In Depth › Science Features

The science of cooking

Cooking a roast dinner is not only a mouth-watering experience, it's also a giant experiment. The scientist and the chef explain the chemistry and share their top tips.

An understanding of the chemical reactions in cooking plays a vital role in the making of a chef. In fact, Adelaide-based celebrity chef Simon Bryant reckons it's imperative.

"The only way you can cook properly is if you know what is going on," he says.

"Knowledge of the science behind cooking means you know how to fix things up if you need to and you can think about how you are going to treat the produce to get the best out of it.

All good cooks are aware of the chemical reactions that occur during the cooking process, even if they don't fully understand them. However, an understanding of the basic science behind cooking helps them produce even better results.

Roast meat is a good example. If the recipe calls for preparing the meat by rubbing it with salt and pepper, there's a good scientific reason.

Associate Professor Hannah Williams, of Curtin University's Food Science and Technology program, says this not only adds flavour, but also the salt draws out some moisture resulting in a slightly crisper surface because of its drying effect.

When the roast is placed in a hot oven, the outside starts to brown and an odour is given off.

Williams says this is the Maillard reaction at work. Named after Louis-Camille Maillard early last century, the Maillard reaction occurs when carbohydrates (sugars) and proteins react to produce browning and a pleasant odour.

When these proteins and carbohydrates react chemically they bind together. They become more dense and change colour (browning) and small chemical components break off the chain to give flavour.

The Maillard reaction produces browning pigments called melanoidal complexes. As these increase in size and density, the meat becomes darker and darker. The reaction also produces a distinctive smell.

"We are hard-wired into that smell." says Bryant. "It goes straight to a place in your head which says: "I'm really hungry", which is why a dog sticks its nose in the air when there's a BBQ going on."

^ to top

Crust me

Browning the meat also changes its texture, because now the proteins are bound to carbohydrates.

Chefs often dust the outside of a roast with flour (carbohydrate) to make the meat's surface brown quicker.

Williams says during the roasting of meat, caramelisation can also occur. This is where simple sugars, such as the glucose in meat cells, form complex sugars and brown on the roast's surface. This caramelisation also contributes to the dish's flavour.

Basting the meat regularly using the juices in the pan is recommended while the meat is cooking.

"This coats the surface with moisture and flavour, building up the flavour profile of the meat and increasing the complexity of its 'crust' while stopping it drying out too much," she says.

By watching the meat brown on the outside, cooks know the meat is changing colour on the inside from red raw to pink to brown.

Williams describes this process as denaturisation, where the protein fibres in the meat change shape.

"The protein starts to unwind and distort because of the heat. The protein loses electrical charge and can no longer bind to water. It also changes colour," she says.

Eventually the denaturisation becomes irreversible — this is when the meat is ready to eat. Williams says the denaturisation process makes the meat easier for humans to digest.

^ to top

Fat burst

But there is also another process at work when cooking a roast.

The fat, which melts from its hard state when heated to release fat-soluble volatile components trapped in it, adds to the flavour. Fat cells often store components that reflect what the animal has been eating, so when they break down there is often an added bonus of flavour. This flavour boost is why, for example, that lamb fed purely on saltbush, is so prized.

But once the meat is cooked, we are often advised to 'rest' it for a while to improve tenderness.

"[This] allows the moisture in the centre of the meat to migrate evenly throughout the cut, resulting in an overall juicier piece of meat. It also allows the heated proteins to relax," Williams says.

Bryant says it's easy to tell if a chef is under time pressure. "In a restaurant if there is a pool of bloody liquid around your steak, you know the chef was in a hurry and didn't rest the meat long enough," he says.

"You know the first few mouthfuls are going to be tough. The same applies to other high-protein products such as fish and chicken."

"All sorts of stuff can go wrong in the kitchen," says Bryant.

"The most common kitchen disaster is overheating and then splitting a hot emulsion sauce, such as hollandaise or béarnaise."

This turns an emulsion sauce into a butter sauce.

"When making an emulsion sauce you are creating water molecules surrounded by fat molecules in suspension. If you evaporate that water molecule the fat molecules have got nowhere to go so you end up with an oil slick."

Bryant's scientific knowledge that overheating can cause a sauce to split means he is careful to avoid the evaporation zone and not heat an emulsion sauce above 80°C.

"If your sauce is getting too hot, throw ice on it and hope like hell that you cool it and add extra water," he says.

"This doesn't always work, but I have used it."

Gravy trainingMaking gravy to accompany the meat is also a complex chemical reaction.



When flour is added to pan juices and heat applied the mixture thickens in a process called starch gelatinisation.



Starch is made of big molecules tightly packed into small granules that swell when water and heat are added. This releases some of the large molecules into the gravy (this process is called pasting).



"The molecules are so big they get tangled around each other and the swollen granules, to form a network that traps the water. This causes the gravy to become thick," says Williams.



If the sauce is left to cool the network becomes set and a gel is formed. Williams says the rate of gelatinisation and strength of the gel depends on the type of starch (cornflour, wheat flour or arrowroot) used.



If thickened gravy is overheated, reheated too many times, or too much acid (lemon juice/vinegar) is added, the network is broken, disrupting the molecular structure. The gravy will become thinner and a good gel will not form.

2011 is the International Year of Chemistry.

^ to top