The effects of microwaves on food safety and shelf life
When microwave ovens first appeared on the domestic market, there was a lot of debate about the radiation hazards or the risks involved in microwave energy leakage. However, microwave ovens emit far less power than mobile phones and they use the same frequency. Is it safe to eat microwaved food?
Using a microwave oven is harmless. When the oven is turned off, the microwave fields are also completely switched off. They help to heat food, and nothing else.
Questions have also been asked about whether or not microwaved food contains radiation after it has been heated up. It does not. Microwaves emit non-ionising radiation, so nothing is left in the food.
However, it may be worth knowing that the food could easily become burnt if the oven is used on full power for too long – and we should avoid eating burnt food.
Effects of heat
It is possible to extend the shelf life of packaged foods by heating them up and then cooling them down again. Vigorous heat treatment can produce sterile food and many years of shelf life at room temperature, but even mild heat treatment combined with cool storage, can extend shelf life by several weeks.
There is plenty of readily available knowledge available about the effects of heat-treatment of food that also applies to microwaves, but microwaves stand out in three ways.
It is not necessary to use a lot of water to cook something by using microwaves since the food cooks from the inside out. Boiling food in a pan of water results in water-soluble vitamins leaching out into the cooking water, etc. and the nutrients are therefore better preserved when using microwaves.
Rapid heating enables heat treatment to take place over a short time at high temperatures. This is particularly beneficial when pasteurising or sterilising vegetables since several important nutrients are best preserved with short heat treatment.
It can be difficult to predict the distribution of temperature in the food concerned. The conversion of microwave energy to heat depends on the geometry, temperature and contents of the product, e.g. water and fat. Overheating at the edges and in the corners can result in burning.
Elsewhere, the temperature may be far lower than planned so that any bacteria that were supposed to be inactivated by pasteurisation may well survive. This can be compensated for by having safety margins such as longer heat treatment, but then some of the benefits of the microwaves are lost.
Consequently much of the research conducted in this area has been focussed on being able to predict exactly the temperatures that develop. The figures below show examples of how a microwave field spreads inside a microwave autoclave and the temperature distribution in a sample.
Non-thermal effects
One question that still needs to be clarified is whether or not microwaves result in changes in food that are not related to the heat that arises, so-called non-thermal effects. For a long time there were questions raised about whether or not bacteria were inactivated by microwaves alone, even without high temperatures. It has been shown that if this is the case then it is completely irrelevant when compared to the effects of the heat generated.
Heat is generated by microwaves by setting water molecules in motion. This happens because water molecules are dipolar, with a positive and a negative charge. Also, several proteins have polar groups of amino acids that are set in motion. When the kinetic energy becomes high enough, the structure of some protein groups is changed.
This affects the properties of the food and makes it easier to extract certain components. Free radicals can also become detached so that the properties of the proteins change in terms of viscosity and solubility, etc.
Sources
- A computational study for the effects of sample movement and cavity geometry in industrial scale continuous microwave systems during heating and thawing processes
https://doi.org/10.1016/j.ifset.2022.102953 - Review Guidelines on reporting treatment conditions for emerging technologies in food processing
https://www.tandfonline.com/doi/full/10.1080/10408398.2021.1895058?casa_token=gF8C2ZOwsgYAAAAA%3AE945xBNUFRSt1cH3PYFVuQfSG_QCQ1_PvBJXDL7BKNlJPXMBzfoEwEI4DT5HT101hrK5UOq_COVB - Influence of non-thermal microwave radiation on emulsifying properties of sunflower protein
https://doi.org/10.1016/j.foodchem.2021.131275
