Can You Trust Your Data For Meal Settings?...

The microwave oven is a peculiar appliance, having characteristics that provide energy transfer directly related to the size and nature of the food substance, and thereby requiring individual heating guideline information. Recent fundamental changes in oven design have created significant variance between ovens, which is making product development more difficult for the ready meal supplier.   Just over a decade ago, the UK food producers and oven suppliers voluntarily agreed to adopt a common method for determining microwave output power, based on British Standard BS EN 60705 (International Standard IEC 705) using a 1000g water test load to establish the 650 watt, 750 watt and 850w performance levels. They also jointly agreed to the implementation of a sliding scale ready meal heating guideline procedure, using A to E rating codes. The UK A to E determination being based on a new method, using a smaller 350g water test load to represent the lower level of power absorbed by ready meal (less than 450g) food products   ******************************************************* Heating categories of domestic microwave ovens when heating small food items, based on power output into a 350g water test load at 240 volts ±1%     Typical Oven                   Heating Category                               Rating Code   Information                               Code                                            Equivalent      * 850/800 watts                       * E                                   * 741 to 800 watts        * 750 watts                              * D                                   * 681 to 740 watts         700 watts                                 C                                      621 to 680 watts         * 650 watts                              * B                                   * 560 to 620 watts      * British Retail Consortium BOP microwave heating guideline requirements.   *********************************************************   These procedures were intended to create some commonality between different microwave models and brands, allowing food suppliers to develop products that could be reliably heated in any size and type of oven.   In view of the very nature of heating a food substance in an electro-magnetic high frequency microwave field, many other user aspects needed to be determined, and especially when using the oven for product development purposes. This includes the power output reduction effects of using an oven with hot internal components (typically experienced after a few minutes use), stability of the electricity supply voltage, product weight and storage temperature, and the effects of meal layout or food component ratio. These factors were introduced in accordance with industry guidelines (Guideline No 14 Guideline on the Verification of Reheating Instructions for Microwaveable Foods) published by Campden & Chorleywood Food Research Association in June 1997. The A to E rating information listed in these guidelines being established by regulating the electricity supply at 240 volts ±1%.   Conditions have moved on since oven standardisation and full implementation of the UK rating procedures, and we now have electrical supply harmonization at 230 volts throughout EU member countries to consider. This change has been brought about to allow all electrical appliance manufacturers to standardise their products at 230 volt operation, and thereby simplifying pan-European distribution.   The difficulty is however, that 230 volts EU harmonization (as far as the UK is concerned) is a paperwork exercise only. In reality the UK 240 volt supply remains unchanged, whereas appliances are now designed for 230 volt application.   With many household appliances this has little or no effect, but the microwave oven is susceptible to changes in the electrical supply, in a similar way to early Black & White televisions, which experienced picture decay during low voltage periods. With microwaves the output power generally increases with voltage rise and decreases with voltage decline, but this is dependent on the components used and the oven manufacturers declared classification. Many classify their ovens at 230 volt, while others 230/240 volts operation, often using 235 volt microwave generating components to help stabilise oven performance under the much higher UK voltage maximum (253 volts) conditions, while some others provide output power determination for both 230 volt and 240 volt operation.   Under these non-standard conditions it is easy to appreciate how difficult it can be to establish meaningful food heating guidelines, and provide for product quality under all circumstances. The microwave ovens used for product testing are, quite rightly so, designed for consumer home use; they are not really suitable for industrial application where other external conditions may apply. Industrial processing equipment can affect the electrical supply, as it regularly pulsates during operation, and together with the addition or removal of on-line equipment, it can often create un-explained detrimental effects on the ovens being used in adjacent QA or product development kitchen areas.   With food retailers demanding better heating quality and back-of-pack information, it is essential to have the microwave performance aspect of product development firmly under control. For example, in most instances a power increase will heat the food item much quicker, or to a higher temperature within the prescribed guideline. It follows that lower power will need more time, and that the guideline may not be sufficient for the required microbiological temperature threshold to be reached. Under extreme high voltage conditions, the relationship between the oven's magnetron and transformer power supply can become un-stable, creating product heating unevenness or food damage where previously none had existed!   In a recent survey (GAMA oven calibration data 2001 - 2003) many 230/240 volt 800 watt E rated ovens were found to be D rated models, at supply voltages below 237 volts some were only just in the E category at 240 volts. The majority of 230 volt 850 watt E rated ovens, at supply voltages above 237 volts, were either very high E rated models or completely off the UK rating scale; some by as much as 2 rating category positions. The same 850 watt ovens when operated at the UK upper voltage limit of 253 volts, being capable of delivering in excess of 950 watts.   There is little point in spending valuable development resources only to have the product rejected at a later stage, and to help alleviate the problem some organisations have installed voltage stabilisation equipment in their test areas. Others have purpose built microwave ovens, which provide for product evaluation across the full 650 watt B to 850 watt E food heating category range, from the same appliance. The majority of these organisations have their ovens regularly calibrated and adjusted on-site, to ensure continuity of product quality www.gamaconsultants.com   It is too late to undo the effects of EU electricity supply harmonization, and with UK legislators unable or unwilling to get involved an immediate solution seems unlikely. Clearly the best way forward is to regularly check and monitor test oven performance, and take remedial action before any adverse heating effect can enter the system.   Author: Gordon Andrews GAMA Microwave Technology Limited.