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Understanding microwave ready meal heating guidelines

Issue No. 3 Part 1 March 2008

We continue the GAMA Microwave Technology series of articles on understanding microwave heating guidelines by explaining the use and functions of both ‘passive’ and ‘active’ food heating containers, plates, pack materials and films. Due to the complexity of this topic it has been divided into two parts: part 1 - ready meal “passive” container heating guidelines and; part 2 - ready meal “active” container heating guidelines.

Part 1 – Ready meal passive container heating guidelines.

Before discussing the merits of various container materials and techniques, it is necessary to remind ourselves of the generation of microwave energy and its transmission, reflection and absorption capabilities; together with the properties of the output power test beaker, which is in effect a microwave transparent food heating container. The microwave oven uses a cooking cavity enclosure known technically as a multimode applicator; multimode meaning a metal box of fixed dimensions that can support a number of resonant modes at a given frequency. The mode patterns created within the cavity are purely theoretically at this stage and represent an empty cavity condition. When the product is placed into the cavity the mode pattern changes and, during heating, the food’s electrical (dielectric) properties change causing the microwave mode pattern to change even further.

To improve the situation, microwave ovens utilise either a mode stirrer and fixed shelf (catering models) heating position or (consumer models) a rotating turn-table tray. The mode stirrer provides continuously variable mode-pattern changes to the incoming microwave energy, helping to provide more even-heating energy distribution. The turn-table, because of its physical distance from the microwave launch position, has less impact on mode-pattern changes and compensates by rotating the food through constantly changing energy fields to achieve a similar heating effect.

Both fixed shelf and rotating turn-table trays are located at a specific distance above the cavity metal base, at a height determined by the oven manufacturer as being the optimum heating position for their appliance. Other suitable even-heating positions may not be present within the oven cavity, and when heating tall-sided food containers significant temperature stratification can be found within the contents. Alternatively, the product can be decanted and heated separately within a serving dish.

The food position and height on the shelf or turn-table tray is of significance. Placing the product off-centre and repositioning it halfway through, will allow the food to rotate through different microwave mode patterns and improve heating performance.

Nowadays, it is common practice to support the product on a plate to facilitate safe removal of the food from the oven when hot. This is a good working practice; providing the support plate is made from a microwave transparent material, which allows energy to pass through from beneath the tray (thus heating the lower food layers) and remain cool during the process. Many plates and dishes can get hot when subjected to microwave energy due to the metal oxide content within the ceramic material, included to make the crockery more durable and suitable for pre-heated thermal retention. It is essential that the food support plate does not get hot and thereby ‘steal’ energy from the product to be microwaved, if consistent performance and heating guideline times are to be maintained.

Heatproof glass and ceramic glass are suitable transparent materials. Some fully-glazed pottery, plain china and heat-resistant plastic materials may be acceptable. To confirm microwave heating suitability place the plate or container into the oven, along with a small quantity (100g) of water in an adjacent beaker, and operate the oven for 1 to 2 minutes. If the water is hot and plate cool after heating it is suitable for use; if the plate is hot, it is probably unsuitable and should not be used …. Note: a microwave ‘safe to use’ plate or container logo does not necessarily mean it is suitable for microwave food heating, always check beforehand!

Similar usage conditions apply to ‘passive’ food containers and film lidding. A few proven materials being borosilicate glass, polypropylene, polystyrene, polyethylene PET, CPET HDPE, PVDC and polyester films, and moisture resistant paperboard trays.

Keeping the food covered during the heating of ‘wet’ products and trapping the steam, will improve the performance and enhance the heated food’s appearance. The film lidding will need to be pierced before heating to release pressure. Alternatively, self-venting laminated films, or pack pressure release seals and valve devices can be used; the early versions of which incorporated an in-built ‘steam whistle’ to indicate when the product was ready!

Where use is primarily based on the microwave requirement, a container should be selected to promote even-heating. In general, a doughnut ring is considered to be the most effective shape but, impractical for most microwave ready meal applications … or is it?

The doughnut ring concept allows energy to propagate evenly from all sides, including the middle portion. Depending on the product, a raised area within the tray base can perform a similar function, by reducing the food depth in the central hard-to-heat areas. It also allows the solids to gather around the tray edges, making it easier for them to absorb energy during heating. Large family pack products can be divided into portions, and heated directly on the serving plate. With dense products e.g. mashed potato or corned beef hash, the central area can be pushed towards the sides of the tray when cold, allowing energy to penetrate from all directions during heating. The process can be completed by thoroughly stirring the product half-way through and at the end of the heating guideline time period.

Food eruption is a topic of concern. This is where liquid or viscous foods such as soups and sweet or savoury sauces can erupt from the pack for no apparent reason, often immediately after heating. Care should always be taken when removing this type of product from the oven.

If we use a glass of beer as an example, where there is often a visible build-up of trapped air bubbles along the inside edges of the glass, similar can occur with a liquid or viscous food substance. Being at the exposed outer surfaces, these air bubbles and the surrounding food mass are the first to be subjected to the intense microwave energy, whereby they heat-up (superheat) and rapidly expand. Sufficient container headspace should be provided for this expansion, and stirring will help minimise these effects. If there is nowhere for the pressure to disperse the effects on water, beverages and liquid or viscous foods can result in delayed eruptive boiling … e.g. see British Standard BS EN 60335-2-25 Safety of household and similar electrical appliances – Part 2-25: Particular requirements for microwave ovens: clause 7.1 Marking and instructions.

Pressure release eruption can also occur when foods are encapsulated in a membrane or skin (e.g. liver or chicken pieces) and covered by a high viscosity sauce. This effect can be more noticeable after reduction of a product’s salt content, which has resulted in deeper microwave energy penetration. Food items having areas of dissimilar specific heat capacity (e.g. water and fat) can react such that the resultant effects of pressure build-up can be observed as a ‘thumping’ noise, localised sputtering or mid-product eruption. Reducing the sauce viscosity and stirring the product (where practical) will help alleviate this condition.

Items to ponder …..

Is there any significant difference in timing and performance between heating our products on a plate or directly on the microwave oven turn table tray?

Could we enhance performance in a reduced heating guideline time, and consequently save electricity costs in the process?

Hot Tips …..

GAMA has specially adapted variable power microwave ovens, and multiple-channel fibre optic thermometry instrumentation, for on-site product temperature profile analysis within a dynamic microwave operating condition.

Pastry pie products can be enhanced by placing them on absorbent paper and piercing the crust in a couple of places to allow the steam from the filling to escape, thereby helping to keep the pie surfaces dry during heating. There are also microwave reactive films that can help keep pastry surfaces crisp and dry during heating!

Use tapered sided soup and sauce containers. As the liquid expands and rises so the surface tension area increases, helping to reduce the overall pressure build up. It also helps if the container inner surfaces are textured, causing the bubbles to burst as they expand, and thereby reducing pressure build-up as it occurs.

On average, the UK Microwave Association website: www.microwaveassociation.org.uk receives 3 millions hits each year. Why not consider becoming an Association member to receive regular industry information and promote your company, products and services?

Coming soon ….

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

© GAMA Microwave Technology

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