Air it is: Swetha Sivakumar, on how oxygen alters food
From turning fats rancid to browning apples and bananas, see how oxidation – and antioxidants – are quietly at work around you. In this week’s Sound Bites.
Oxygen is an electron-grabber, also known as a free radical.
A free radical is a highly reactive atom, ion or molecule that is unstable because it has at least one unpaired electron. The free radical is desperate for another electron to make it stable. The ways in which oxygen grabs on to other materials to gain this stability gives us the term “oxidation”.
How does all this come into play in the kitchen? Let’s start by looking at cooking oils. It is often said that one should store oils in a cool, dark place. Why? Because light and oxygen are enough to kickstart the process of rancidity.
The energy from a photon of light can break the bonds that hold fatty acid intact. This makes the fatty acid molecule unstable. Any oxygen molecules nearby will react with the unpaired electrons in these atoms, starting a chain reaction that can cause the entire bottle to break down.
Foods high in fat are particularly susceptible to oxidation. These include nuts, fried snacks, cheeses and cooked meats. One way to avoid rancidity in these foods is to remove the fat content. This is why skimmed-milk powder lasts longer than most milk powder variants.
Packaging can help, of course, since it can keep light and air away, over long periods. Seal potato chips in multiple layers of plastic and aluminium foil and fill the bag with an inert gas such as nitrogen and a crisp wafer can last for months.
Another effective way to prevent rancidity is to add an “antioxidant”.
Antioxidants are also called stable radicals. They do not steal electrons from nearby stable molecules, but will happily accept an electron from a free radical. Thus, rather than starting a chain reaction, they put an end to one.
Plants are a major source of natural antioxidants, because they have to be. There is no greater oxidative stress than the one they undergo when performing photosynthesis (in which they use sunlight to split water molecules into hydrogen and oxygen atoms in order to make food).
Which is why leaves and other plant parts are chock full of antioxidants such as chlorophyll, lycopene and the Vitamins C and E, to prevent this intense reaction from wreaking havoc.
But there is a flip side to oxidation too: Plants that aren’t chock full of antioxidants can use this reaction as a shield.
Think of why it is that some produce browns when it is cut and some does not.
Tomatoes, carrots, pumpkins and berries — those brightly coloured foods with high levels of antioxidants — do not undergo browning because their antioxidants keep oxidation from occurring.
Produce that is not as rich in antioxidants — such as apples and bananas — brown in self-preservation. When cut or bruised, a reaction begins that combines enzymes they contain with oxygen from the air to “seal off” the wound.
In an apple, for instance, the enzyme polyphenol gets to work when it is cut, combining with oxygen to produce a brown molecule called melanin that forms the brown skin and delays further oxidative damage.
Should one wish to turn off the oxidation reaction, in order to help fruits and vegetables retain their original textures and flavours, this is surprisingly easy to do. One can just denature the enzymes that kickstart the melanin formation process, by adding heat (via blanching) or by adding an acid such as lemon juice.
Incidentally, this is the same melanin that protects our skin from harmful ultraviolet rays, though exactly how it does that is perhaps a subject for a different column.
Look around you right now. In how many ways is the oxidation process currently in play, where you sit? It’s another interesting way to view the world, isn’t it?
(To reach Swetha Sivakumar with questions or feedback, email email@example.com)