Why does microwaving fish smell so bad?

The work break room can be a place of rest, gossip, fun, or recovery. But it can also be a cause of complaint when yesterday’s fish dinner becomes a smelly microwaved lunch.

One of us (Nathan) learnt that the hard way when reheating a Korean mackerel and kimchi stew – a potent mix of fermented, fishy and garlic smells were all released into a small space. The look on a colleague’s face will not easily be forgotten.

For most people microwaving fish at work is a big no-no. It has even led to armed confrontations.

But what is happening in the microwave? Is there something specific to reheating fish in the microwave that makes it particularly smelly?

Smells fishy

One of the reasons it smells when you microwave it is because fish smells fundamentally fishy. Among the key chemicals that contributes to that characteristic smell is trimethylamine.

A precursor to this chemical, called trimethylamine oxide, is concentrated in fish flesh. It helps maintain the balance of minerals and chemicals inside the fish in their water habitat.

Trimethylamine oxide isn’t that smelly itself, but it can be modified by bacteria to the extremely volatile and smelly trimethylamine (which has a boiling point of only 3–4°C).

Trimethylamine gas has a signature smell – fishy! It is also produced in the blossoms of some flowers. A beautiful looking hawthorn bush can have a very unpleasant smell – somewhere between fish and urine.

The longer a piece of fish is kept after purchasing, the more opportunity bacteria have to thrive and produce trimethylamine; in other words, fish is often strong-smelling even before it gets to the microwave.

The smell of trimethylamine can be reduced by acids. The acid turns the volatile gas into a non-volatile salt (volatile means the ability to spread into the air). So it’s not surprising many fish dishes make use of acidic vinegar, or lemon juice – it makes the fish less smelly.

Cooking is chemistry

Other key reactions in cooking fish are lipid degradation and oxidation – which is the breakdown and reactions of fats at high temperatures.

This breakdown often sends lots of small molecules into the air, which means they are more easily smelled. That’s one reason fishes with higher fat content (such as salmon or mackerel) generally have stronger smells and flavours when you cook them. This chemical process can smell quite delicious, as is the case for many meats when they undergo the same processes during cooking.

However, not all compounds produced during this lipid degradation process smell good. Some aldehydes – a range of compounds often released when you cook fish – are particularly “fishy”.

Many cooking methods for fish – such as steaming or poaching – are relatively gentle. Frying uses a hotter, more direct heat. The amount of lipid degradation will vary with the cooking method. The more heat, the more aromas.

Enter the microwave

Microwave ovens are a remarkable innovation for heating food rapidly and efficiently, while also retaining nutrients.

They work by heating up what’s known as polar molecules. These are molecules that have an uneven distribution of electrons across their three-dimensional structure.

Microwaves will make water molecules and salts move in slightly different ways, and this motion is how the electromagnetic energy of the microwave is converted into heat.

Non-polar molecules, where the electrons are more evenly distributed, are not heated by microwave radiation. A striking demonstration is the microwaving of water and kerosene (please don’t try it at home or in the office). The polar water molecules get hot while the non-polar kerosene stays close to room temperature.

You might have heard about microwaves heating up water molecules, but they are not the only ones that get moving. Fats, sugars and things like the components of salts will also respond to microwave radiation.

Localised heating

Microwave ovens are well known for their tendency to heat things unevenly.

Fish is more susceptible to inconsistent heating due to its high moisture, dissolved salts, and the presence of fats. In other words, it easily absorbs microwaves and turns that energy into heat.

As the temperature increases, the fish can more effectively turn the microwaves into heat. This can create a potential “thermal runaway” feedback mechanism where hot regions absorb increasing amounts of microwave energy, potentially accelerating the release of unpleasant smelling compounds.

Parts of the fish may get well above 120°C long before the entire dish is heated to the desired temperature. These temperatures are high enough to trigger the fat degradation and oxidation we talked about earlier, and cause proteins in the flesh to break down – often giving the fish a rubbery texture.

This heat also creates steam that carries the volatile odour compounds out of the fish more rapidly than under conventional heating.

Break rooms generally don’t have sufficient ventilation to handle the rapid release of the volatile aromas into a small space. The result: an unevenly heated lunch and disgruntled colleagues.

Next time your colleague funks up the break room, consider leaving a passive aggressive copy of this article nearby. It will hopefully get them to reconsider their lunch choices and to understand some of the chemistry involved.