heating compared with microwave heating
| Aspect | Ohmic heating | Microwave heating |
|---|---|---|
| Energy used | Electric current passed through the food | Electromagnetic waves, microwaves |
| How heat is made | Electrical resistance of the food makes heat | Water molecules vibrate in the wave field and make heat |
| Where heat is made | Throughout the food wherever current flows | Where the waves reach, strongest near the surface |
| Evenness | Even, because current flows through the whole piece | Can be uneven, with hot spots and cold spots |
| What the food needs | Electrical conductivity, so current can flow | Water or other polar molecules that respond to microwaves |
| Heating speed | Fast, because heat is made inside the food | Fast, because heat is made inside the food |
| Solids and liquids | Heats solids and liquids together at a similar rate | Liquids and thin pieces heat faster than thick solids |
| Equipment | Electrodes in contact with the food, and a power supply | Magnetron, waveguide and a cavity, with no contact needed |
| Main problem | Needs the food to conduct, and electrodes must touch the food | Cold spots where waves do not reach, and limited penetration depth |
| Typical foods | Soups, sauces, liquids with particles, ready meals | Ready meals, reheating, pasteurisation of liquids, tempering |
Scientific explanation
Definition
Ohmic heating is the heating of food by passing an electric current through the food itself, so the electrical resistance of the food turns the current into heat throughout the food. Microwave heating is the heating of food by electromagnetic waves, which make water and other polar molecules vibrate, so heat is made inside the food. Both are rapid internal heating methods, and Rahman gives a chapter to each.
Working principle
In ohmic heating, electrodes touch the food and current flows through it. Every part of the food that carries current makes its own heat, so a soup and the pieces in it heat together at a similar rate, and there is no hot surface and cold centre. In microwave heating, the magnetron makes microwaves that travel into the food and are absorbed as they go, so the outer part absorbs more than the deep part, and standing waves make hot spots and cold spots. That is why microwave heating is fast but uneven, while ohmic heating is fast and even where the current flows.
Apparatus
The books used here name electrodes and a power supply for ohmic heating, and a magnetron, waveguide and cavity for microwave heating. No laboratory apparatus for telling the two apart is named, because the distinction is in the energy source, not in a test.
Measurement procedure
The distinction is made from the process record: current and voltage for ohmic heating, and microwave power and time for microwave heating. Temperature is measured with probes in both cases, and cold spots are the main check in microwave heating. The books used here give no worked calculation with numbers for this comparison, so none is given.
Factors affecting readings
For ohmic heating, the electrical conductivity of the food decides how fast it heats, and conductivity changes with temperature, salt and composition. For microwave heating, water content, thickness and shape decide how far the waves reach and where hot spots form. In both cases, food composition and piece size change the result.
Formulas and calculations
The books used here give no formula for this comparison, so no formula or worked calculation is given.
Industrial applications
- Ohmic heating is used for soups, sauces and liquids with particles, where even heating is wanted.
- Microwave heating is used for ready meals, reheating and pasteurisation of liquids.
- Ohmic heating is used where solids and liquids must heat together at the same rate.
- Microwave heating is used for tempering frozen food and for fast cooking in catering.
- Both are used to pasteurise and sterilise food faster than conventional heating.
Advantages and limitations
- Ohmic heating: advantage: even heating of solids and liquids together, with no cold centre. Limitation: the food must conduct electricity, and electrodes must touch it.
- Microwave heating: advantage: fast heating with no contact and no electrodes. Limitation: uneven heating with cold spots, and waves only reach so far into thick food.
- Ohmic heating: advantage: rapid and uniform where it applies. Limitation: needs a conductive food and a special heater.
FAQ
Is ohmic heating the same as microwave heating?
No. Ohmic heating passes current through the food, while microwave heating sends waves into the food.
Why does microwave food have cold spots?
Waves are absorbed as they travel in, and standing waves make hot and cold spots, so thick pieces heat unevenly.
Can ohmic heating heat a soup with pieces?
Yes. That is its strength: the liquid and the pieces heat together because current flows through both.
Does ohmic heating need water in the food?
It needs electrical conductivity, which in food usually comes from water and dissolved salts.
