Christmas Chemistry

As Christmas draws ever closer we take a look at the most important - and stressful - part of the festivities through science-tinted specs. Welcome to the season\'s tastiest chemical reaction...

It takes hours – even days – to defrost and then ends up so dry that no amount of gravy makes it easy to swallow; the turkey is the nightmare facing any chef on Christmas morning. Then there’s everything else that goes with it: roast potatoes; Yorkshire puddings; stuffing; and if that’s not enough, the Christmas pudding. Leave any one of these vital ingredients out and it’s just not Christmas, but other than leaving the chef somewhat frazzled, what happens while your Christmas feast is being cooked?

The task of cooking the mammoth-sized turkey that’s far too big to feed even the Brady Bunch is usually first on the checklist. It’s not rocket science, but what is it that makes the turkey so damn difficult to get right? Even if you follow the cooking instructions to the letter, your turkey either ends up dry and overcooked, or it’s still flapping around the kitchen.

Turkey is 75% water – mostly bound to proteins, which make up around 20% of the meat. The remaining 5% is fat. Turkey has both light and dark meats which cook differently: the legs and wings take a bit longer because their meat is darker and contains more fat but the lighter more plentiful meat consists of muscle fibres – notably the proteins myosin and actin.

When heated above 40°C, the proteins start to denature– the more the meat is heated the more the proteins unravel and the more tender the meat. But don’t heat the meat for too long because the denatured proteins shrink, causing the meat to become tough. Connective tissues like collagen join muscles to the bone and wrap around muscle fibres. It’s too tough for us to eat, but once heated above 60°C the collagen triple helices in the protein break down and unwind becoming soft gelatine¹.

Most of the meat’s water remains bound to proteins, but a small amount escapes during denaturing. The meat becomes dry because the meat is cooked in a dry heat and proteins begin coagulating with the meat. Basting the meat (and potatoes too) with the escaped fat and juices serves to reduce the amount of moisture lost through evaporation

“You will need to make a series of compromises to cook the ‘perfect’ turkey. The outside needs to be heated to between 140 and 200°C,” Dr Peter Barham, a physicist and molecular gastronomist from the University of Bristol told Physorg.com, “The tender breast meats wants to be heated to no more than 55 to 58°C to keep the muscle proteins from contracting and becoming tough. The tougher, collagen rich legs and wings need to be heated to a higher temperature (say around 65 to 70°C to denature some of the collagen.”

So it’s all about balance – but how do you achieve it? Some people remove the legs and cook them separately while others use aluminium foil to reflect heat from the breast and heat the legs faster. Among the more interesting ideas Laboratory News came across was from food science writer Howard McGee who uses ice packs on the breast of the bird to keep its temperature lower than the legs while thawing so the legs get a ‘heat start’ on the rest of the bird².

The turkey’s in the oven and the house is beginning to fill with the familiar meaty aroma, add in the potatoes and its all beginning to smell rather tasty. These aromas are generated by the Maillard reaction – a series of chemical reactions between the amino acids in proteins and reducing sugars – and each food has a distinctive set of flavours compound which are formed during cooking³. The carbonyl group of the sugar reacts with the nucleophilic amino group of the amino acid to form a complex mixture of poorly characterised molecules which give rise to odours and flavours.

Shame it all gets spoiled when you suddenly realise your Brussels sprouts are overcooked. The reason many people say they hate the hardy vegetable is the sulphurous smell – and taste – which accompanies the release of glucosinolate singrin when they’re overcooked.

“The turkey’s in the oven and the house is beginning to fill with the familiar meaty aroma, add in the potatoes and its all beginning to smell rather tasty...Shame it all gets spoiled when you suddenly realise your Brussels sprouts are overcooked”

Cooking myths or handy hints – is goose fat better than ordinary lard and does sprinkling flour on the potatoes during cooking really crisp them up.  The Laboratory News team are far too busy to test these hypotheses – or rather too lazy to cook a roast – so we turned to the power of the World Wide Web and found The Accidental Scientist – The Science of Cooking message board:

“For the ultimate crunchy roast potatoes cook them as normal and then half way through roasting sprinkle with plain flour – the results are fantastic,” said brighton girl, “Cooking in goose fat works a treat, it is a little more expensive but well worth it. Also, roughing the edges after boiling is good for extra crispiness if you like them like that,” said pete.g⁴.

So that’s cleared that up –rough them up a bit, cook them in goose fat and sprinkle with flour and you’ve got the perfect roast potato.

But what of the gravy? Traditionally the gravy is the un-thickened juices of the meat perhaps with a bit of gravy browning for flavour, but according to scientists from the Royal Society of Chemistry, adding a teaspoon of dark soy sauce makes the perfect gravy. The monosodium glutamate (MSG) in the soy sauce brings out the meaty flavour – otherwise known as umami – and mops up important ingredients which seep out of the meat and would otherwise be lost⁵.

“Gravy is a way to recover proteins, essential vitamins and minerals that may be lost on roasting a joint of meat and when cooking vegetables,” said Dr John Emsley, lecturer and scientist at the University of Cambridge told The Telegraph, “It can also be used to provide other essential minerals such as iodine which is needed for key hormones and for the brain.”

And so simple to make: add a sprinkle of flour to the juices and stir, adding water that your vegetables were cooked in – plus the soy sauce – and simmer. So there it is – a juicy turkey, crispy roast potatoes and lashings of perfect wholesome gravy – let’s just hope the dinner hasn’t gone cold by the time it’s ready.

Dr Emsley was also asked to come up with a formula for the perfect Yorkshire pudding to ensure it rises by four inches. His answer:

carbohydrate + H2O + protein + NaCl + lipids = perfect Yorkshire puds

So that’s flour, water, egg milk and fat then. According to Dr Emsley, the fat should be really hot before you add the batter, and the batter should never be stored in the fridge – "Some amateurs even place the batter in the fridge first. What kind of foolish act is that?" he said⁶.

Well, who cares how you cook your Yorkshire puds – and the rest of your Christmas dinner – as long as it tastes good. But while you’re raising your glasses and tucking into your feast, spare a thought for the frazzled chef who prepared it for you and the science behind it.

Christmas Dinner facts7: • Turkey was considered a luxury in the UK before World War II when goose or beef was the more common meat for Christmas dinner. • The domesticated turkey is descended from its wild ancestors, and the dominant breed reared is the Broad-breasted White, which is selected for its size and the amount of meat they produce. • In 2009, 7,734,000 turkeys were consumed in the UK on Christmas Day. • Although it does have high levels of tryptophan, it’s unlikely that your Christmas turkey will make you fall asleep – it’s the rest of the meal and alcohol combined with a relaxed atmosphere that makes you sleepy. • According to tradition, you shouldn’t serve your Yorkshire puds with dinner – hard-up housewives used to serve them as a starter with gravy so diners didn’t eat so much of the expensive meat.

References 1. How to cook the perfect turkey (the scientific way) http://www.physorg.com/news9327.html 2. Physics of Turkey – thermodynamics and the cooking of a turkey http://physics.about.com/od/physicsintherealworld/p/TurkeyPhysics.htm 3. Maillard Reaction http://en.wikipedia.org/wiki/Maillard_reaction 4. The Accidental Scientist – The Science of Cooking http://apps.exploratorium.edu/cgi-bin/yabb2-cooking/YaBB.pl?num=1264376250/28#28 5. Scientist cooks up the perfect gravy http://www.telegraph.co.uk/science/science-news/6407660/Scientist-cooks-up-the-perfect-gravy.html 6. Scientists crack the perfect Yorkshire pudding formula http://www.telegraph.co.uk/foodanddrink/foodanddrinknews/3447945/Scientists-crack-the-perfect-Yorkshire-pudding-formula.html 7. Domesticated turkey http://en.wikipedia.org/wiki/Domestic_turkey