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Christmas feature

The chemistry of delicious

19-Dec-2005
Last updated on 30-Jul-2013 at 20:36 GMT

The chemistry of delicious

If only they taught chemistry in schools as it happens at Christmas tables across Europe. For here lies the secret of delicious. Come next weekend, the centrepiece from north to south will be a roast. A British turkey, a German goose, a French roast beef: all amount to a grand fiesta of chemical reactions that few other methods of cooking can match.

Yet with temperatures selected, cooking times calculated, safety mittens donned, and the telephone number of the Emergency Services (Mother) close to hand, do we actually understand the chemical reactions we are about to initiate?

Whatever the meat, the chemistry will be the same.

Meat is the muscle tissue of the animal, some 75 per cent water, 20 per cent protein and five per cent fat and carbohydrates. The proteins in the raw meat are coiled and held together by bonds. On heating, the bonds break and the proteins unravel in a process known as denaturation. This makes the meat tender.

At higher temperatures, or longer cooking times, more and more water is lost, and the meat becomes overcooked, dry and tough. Basting replaces lost fluids, helping to keep that festive fowl juicy.

But why does roasted turkey taste so good? The answer is the Maillard Reaction. Discovered in 1912 by Frenchman Louis Camille Maillard, the reaction occurs when sugar molecules (carbohydrates) are heated with amino acids (proteins). Hundreds of different flavour compounds are formed during the reaction, which can then go on to form other flavours.

The Maillard reaction, initiated above 154 °C, always starts at the surface of the meat. The carbonyl group of the sugar reacts with the amino group of the amino acid to form N-glycosylamine, which is unstable and, via the "Amadori rearrangement", produces ketosamines.

These so-called Amadori compounds are involved in a cascade of further reactions that eventually result in a complex mixture of volatile compounds, which give the roasted turkey/goose/duck its aroma and flavour, and brown compounds called melanoidins, which gives that golden brown colouring.

The important aroma compounds formed by the Maillard reaction include thiophenes, pyrroles, thiazoles, pyridines and pyrazines. Some of these compounds give a 'nutty' taste, some a 'roasted' smell, and others a hint of chocolate or green vegetables.

This explains why boiling a turkey in water (100 °C) produces pale and plain meat - the Maillard reaction never kicks in.

To make sure it does, the British Turkey Information Service (BTIS) recommends a temperature of 195 °C (375 °F), guaranteeing the chemical reactions behind a perfect Christmas turkey.

Timing is also of the essence. This should be 18 minutes for each 450 g (per pound) of bird, says the BTIS. If you think that sounds like a long time, spare a thought for Tyson, the 40 kg (86 lb) turkey from Northamptonshire who in 1989 made it as the world's biggest turkey. At 195 °C, Tyson would have taken 26 hours to cook.

For that kind of marathon, even the most robust of Maillard reactions is going to be struggling to deliver taste up against the dehydration involved.

Nor does the chemistry end in the oven. There is also the worthy apres-roast. Why does grandpa always fall asleep after Christmas dinner? Turkey contains an essential amino acid called L-tryptophan, which is a precursor for serotonin, a neurotransmitter that controls mood and regulates sleep.

So the turkey is to blame for grandpa's snoring? Actually, no. L-tryptophan only works in the absence of other amino acids and on an empty stomach. The reason for grandpa's drowsiness probably has more to do with the massive calorific intake after the sherry he's been sipping all morning.

Merry Christmas!

Stephen Daniells, Food Science Reporter.