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PRESS RELEASE - September 2008


Recent reports on the quality of the foods that we eat and their effect on our fitness have prompted many manufacturers to review their labelling claims and recipes, and in consequence some may find it necessary to reformulate their products. In such instances, the scale of changes that may need to be made so as to ensure these products comply with new health and nutrition expectations could significantly alter the microwave heating balance of their ingredients. It should also be considered that salt, sugar and fat are three of the most microwave reactive ingredients likely to be used in a microwaveable food product.

Microwave ovens create high frequency electromagnetic energy and it is the interaction between the energy generated and the different substances that results in heat being created. Radiated microwaves are absorbed by many different types of semiconductor and insulating (dielectric) materials, which thereby produces temperature rise. In common with other heating methods the heat is created at the surface. It then penetrates and conducts towards the centre, with the speed of energy dissipation and depth of penetration being dependent on the composition of the item being heated.

Microwaves can be used to heat many different substances from Formula One motor racing tyres to microwave meals. Both are fast, but with the reaction in each instance being different due to the specific nature of the substance being heated.

By using network analysing 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 commonly used food industry half-power absorption determination method, penetration depth into mashed potato or a portion of cooked peas for example is around 7mm.

Salt significantly reduces microwave penetration, and salt reduction would therefore potentially increase energy penetration depth. For instance, using the same method, the penetration depth into potable water at ambient temperature is approximately 11mm, reducing to 5mm if one percent salt is added and 2mm if this is increased to five percent. Conversely, the 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 the dehydration associated with microwave cooking of delicate green vegetables, commonly experienced due to the energy being more concentrated at the food’s surface.  

Many foods fall into similar groupings 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 energy penetration further reduced within ham as its temperature rises. Usually the ham can be thinly sliced or diced within a microwave meal to help evenness of heating, rather than being applied in large chunks.

Often cheese products can produce inconsistent results, dependent on their salt and fat content. Fats and oils are poor absorbers of microwave energy but, fortunately this is off-set somewhat by them having a relatively low specific heat capacity, requiring less energy to raise their temperature.  On a cautionary note, while fats and oils can be difficult to heat (the reason for a susceptor heating patch within microwave Pop-Corn packs) once their temperature starts to rise thermal runaway can occur. This includes non-food items such as unwanted hair removal depilatory waxes, intended for heating in a microwave oven.  

Sugar is a very good absorber of microwave energy. For example, high sugar jam or fruit-filled pie fillings can easily reach temperatures sufficient to burn a person’s mouth, even though the pastry exterior may appear relatively cold. Never-the-less, sugar can be used to balance the heating effects of poor microwave absorbing components within microwaveable deserts and puddings.

In a multiple component chilled meal product, controlling the rate at which individual food components absorb energy and heat up is essential to achieving overall heating uniformity. Individual component shape and consistency, evenness of meal component size, liquid content and viscosity, are all important factors in this respect. With an equivalent frozen product, and in addition to the product composition, it is the control over the latent heat changes experienced during the defrosting process which influence heating uniformity. Ice is almost transparent to microwave energy, making it difficult for a frozen product to effectively absorb energy until defrosting occurs. When this happens the defrosted area can increase disproportionately in temperature, leaving the rest to catch-up as the heat conducts throughout, often leading to an unevenly heated result.

In consequence, should any microwave products containing the above ingredients be reformulated as described, it would be necessary to evaluate their performance under actual and abuse heating conditions, then adjust the back-of-pack usage guidelines where required.

Profile: Gordon Andrews is Managing Director of GAMA Microwave Technology Ltd. a microwave appliance development organisation, which also supplies regular on-site oven calibration and heating technology services to the food industry. Gordon can be contacted as follows:
E-mail: mail@gamaconsultants.com  Tel: +44 (0)20 8309 9222 

He is also the Technical Officer of the UK Microwave Technologies Association (MTA) see website: www.microwaveassociation.org.uk

Ed. For more in-depth microwave ready meal heating guideline procedures, go to the GAMA Food Industry Information website: www.gamamicrowave.com 

© September 2008 GAMA Microwave Technology Ltd.

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