


Understanding microwave ready meal heating guidelines

Issue No. 2 February 2008

What does “Microwave Power and Declared Oven Wattage really mean?’ …

Useful microwave energy is measured in Watts. It is determined by measuring the temperature rise of a known quantity of potable water, after it has been heated in an oven using a regulated electrical supply voltage for a given time, in a round, thin-walled borosilicate glass beaker of predetermined dimensions … the specific details are given in British Standard BS EN 60705, Clause 12.

Why water?  Internationally, potable water can be obtained locally and thereby used as an international standard for the harmonisation of power determination. It also has some heating similarities to those of food.

Why use temperature rise? By using an agreed calculation published in the International Electrotechnical Standard IEC 705, microwave power absorbed can be deduced from the temperature rise and expressed in watts.

Why a borosilicate glass thin-walled beaker of predetermined dimensions? Borosilicate glass is reasonably transparent to microwave energy. It enables power to be determined from temperature rise in the water, and is unaffected by any appreciable power loss attributed to the container material. The beaker dimensions have been determined to be compatible with the dimensions of the microwave wavelength, so as to provide an acceptable and suitable method for sales comparison purposes. If the test is carried out with a different container material, different dimensions, or a different volume of water load, it will produce a different output power result.

Why a regulated UK electrical supply voltage? The performances of all microwave ovens are significantly affected by electrical supply voltage variations. Generally, low voltage means lower power relative to the wattage declaration, whereas high voltage usually means higher power determination. See MAFF Science Series Number 11.

Until 1996, the UK electrical supply voltage was declared at 240 volts ±6% (225 to 254 volts). Thereafter, throughout the EU, electrical supply voltage was harmonised at 230 volts, with a revised standard deviation of -6% +10% (216 to 253 volts) for the UK. Since then, all common electrical appliances, including voltage sensitive microwave ovens, have been rated as using 230 volts.

Through our experience of calibrating ovens, we have found it rare to find electrical supplies in the UK much lower than 227 volts, yet frequently above 245 volts, and often as high as 253 volts. In consequence, this bias towards a higher voltage can result in a seemingly 850 watt microwave oven (declared at 230 volts) generating more than 950 watts from a 245 volts supply. This means, that if the product heating guidelines were inadvertently established using these higher power conditions, it may not reach the 72°C (held for 2 minutes) microbiological safe threshold when heated under lower oven power conditions! Conversely, if the heating guidelines were established using lower power, the product may overheat and become organeoleptically unacceptable at a higher power level. It is therefore essential to test the food product thoroughly, and ensure it is sufficiently robust to meet with all typically available UK oven and electrical supply conditions …. see chapters 2, 3, 4 and 5 of the C&CFRA  (Campden & Chorleywood Food Research Association) Guideline No. 14 …Guidelines on the Verification of Reheating Instructions for Microwaveable Foods.

In addition, there are also local day-to-day and seasonal supply voltage fluctuations to consider, especially within a typical food factory environment where electrical demand can intermittently vary. Under these circumstances the problem can usually be alleviated by installing voltage stabilisation equipment, or through making oven adjustments, so as to meet with the specific microwave product development kitchen requirements.   

Microwave power is also dependent on the container and test load size, and this leads to a situation where most microwave ready meals absorb less than the declared oven wattage. A food industry guideline has been published in the UK that specifically addresses the heating of microwave chilled and frozen products. This uses an alphabetical A to E code that refers to the individual oven performance, and advises on the heating category to be applied for a typical microwave sized food product requirement … see chapter 4 of the C&CFRA (Campden & Chorleywood Food Research Association) Guideline No. 14 … Guidelines on the Verification of Reheating Instructions for Microwaveable Foods.

Finally, a tip when developing heating times for new products. Initial power output reduces (up to 12%) during the time that the internal microwave oven components heat up. For instance, after the first product heating test, it is not unusual to find subsequent samples do not reach the same temperature as the first sample tested when using the oven from cold … e.g. the last family member receives the lowest temperature meal!

Top Tips …..

To minimise electrical supply voltage reduction, do not operate the dishwasher or kettle from the same electrical sockets as the product evaluation microwave ovens; especially during the product sampling process!

In addition to voltage regulation and oven calibration services, GAMA can fit forced-air cooling facilities to the internal oven components, thereby providing repetitive product evaluation instead of waiting the 6 hours natural cool down period stipulated!

Coming soon …

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

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