Microwaveability

Canmaking News.com
January 2008

Under normal conditions of use, shallow and wide open steel and aluminium containers are safe to be used in microwave ovens. This is the outcome of an independent study by the renowned Fraunhofer Institute. Researchers also found that food in metal containers is more uniformly heated than in plastic containers. The findings open up a wealth of opportunities for consumers and brand owners alike.

Aimed at looking into the safety and performance of microwave heating food in rigid steel and aluminium containers, the survey was conducted by the renowned Fraunhofer Institute for Process Engineering and Packaging on behalf of the Metal In Microwave Industry Group (MIMIG) composed of Alcan Rhenalu, the Association of European Producers of Steel for Packaging (APEAL), Crown Food Europe, Fördergesellschaft Metallverpackungen (FGM), Impress and Novelis.

The heating experiments were performed with food packed in five metal containers of different dimensions and shape and with four different popular household microwave oven models.

The following metal containers were tested:
a round steel bowl (99 mm diameter x 35 mm height), used with 200 g filling,
a round steel bowl (127 mm diameter x 30 mm height), used with 250 g filling,
a square steel container (125 mm x 125 mm x 25 mm), used with 300 g filling,
a rectangular aluminium container (160 mm x 99 mm x 35 mm), used with 400 g filling,
a round steel container (153 mm diameter, 36 mm height), used with 425 g filling.

These containers were chosen because of their large open surface and shallow profile. This is considered as preferable for use in microwave ovens. Comparison experiments were performed with plastic containers of similar shape and size, filled with the same quantity of test food. The plastic containers were made of CPET and were specified for microwave heating. Test fillings in heating experiments were tap water, egg batter, chili con carne and an infant meal (pasta with vegetables and small meat balls in sauce). All the test materials were liquid or semi-liquid and filled the containers completely from side to side.

The microwave nominal power ratings for the four ovens were 700 W, 800 W, 900 W and 1000 W. The oven constructions were of the standard household type with glass turntable and with the opening of the microwave waveguide in the right side wall of the oven cavity.

Safe for use
During about 1000 microwave heating experiments with normal handling of metal containers, not a single incidence of a spark or a potentially risky situation was observed. In addition, no functional oven damage or unusual degradation of oven power was found after this large number of heating experiments with metal containers.

"We therefore conclude that the use of shallow and wide open metal containers for heating of food in microwave ovens is perfectly viable from a safety point of view, as long as some basic rules of microwave oven operation are respected" says Thomas Pfeiffer, a Fraunhofer researcher. A few basic guidelines, such as fully removing the lid of the container, placing only one filled container at a time in the centre of the glass turntable and leaving a gap between metal container and oven wall, are specified in the Fraunhofer report.

More uniform heat
Although microwave heating times for food in steel and aluminium containers were longer than in similar plastic containers, there was generally less temperature variation and therefore better heating uniformity in the tested metal containers. The study recommends the use of shallow metal containers with a wide open surface area to reduce the heating time.

Conclusions of the study 
 

• Microwave heating of food in steel and aluminium containers with a large  open surface and shallow profile is safe, provided that some basic rules of microwave operation are respected.
  
• No functional oven damage or unusual degradation of microwave power has been  observed.
  
• Microwave heating times for food in metal containers are longer than for food in similar plastic containers. The difference decreases for larger containers. Therefore, it is recommended to use shallow metal containers with a large opening surface.
  
• Temperature distribution was generally more uniform in the tested metal containers than in the plastic containers. 
   

BASIC GUIDELINES FOR SAFE USE OF STEEL CONTAINERS IN MICROWAVE OVENS  

• The metal lid of the container must be removed completely prior to microwave heating.
  
• As with containers of other materials, only full containers must be put into the microwave oven.
  
• Only one metal container must be heated at a time.
  
• Place the metal container in the centre of the glass turntable. An insulating air gap of at least 2.5 centimetres between metal container and oven walls or oven floor should be maintained. For ovens without glass turntable, the container must be placed on a ceramic dish.
  

New market potential
The microwaveability of shallow metal containers opens up a wealth of new opportunities for brand owners and consumers alike. For the consumer, it offers the added convenience of dual ovenability with both microwave and traditional ovens. For brand owners it provides new opportunities to segment their product range further by developing suitable food products such as ready meals and soups for microwaveable metal packaging.

Microwave ready meals fit the convenience megatrend
Indeed, latest Datamonitor consumer trend studies show that consumers of all ages increasingly opt for convenience in their day-today food preparation choices. According to this same study, the convenience mega-trend reflects time pressures, stresses and work-life balance issues which consumers are increasingly experiencing. Consumers constantly seek out more efficient and effective products that can help facilitate crammed lifestyles and respond to the need to maximise leisure time. Traditional mealtimes are evolving, being replaced with lighter and quicker meals.

As a consequence, the ready meals market is growing in every global region, but particularly in Europe, reflecting the consumer's desire for convenience and quick easy meals.

This growing importance of pre-prepared meals is also reflected by the high frequency of microwaving. According to Datamonitor figures, the average consumer now microwaves around 19.3 meals a month in the US and in Europe, nearly 4,5 times a week.

The microwaving popularity demonstrates the ascendancy of ready meals and a focus on time and labour-saving.

Heating experiments were performed on 5 different metal containers, among which a round steel bowl (153 mm x 36 mm) (left, top & bottom); a round steel bowl (127 mm x 30 mm) (right, top & bottom).

KEY ADVANTAGES OF STEEL CONTAINERS IN THE MICROWAVE 

1. Dual ovenability: the same steel containers can be used either in the microwave or in conventional ovens
2. Rigidity, especially after heating, allowing easier handling
3. Less risk of over-heating; no risk of melting unlike plastics
4. Better heating uniformity than with plastics
5. Best recycling performance amongst food containers
   

LEGENDS
Under normal conditions of use, shallow and wide open steel and aluminium containers are safe for use in microwave ovens.

Temperatures are shown as percentage of average end temperature. Each measurement consists of two planes (13 mm measurement is near to filling surface, the 8 mm measurement is near to the bottom of the container).

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