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FOOD INDUSTRY INFORMATION SERIES

Understanding microwave ready meal heating guidelines

Issue No. 1 January 2008 - see also Addendum to Issue No. 1 January 2008

How microwaves interact with food substances’ …

Microwaves are short high frequency (non-ionizing) electromagnetic waves of energy that, in common with household electricity, have both electrical and magnetic properties. They are called microwaves due to their wavelength, which is approximately 12.25cm in free space; wavelength being the distance between each similar wave position of a complete wave form. There are a number of permissible ISM (Industrial, Scientific and Medical) frequencies available for food heating use, where both domestic and catering microwave ovens have been allocated the 2450MHz ±50MHz frequency waveband.

Let me first explain that microwave ovens create energy, not heat, and it is the interaction between the energy created and the different substances that results in heat. Radiated microwaves are reflected by metals, but absorbed by many semiconductor and insulating (dielectric) materials, thereby generating a temperature rise. In common with other heating methods heat is created at the surface, which penetrates and conducts towards the centre, speed of energy dissipation and the depth of penetration being dependent on the composition of the substance or material to be heated.

You will note as yet no mention of food. Microwaves can be used to heat many different substances, from Formula One racing tyres to microwave meals. Both arefast, but the reaction in each instance is different due to the nature of the substance being heated.

Without going into too much detail, and by using specific laboratory instrumentation, it is possible to determine the energy absorption and dissipation rate of a food substance, and from the data obtained calculate its penetration depth. Using the food industry half-power absorption method, penetration depth into mashed potato or a portion of cooked peas is around 7mm.

Similarly, the microwave penetration depth into potable water at ambient temperature is approximately 11mm … add 1% salt and this reduces to 5mm … add 5% salt and this reduces to less than 2mm. Conversely, removal of salt aids energy dissipation and allows deeper penetration. Most good quality microwave oven Use & Care booklets suggest seasoning should be added after heating. This is to minimise the effects of localised food burning (sometimes sparking under frozen conditions) and dehydration, especially when cooking delicate green vegetables, due to energy being concentrated at the food surface.

Many foods fall into similar groups but, equally many do not. Although pork and ham come from the same animal they heat at differing rates due to the curing process, with penetration becoming less into ham as its temperature rises. To help evenness of heating the ham can be sliced or diced, rather than being applied to the product in large chunks.

Cheese products can also produce inconsistent results, dependent on salt and fat content, where typical fats and oils are poor absorbers of microwave energy. Fortunately, this is compensated somewhat by them having a low specific heat capacity, thereby requiring less energy to raise their temperature. On a cautionary note, fats and oils in common with high sugar content food items can sometimes heat up very quickly, leading to thermal runaway .... e.g. this includes non-food items such as depilatory waxes intended for heating in the microwave, and very hot fruit pie fillings that can burn the tongue, even though the pastry exterior appears relatively cold!   

In a multiple component meal product, controlling the rate at which individual food components absorb energy and heat up is critical to achieving overall heating uniformity. Individual component shape, evenness of main component size, and liquid content viscosity, are all important factors in this respect. With a frozen meal equivalent it is the microwave energy distribution, product composition, and control over the latent heat changes experienced during the defrosting process, which influences heating uniformity. Ice is almost transparent to microwaves, making it difficult for the frozen product to absorb energy until defrosting occurs. When this happens the defrosted portion can increase disproportionately in temperature, leaving the rest to catch-up as the heat conducts throughout, often leading to an unevenly heated situation … e.g. it’s generally the exposed food tray corners and edges that heat-up first!

Additional to oven performance, it is the shape and overall presentation of the product which influences this process, and thereby provides an evenly heated result. Exposing the difficult to heat foods to the microwave energy, and using them to help shield the easy
to heat items, is a good process objective. Re-positioning the food tray within the oven during heating, stirring or rearranging the food items throughout, and allowing the product to rest a short while after heating, are also useful techniques for obtaining an evenly-heated result. Alternatively, there are purposed designed food containers that can provide more even heating performance; often in a much shorter energy saving time! 

Hot Tips …..

As you can see, salt and other food product ingredients can significantly affect heating performance, sometimes leading to overheating and eruptive results. Always complete a comprehensive product evaluation review when changes are made to the food ingredients, or packaging shape and materials used, to ensure back-of-pack heating guidelines are un-changed. Whenever possible select a variety of different oven brands, power outputs, and cavity sizes, to obtain the maximum information throughout the review process.

Remember to document the results, photograph the heated product, and safely file the data for future QA comparison and due diligence reporting purposes. 

Coming soon …

... Ready meals active container heating guidelines
... Industrial microwave food production considerations
... Breakthrough in Even-Heating Microwave Technology

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