To some extent, today’s microwave ovens are self-regulating and withstand a certain amount of reflection. This makes it possible to use a wide range of packaging materials.

For ready-made meals, for example, the energy can be focused or reflected away from parts of the meal so that all the ingredients in the dish are sufficiently cooked.  

Because there are still ovens in use that are vulnerable, most manufacturers choose to use materials that do not reflect or absorb the microwaves. Therefore, the most common choice is a few types of plastic materials that are able to withstand the temperatures that can occur in a microwave oven without there being a risk of migration from the plastic to the food.

Polypropylene (PP), crystallized polyethylene terephthalate (CPET) and mixtures of polyphenylene oxide and polystyrene (PPO/HIPS) provide lightweight, thermally stable and strong bowls with good barrier properties. 

Cellulose-based materials

Due to increasing environmental awareness, there is great interest in cellulose-based materials instead of plastic.

However, such materials must have an internal coating of plastic or similar substance that acts as a gas barrier, and to prevent the material from absorbing water or grease and swelling.

In principle, such packaging is suitable in combination with microwaves, but has not yet been systematically tested and documented. 

Sharp edges

Sharp edges and corners are generally a source of overheating in microwave ovens and this also applies to packaging.

Typical examples are bags of sauce that narrow towards the edges where the bag has been joined together.

Overheating in these zones can cause the plastic to melt, which may lead to leakages. In a worst-case scenario, the leak might be so small that it is not noticed until the product reaches the consumer.

Plastic packaging is usually moulded or extruded which means that it is easy to make rounded edges and corners. 

When choosing a packaging size, it is important to remember that the microwave penetration depth into foods is generally less than 1 cm. The height or width of the food should therefore be limited to 2 to 3 cm to avoid cold zones and long periods of heat treatment with unnecessary heat loads on the outer edges.

Sources (in addition to personal experience):

• S.Chandrasekaran, S. Ramanathan, T. Basak, Microwave food processing - A review. Food Res Int52, 243-261 (2013).
• European Parliament Council (2018). DIRECTIVE (EU) 2018/852 OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL OF 30 May 2018 amending Directive 94/62/EC on packaging and packaging waste.
• Deshwal, G.K., Panjagari, N.R. & Alam, T. An overview of paper and paper based food packaging materials: health safety and environmental concerns. J Food Sci Technol 56, 4391–4403 (2019). https://doi.org/10.1007/s13197-019-03950-z
• http://wwwcourses.sens.buffalo.edu/ce435/2001/Plastic_Microwave/Microwa…