What’s the Chemistry of Cooking?

A lot of cooking involves chemistry and is essentially a series of chemical reactions. Knowing about this kitchen chemistry can help you to understand much more about what’s happening – and why your recipes sometimes go wrong.

Why does Food go Off?

Complex chemical reactions go on in food all of the time – even if you’re not cooking them. Some of these reactions make food go off.

For example, bananas turn brown because a hormone within them triggers the release of ethylene gas (C2H4). This accelerates the ripening processes until the banana becomes over-ripe.

Ethylene has an accomplice called polyphenol oxidase (or PPO for short). It’s found in apples and potatoes, so leaving your banana next to an apple will hasten the ripening process. Food companies use these same chemicals to bring unripe fruit up to speed for consumers, and use other chemicals to slow the ripening process down.

Why are Curries ‘Hot’?

Capsaicin is what makes spicy foods like these chilli
peppers hot. (Image James Bowe).

When you’re sweating through that green curry, have you ever wondered what makes it taste so hot?
The short answer is capsaicin. The long answer is more of a mouthful:


Receptors in your mouth and throat can detect even tiny amounts of capsaicin – just 10 parts per million (or ppm for short) will cause that familiar burning sensation on your tongue. The capsaicin allows calcium ions to flood cells. This trips a pain signal and before you know it you are reaching for a glass of water. This doesn’t work very well, because capsaicin is not soluble in cold water. Milk and other dairy products such as yoghurt provide the greatest relief – they contain casein which effectively washes capsaicin molecules away.

What Happens to Meat When You Cook it?

Red meat contains myoglobin which turns brown during
the cooking process. (Image Andy Ciordia).

Meat is animal muscle, 75% of which is water. The rest is protein (about 20%) and fat (5%), as well as small amounts of carbohydrates, acids and minerals.

So what happens when a piece of raw meat goes on a hot frying pan?

  • The protein molecules are in bonded coils, but as heat is applied the bonds break and the coils start to unwind.
  • Meanwhile much of the water content in the muscle fibres leaches out – that’s why your fillet steak or chicken breast is smaller after cooking than when it is raw.
  • If it’s red meat (lamb, beef) it begins to turn brown as the myoglobin reacts to the heat. Similar to haemoglobin, myoglobin is a protein that stores oxygen in red blood cells. Heat triggers iron atom oxidation. The iron atoms in the protein lose an electron and this gradually changes the colour from red to brown.
  • White meat (chicken, turkey) has far less myoglobin, so it is pink when raw and turns white when cooked.

Why Does Popcorn Pop?

Salted? Buttered? Plain? Popcorn is an extremely lucrative industry. Exports are worth more than €40 million to the US economy and a night at the movies isn’t complete without it. But why does it pop?

The secret is in the grain. Many other grains – like wheat or rice – won’t pop, because their outer shell or hull is porous. Popcorn kernels are around 13.5% water, and when you heat the popcorn the water in the kernel boils and turns to steam.

The hull is sealed so the steam creates pressure inside. When the temperature rises to 180°C and the pressure is at nearly 1000 kPa (kilopascals), the shell explodes – or pops.

If your popcorn isn’t working out, it’s probably because one of these factors is off. If the kernel contains more than 13.5% water it will pop into a thick dome shape instead and won’t be as fluffy and crisp. On the other hand, if it doesn’t have enough water it won’t build up enough steam. If the kernel is cracked or damaged in any way, the steam will find a way out and your popcorn won’t pop.
See popcorn popping in slow motion » 

Why is Honey Good for Sore Throats?

Honey has four attributes that help it fight infection.
(Image Hillary Stein)

Nothing soothes a dry cough quite like honey and scientists are now looking at how to use the special qualities of Manuka honey and ‘medical grade’ honey to battle more than just colds and flu.
They are now being used to treat MRSA – the ‘hospital superbug’ which has developed a resistance to penicillin and other antibiotics. Honey has four attributes that make it effective at battling infection:

  • H2O2 which may be more familiar as hydrogen peroxide or bleach. Consuming or applying large quantities of H2O2 is not a good idea because it’s highly corrosive, but scientists have found that honey contains low levels of the chemical and it can kill MRSA bacteria.
  • Dextrose and fructose (C6H12O6 in chemistry-speak, or sugars in chef speak). This makes up 75% to 85% of honey, which means that it doesn’t contain much water – and bacteria can’t thrive without water.
  • Bee Defensin-1 This protein, also found in the bee secretion royal jelly, is a natural antibiotic.
  • Methylglyoxal (C3H4O2, or sometimes simply ‘MGO’). This chemical compound inhibits bacteria’s ability to produce the proteins needed to survive, and makes honey a powerful remedy to infection and illness.

So that’s plenty of food for thought the next time you have a slice of toast and honey.