What’s that smell coming from my lunchbox?

John Woodliffe
23 November 2017

Above: Image © shironosov, istockphoto.com

Have you ever opened up your lunch, gotten a strange whiff of it and thought, “Will I regret eating this tomorrow?”

Food going bad actually involves quite an interesting bit of chemistry. Certain chemicals in the food can react with the oxygen in the air to form new chemicals. These new ones don’t smell or taste half as good as the original ones did! Scientists call these redox reactions - that is, reduction or oxidation reactions.

One cause of all the rotten smells and tastes is a reaction called oxidation. We're going to look at what this is, at how it happens in food, and at some other real-life examples of oxidation chemistry.

Oxidation Reactions

In oxidation, a compound either loses electrons, loses hydrogen (H), or gains oxygen (O). Compounds that cause others to be oxidised are called oxidizing agents. They’re useful for some things, like water treatment and energy storage in batteries. They are unfortunately not at all useful in food. In fact, they cause many foods to go bad!

One oxidizing agent is simply O2, oxygen in the air!

Below is an example of a non-food-related oxidation reaction. It’s often used in the iron industry to get iron from iron ore:

Fe2O3  +  3CO     →     2Fe  +  3CO2

The CO (carbon monoxide) is being oxidized to CO2 (carbon dioxide) by gaining an O atom.

But wait. There’s also Fe in there, which is the symbol for iron. What’s happening to that iron?

Just like in mathematics, every type of reaction has an opposite. The opposite of oxidation is reduction.  This is the reaction when a substance gains electrons, loses an O, or gains an H. So, the Fe2O3 in that equation above is being reduced. It’s losing oxygen and becoming iron (Fe).  

In all redox reactions, one chemical is oxidized while the other is reduced. You can’t have one without the other!

Here’s a mnemonic to help you remember the difference between oxidation and reduction reactions:
“LEO (the lion) says GER.”
LEO = Loss of Electrons is Oxidation; GER = Gain of Electrons is Reduction

What about my lunch?

Let’s get back to food. Imagine that in your smelly lunch bag, you find a browning apple. When food browns, it’s usually some redox reactions at work. Here’s a simplified version of one of the steps in the fruit browning process :

step in the fruit browning process

In many fruits, compounds which contain an alcohol group (OH) are responsible for the colour.  In the browning process, the compound with the OH is losing the H (hydrogen) so that now the C (carbon atom) has a double bond with the O (oxygen atom).  Further reactions happen to this compound, eventually causing the brown colour you see on fruit going bad.

Oxidation reactions are also responsible for the bad odour you can smell in oils (such as butter) which have gone rancid.

Antioxidants can help!

So, if many foods are oxidized when exposed to oxygen in the air, what can you or I do about it? Not expose it to the oxygen found in our air? That’s a little impractical, I think!

Food manufacturers have a more practical solution. They add chemicals called antioxidants to food. An antioxidant does what the name suggests: it works against oxidation! Vitamin C is actually an antioxidant, and it works by being easily oxidized itself. In other words, the oxidizing agent comes along ready to oxidize your food, but vitamin C gets there first and says “Oxidize me instead!” This way, your food can last longer before the oxidizing agents eventually get through all the antioxidants and start to oxidize your lunch. Neat, huh!?

Redox Reactions: A Summary

And there you have it. Your food goes off thanks to the oxidation of the chemicals in your food, which is a reaction with the oxygen in the air. You now know that oxidation can be defined as losing electrons, gaining O atoms or losing H atoms, and that its opposite is called reduction.

Did you know? Both photosynthesis and respiration contain redox reactions. This means that without redox reactions, there would be no animal or plant life!

So the next time your friend is complaining about their rotten tomatoes, you can wow them with some chemistry!

Learn more!

John Woodliffe

John Woodliffe

My scientific studies really began when I attended the University of Cambridge, United Kingdom, studying natural sciences. After completing my bachelor’s degree, I went on to do a master’s degree in chemistry, staying on at Cambridge. Having graduated, I then worked for a year at a small R&D pharmaceutical company, developing and testing new drug formulations. Following that I got married to my lovely wife, Jess, and together we moved to South Korea as a sort of gap year, teaching English. Now nearing the end of that year we will soon return to the UK and I shall hopefully continue my studies with a PhD in inorganic chemistry. My hobbies include racket sports, playing the guitar and attending church!







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