Who’d of thought lonely electrons could be so bad for you!

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Belying their reputation as uncommunicative nerds, physicists appear to have pulled off a bit of a public relations coup.

Take a look around popular culture. There’s Einstein’s goofy face staring back at you from T-shirts and posters. They made a movie about Oppenheimer, and everyone knows media-savvy physicists like Carl Sagan, Neil deGrasse Tyson and Stephen Hawking.

In fiction it’s almost always a physicist as well: Bruce Banner, Hank Pym, Doctor Manhattan, Dana Scully and Hubert J. Farnsworth. All physicists. The Big Bang Theory, physicists.

I’d like to ask though, has anyone walked into a kitchen and thought to themselves, gee I wish I knew more about general relativity or wave-particle duality before making this mayonnaise?

The answer to that question is no.

It is chemistry that we all use in a very practical way every time we step into a kitchen. When we cook a steak, make some mayonnaise or whip up some fudge we’re doing things quite similar to what a chemist does in their lab.

And it’s not just cooking, what we eat and the effect it is has on our bodies is often down to chemistry. We’ve seen that with bacon.

So, today I want to talk about some chemistry that really matters for our health and well-being. It’s a chemistry that strikes fear into the hearts of students but I promise to keep it simple. It’s the chemistry of redox reactions.

Reduction-Oxidation Reactions Are Everywhere

Redox reactions are one of those things that, once you know about them you see them everywhere.

They are responsible for the heat we get from a fire, the rust that ruins our cars, the steel we use to build our buildings, the photosynthesis that captures the energy in sunlight and the batteries that power all our devices.

In the kitchen they brown our apples, and our steak, and they caramelise our sugar. They are responsible for fermentation, digestion and fats going rancid.

They are everywhere.

What is a Redox Reaction

Simply put a redox reaction is just a chemical reaction in which electrons are transferred between molecules.

To understand this let’s begin with a piece of burning wood.

When you burn something it generally becomes lighter. If you burn wood it turns to charcoal, which is lighter than wood, and charcoal, in turn, becomes even lighter when it burns and becomes ash.

For quite some time this was a mystery. Where was this mass going?

In the early 1700s a guy called Georg Ernst Stahl proposed a theory that involved the existence of a substance called ‘phlogiston’.

Phlogiston

Phlogiston was a ridiculous word for what, ultimately, proved to be a slightly ridiculous theory.

For what it’s worth, it was believed that phlogiston was a substance that was released when things burnt. It’s absorption by the surrounding air explained why things got lighter when you burnt them.

There was a problem though. When you burn metal, it gets heavier. Its ash, called calx, is actually heavier than the original metal.

Supporters of phlogiston tied themselves in knots trying to explain this suggesting, for example, that phlogiston could have negative weight. That its buoyancy explained why metal was heavier when it was gone.

But when Joseph Priestley discovered oxygen in 1744, a French scientist called Antoine Lavoisier put everyone out their misery and provided a much better explanation.

Combustion is a Redox Reduction

Lavoisier theory was that combustion was a chemical reaction between oxygen and the burning material.

He proved this theory using a meticulous series of experiments during which he initiated the chemistry we now call redox reactions.

In Lavoisier’s theory oxidation is the addition of oxygen and reduction is the removal of oxygen. When a block of wood burns, oxygen in the air reacts with hydrogen and carbon in the wood to produce carbon dioxide (CO₂) and water (H₂O).

Wood ash is thus lighter than wood because CO2, a gas, and water, turned to steam by the heat, rise into the air and are dissipated during combustion.

It also explained why calx weighs more than its original metal. When you burn a metal you don’t get gases you get solid metal oxides. Nothing dissipates as a gas.

Calx weighs more than the original metal because of the added mass of the oxygen in the metal oxide.

Process	Definition (Electron View)	Definition (Oxygen View)	Kitchen / Real-World Example
Oxidation	Loss of electrons	Gain of oxygen / Loss of hydrogen	Apple browning, iron rusting, burning wood
Reduction	Gain of electrons	Loss of oxygen / Gain of hydrogen	Iron smelting, Vitamin C neutralizing free radicals

It’s Actually About Electrons

Over time, Lavoisier’s redox theory was generalised and altered to encompass the addition and removal of electrons, not just oxygen.

Lavoisier’s removal and addition of oxygen become a specific case of a more general process of electron movement between molecules.

This has enraged students ever since because Lavoisier’s original terms were retained. This means that reduction is now the gain of electrons (originally the removal of oxygen) and oxidation is the loss of electrons (originally the gain of oxygen). Clear as mud, right?

Give me some more chemistry – Smelting Iron

Okay, iron smelting is not something you usually come across in a cooking blog. But it’s a very simple reaction, doesn’t involve enzymes or other complicated molecules, so it’s good as an example.

In the wild most metals are found in their oxide form, that is they are combined with oxygen.

To obtain pure iron from iron oxide you need to reduce the iron oxide and remove the oxygen. This is what happens during the smelting process.

During smelting, carbon, in the form of coking coal, is used as a reducing agent. The carbon is oxidised (it receives oxygen molecules and loses electrons) and the iron oxide is reduced (it loses oxygen and gains electrons).

If we are tracking electrons instead of oxygen, we say that the carbon has lost electrons because oxygen is more electronegative than carbon. The oxygen in CO₂ ‘hugs’ the electrons more closely than the carbon (I discussed electonegativity in the emulsion post if you want a refresher).

If you want to regenerate iron oxide you can burn the pure iron in which case you are oxidising it, adding oxygen from the air and causing the iron to lose electrons because of the electronegativity of oxygen.

What’s This Got to do With Food?

It isn’t just naming conventions that has confused chemistry students. There are a whole bunch of rules and processes for tracking electron movement in redox reactions that makes them great exam questions.

For us, though, we don’t need to understand all that. It would be a very rare day indeed if a cook needed to balance a redox reaction.

But it’s worth knowing about redox reactions for a couple of reasons. One is that it explains why certain foods act they way they do when their internals are exposed to oxygen.

The other is that redox reactions, and the interplay of free radicals and the antioxidants that we get from our food, are vitally important for our health.

Redox Reactions, Antioxidants and Free Radicals

As we go about our day, a nasty by-product of our internal biochemistry, lifestyle and diet are molecules called free radicals. These free radicals cause all sorts of problems because they have a spare electron on one of their atoms.

I don’t want to go into valence shells and electron orbitals, so you’ll just have to trust me when I say that electrons generally like to be paired with other electrons when they are rotating about the nucleus of an atom.

Unpaired electrons are energetically unstable, so a free radical really wants to pair up its unpaired electron. One way it can do this is by stealing an electron from another molecule.

This damages the other molecule which can itself become a free radical that then goes looking for yet another molecule to mug for its electrons.

Because these reactions involve the transfer of an electron they are redox reactions. In other words, a free radical is a powerful oxidising agent. It oxidises (removes electrons) from other molecules.

Free Radicals and Our Health

Free radicals can be a by-product of our normal biochemistry but they can also be a side-effect of things like smoking, pollution, excessive sunlight, chronic stress as well as dietary choices. Ultra-processed foods, alcohol and cured meats can all increase the amount of free radicals in our bodies.

We really don’t want a lot of free radicals floating around in our bodies. They are loose canons. If uncontrolled they’ll just move around randomly damaging all sorts of molecules we need for the proper functioning of our cells.

Oxidative stress, when we have a lot of free radicals in our body, has been linked to cancer, neurodegenerative diseases (like Alzheimer’s), cardiovascular disease, accelerated ageing and arthritis.

Free radicals are not our friends.

Antioxidants Save Us from Free Radicals

It’s not all bad news though.

We’ve all heard about antioxidants and free radicals are why we’ve heard of them. Antioxidants are strong reducing agents. They reduce other molecules by giving them an electron.

If an antioxidant runs into a free radical they can donate an electron without becoming a free radical themselves. This means that antioxidants can reduce oxidative stress by “soaking up” free radicals.

Chemically, there a few ways that they are able to do this, which I wont go into here, but antioxidants like vitamins C and E and anthocyanins are all abundant in fruit and vegetables. They are one of the main reasons that we should all be eating more fruit and vegetables.

If you are experiencing oxidative stress because you’ve eaten a pound of bacon, antioxidants are what you are looking for and you’ll find them in fruit and vegetables.

Preserving Antioxidants in Fruit and Vegetables

In the kitchen, given all those plant antioxidants are so good for us, we really want to preserve them when we are preparing our fruits and vegetables.

It is relatively easy to lose the antioxidants while cooking fruit and vegetables. Some of them are sensitive to heat and can break down during cooking. Boiling fruit and vegetables also tends to leach out many of the antioxidants into the water.

Some general rules of thumb are that raw is better than cooked, and the shorter and gentler the heat the better. This usually means steaming or microwaving is the best way to cook them.

Skins are good. Many antioxidants are in the skin or just underneath the skin. In fact many antioxidants are pigments, so the peels of brightly coloured fruits and vegetables can be a rich source of antioxidants.

Making a smoothie is better than juicing as many antioxidants remain bound to the fibre in the plant. Fibre is also very good for you, so it’s a double win.

Cutting fruit too early can also reduce the antioxidant content as they will react with oxygen in the air and lose their capacity to act as reducing agents. Generally you can see this happen as the fruit browns, like apple slices that have been sitting around too long.

Finally older fruit, especially if they are starting to brown are likely to have had some reduction in their antioxidant content.

Why does my apple go brown when I cut it?

Anyone who’s spent five minutes in the kitchen knows that some things start changing colour when you cut them up. Apples, bananas, lettuce and potatoes are just some of the fruits and vegetables that display this behaviour.

It’s probably not a surprise now, but this is another example of redox reactions.

In this case it is an example of enzymatic oxidation as polyphenol oxidase (PPO) in the fruit catalyse the oxidation of naturally occurring phenolic compounds in the presence of oxygen.

When something like an apple is cut, or begins to age, the cells of the fruit are exposed to oxygen and the phenolic compounds are oxidised into molecules that undergo further non-redox reactions to become pigments. These pigments are what discolours the fruit.

If you want to stop the browning, put the sliced fruit into water. Under water, oxidation can’t occur because of the lack of free oxygen. An acid, like lemon juice, will also stop discolouration because the acid denatures PPO and prevents it catalysing the oxidation reaction.

Conclusion

I’ve only touched on the different ways that redox reactions are important in our food.

Rancid fats are the result of light sensitive redox reaction pathways, winemakers have to pay careful attention to the redox potential of their wines and the whole fermentation thing is powered by long chains of redox reactions.

Not to mention all the other ways that redox reactions affect our health and well-being as both necessary parts of our biochemistry and as a consequence of our diet and lifestyle.

Will knowing all this chemistry make you a better cook? Probably not. But you might eat more fruits and vegetables and make better choices when preparing them. So, it might make you healthier.

Which, at least, is more than you can say for the general theory of relativity.