Chemistry makes Christmas magical!

From the enticing aroma of the turkey in the oven to the ‘whoosh’ of the flames as the brandy-soaked pudding comes alight, Christmas is a wonderful time for the senses. But have you ever considered the science behind our best-loved festive traditions?

Most people know that cooking involves chemistry, and where better to start than the Christmas Day turkey? The turkey meat you cook is muscle tissue, about 20% of which is protein – nearly all the rest is water, with a small but important amount of carbohydrate. If you ‘hang’ the meat and allow it to age, enzyme catalysts naturally present in the muscle start to break down the proteins so that they lose their naturally rigid structure and the meat becomes more tender.

You can speed up the tenderising process by heating the meat, but above a certain temperature the enzymes stop working (the active site loses its shape, the technical term being ‘denatured’). This is why many chefs cook the turkey for a long time at a low temperature – if they just stuck it in a hot oven, the protein chains would tend to bunch together, which, coupled with the loss of water, results in tough and dry meat.

Simply cooking the meat at low temperatures wouldn’t give the meat its brown colour and the wonderful smell and taste that go with roast turkey – that is down to a chemical reaction known as the Maillard reaction, which kicks in above 140°C. It’s named after the discoverer, Louis Camille Maillard (that’s Maillard, not mallard, which would be a duck, not a turkey!). Roast potatoes are cooked at a higher temperature than boiled potatoes, and traditionally in animal fat too, which allows Maillard reactions to occur, generating the smell and also the browning.

If you’re one of those people who find Brussels sprouts bitter, there may well be a genetic reason for it. Scientists have found that the presence of the TAS2R38 gene leads to a receptor that comes up with the ‘bitter’ response when tasting Brussels sprouts and other brassicas. This bitter taste is down to compounds called glucosinolates, such as sinigrin, which are there for a reason: they help plants like Brussels sprouts, horseradish and mustard fend off insect predators.

It’s not all bad, though, as scientists have also found that the glucosinolates in sprouts break down to give a molecule called sulforaphane, which has promise in fighting some cancers. So eat up those greens!

Oranges are traditionally eaten at Christmas. The smell of orange peel comes from very small amounts of aldehydes, including the 8-carbon molecule octanal, which is slightly smaller than its brother aldehydes used to impart that amazing aroma to perfumes such as Chanel No 5.

Over 90% of the oil you get from the peel is made up of the hydrocarbon limonene, found in lots of other fruits such as lemons and grapefruit. Because it’s a hydrocarbon – slightly larger than the hydrocarbon molecules that make up petrol – it is rather flammable, so don’t try squirting your orange oil at the candles on the table.

The more obviously flammable part of the Christmas meal is the Christmas pudding. Brandy is normally used to provide the fuel: ethanol. The ethanol molecule contains some oxygen, so it burns with a clear, hot, blue flame, unlike the hydrocarbons in candle wax, which give a yellow flame. Over half the brandy is actually water, so some of the heat from the fire is used to evaporate the water, which stops the pudding from getting too hot and burning to a crisp – it keeps that moist, chewable consistency.

And what about the crackers you pull at the meal? How do they work?

Some crackers use a chemical called silver fulminate. It is a very shock-sensitive substance, a cousin of chemicals such as lead azide, used in detonators. As you know, a cracker contains two long, narrow strips of card. One is painted with a tiny amount of silver fulminate, while the other is coated with an abrasive – a sandpaper-like material. They are in contact with each other so that when the cracker is pulled, the two strips of card slide past each other and the friction from the abrasive detonates the silver fulminate. There is only a tiny amount – micrograms – of the silver fulminate: any more and the ‘crack’ would be a ‘kaboom!’

Christmas trees are very flammable, for the same reasons that forest fires can spread quickly. One of the culprits is a molecule called pinene. As you’d guess from its name, it’s found in pine trees and contributes to their special smell.

Here’s something involving pine cones you can try that will put a bit of colour into your Christmas.

Make up solutions of different salts dissolved in water, such as copper sulphate solution or sodium chloride (common salt) solution. Soak the pine cones in one of these solutions overnight, then take them out and let them dry out. When you put them on top of a coal fire, they’ll burn with a coloured flame: yellow for common salt, turquoise for copper sulphate – a taste of the chemistry-related magic of Christmas.

Dr Simon Cotton
Honorary Senior Lecturer in Chemistry, University of Birmingham