What chemicals will we be eating and drinking this Christmas?

As we head into the festive season, what sort of chemicals will we be eating and drinking?

Chemicals are a menace. Processed foods are bad for us. Or, at least, this is the logic behind a "chemical-free" craft-beer on sale in some pubs in Ireland.

But, as absolutely everything that exists – you, me, plants, televisions, liquids, stars, air, chairs, food, water – is made of chemicals, a chemical-free beer, by definition, is a beer that does not exist. Why are so many people afraid of chemicals? And as we head into the festive season, what sort of chemicals will we be eating and drinking?

"We instinctively regard processing and chemical preservatives as negative and to be avoided, but many Christmas food staples are great examples of traditional approaches to both processing and preservation," says Alan Kelly, professor of food and nutritional science at University College Cork.

"Some very powerful chemical preservatives include salt, alcohol, spices and sugar, all of which feature heavily in Christmas foods, and have been used for centuries because of their ability to inhibit nasty bacteria from growing, long before any science of chemical preservation was understood. In addition, processes such as heating (cooking the turkey), freezing (ice cream, desserts) and smoking are also fundamentally food processes designed to inhibit spoilage and render food safe to eat. So, we have applied processing and preservatives for centuries, and because these are accepted we don't consider them as what they are really for. And if you look at the list of chemicals identified in turkey or its odour when cooking and put them in an ingredients list, they sound scary, but this is because they are complex chemical structures. "

Dr Peter Crowley, a protein chemist at NUI Galway, says that the scientific community needs to do better job of communicating chemistry. "Of course there are some chemicals that, in particular doses, are bad for you, and some that have been dubiously. But there is an ignorance of some basics."

Indeed, all foods and drinks are composed solely of chemicals and nothing else. In 2014, chemists Alexander Goldberg and CJ Chemjobber, writing in Nature Chemistry, drew attention to chemophobia – an irrational fear of chemicals – by compiling a list of consumer products, including food and beverages, that are inaccurately labelled as "chemical free". Writing in Odou, a magazine focused on perfume and scent, journalist and perfumer Pia Long points out that, of the 60 million known chemical compounds, it is irrelevant to our bodies whether they are synthetic or naturally occurring, that the plants and meats we eat are chemical factories and so are our bodies, and that the use of the word "chemical " is too often followed by "industry", whereas those who speak of the benefits of naturally occurring raw materials are often labelled as hippies.

In our Christmas dinner, however, a whole variety of chemical processes are working together to create the greatest meal of the year. Kelly says that because turkey is nutritious to us, it is also nutritious to harmful bacteria, so the first priority is to make it safe to eat; flavour is a secondary consideration. This means subjecting it to sufficient heat that it kills the bacteria at the core of the bird.

The heating process is a fundamentally chemical process and it is from here that the bird gets its flavour, so understanding these chemical reactions can lead to a tastier turkey. "Proteins are long chains or coils of amino acids, and in nature they have a defined structure," he explains. "When you heat them, you break down the bonds that hold these proteins together. They unfold, adopt new structures and interact with each other and with sugars in the bird. As the proteins 'denature', they lose moisture. Meanwhile, the Maillard reaction – involving sugars and proteins – causes the browning of the turkey. "

Chemistry may also give us the formula for a perfect Christmas dinner. Psychologists Lewis and Yufera-Leitch say that the perfect Christmas dinner is composed of 150g of white roast turkey meat, 110g of stuffing, 155g of steamed sprouts, 170g of carrots, 150g of red cabbage and 100g of gravy (although presentation, smell, and even the music being played over dinner have an impact on our enjoyment).

Dr Crowley of NUI Galway points to how starch granules in the flour we use to thicken sauces and gravy burst at a particular heat level, causing the liquid to thicken suddenly. “I love how flavours arise from amino acids and sugars chemically reacting in the meat or the veg. I love how the sprouts turn a more vibrant green but then get duller in the cooking. I love how the oil brings out a richer flavour. I love how the spices, when heated, change their chemical composition to give out their distinctive aroma. That is chemistry, and that is Christmas.”

The nostalgia effect: How chemicals and scent make Christmas

Christmas is heavily associated with scent: the smell of the tree, the blast of the turkey from the oven, the cloves in our hot ports and whiskey. Even thinking of these smells evokes nostalgic memories of the festive season. Why is this?

“Plant-based chemicals are some of the strongest sensory provoking compounds,” says Dr Rena Barry-Ryan of DIT. “This is due to their function in nature: to attract insects, birds and animals. They are normally airborne so we can smell them. Heating them by cooking gives them more energy thus amplifying their smell or flavour. The distinctive smells of Christmas are part of a very large catalogue of more than 10,000 compounds in your brain’s aroma memory.”

The Proust phenomenon – named after the author Marcel Proust's seven-volume novel In Search of Times Past, which explored memory – is a scientific affirmation of the folk knowledge that food and smells evoke a sense of place and occasion. Many of the spices we associate with the festive season, including cinnamon, cloves, and ginger, were formerly expensive and luxurious treats that we only got to consume at Christmas.

“Many evoke a warming luxurious aroma and flavour such as cinnamon (cinnamaledehyde) and others a relaxing and even numbing effect such as cloves due mainly its high eugenol content,” says Barry-Ryan. “The smell and taste of citrus fruits such as oranges, lemons and grapefruit comes from the hydrocarbon limonene from the peel. All these aroma compounds induce saliva flow, the release of gastric acid and an increase in hormone levels which stimulate appetite and satiety.”