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Image: Raw steak

Low-temperature cooking

The Sunday roast may be the national dish, as well as a national institution, but it's not cooked without controversy. Here's what the high-temperature/low-temperature debate is all about.

It's not for nothing that the French refer to the British as 'rosbifs'. The traditional roast joint, dark brown on the outside and warm and pink on the inside, has long been a potent symbol of Britishness where food is concerned.

So, if the dish has been around for centuries, surely it must have been perfected long ago? Not necessarily. There's more than one way to roast a joint.

Low-temperature cooking

Most people think that it's necessary to first brown the meat at a high temperature before letting it cook through. Although this is the way that most roasts are still cooked, it doesn't actually have to be cooked this way. In recent years a number of chefs, including Heston, have realised that cooking meat to make it tender can take place at relatively low temperatures, before the joint is browned on the outside. Understanding some of the chemistry and physics that happens when we cook meat will explain why this is so.

Actually, cooking meat at low temperatures is not a new invention. The English scientist Benjamin Thompson in the late 18th century described how he had left a joint of meat in a drying oven overnight and was amazed when, the next morning, he found that the meat was tender and fully cooked, although it hadn't browned. He was totally at a loss to explain why this had happened.

His experiment was repeated by Professor Nicholas Kurti from The University of Oxford during a lecture at the Royal Institution in 1969, when he showed that the temperature of the meat in Thompson's trial never went higher than 70C, far lower than the temperature at which most of us roast meat, at up to 200C or more.

Meat myths

The idea that meat has first to be browned to 'seal' in the meat juices during cooking is totally wrong - despite what you may have read or heard, even from notable chefs. To prove it, try this simple experiment.

Download an experiment

Basically, there are three main reasons why we cook meat: to make it tender, to give it a 'roasted' flavour and to kill any harmful bacteria that may be present. However, despite popular belief, the chemical changes that happen when meat is cooked to make it tender, and those that give it the dark brown colour and its rich roasted flavour, are quite unrelated and the two processes happen at very different temperatures. Harmful bacteria are killed off at temperatures in between the two extremes.

It is therefore useful to understand the difference between these three processes - making the meat tender, giving it a roasted flavour and killing harmful acteria.

Muscle power

Meat is, of course, the muscles an animal uses to move around. These are attached to its bones by strong tendons. Meat contains different types of protein: each is designed to do quite different jobs.

Muscles owe their 'pulling power' to long protein fibres called actin and myosin. These fibres slide along each other when stimulated by nerve impulses, which make the muscle shorter and fatter. If muscles are used for short, fast bursts of energy, then glucose from the blood provides the fuel. If the muscles have to give long sustained activity, then fat provides the energy and in this case another protein, called myoglobin, is needed to help oxidise the fat and provide the energy.

You can tell what kind of muscle tissue the meat you're cooking is made from by just looking at it. Myoglobin is bright red, so muscles that work a lot are red while those that are used for less regular, sudden movements are pale or even white.

The fibrous tissues that surround the muscles and connect them to the tendons and bones have to be very strong, tough enough to move the animal around without them stretching or breaking in the process. This tissue, and the tendons themselves, are made from another protein molecule called collagen; in general the more collagen there is in the joint of meat, the tougher it is.

Cooking meat

The different proteins in meat are affected by heat at quite low temperatures, far below the temperature of a normal roasting oven. The myosin molecules start to shrink and squeeze out fluid from the muscle cells at about 55C while the collagen molecules start to change when the meat is slightly hotter, at 60-65C.

Although the shrinking of the fibres tends to make the meat more firm and chewy, the collagen reacts in a different way. It starts to break down to form gelatin. This is what happens when meat is cooked for a long time at a low temperature. The tough collagen molecules which hold the muscle fibres together slowly disintegrate and the gelatin dissolves into any liquid added, which helps give a rich gravy. See for yourself with this experiment.

Download an experiment

Flavours in meat

Meat cooked in hot water will not look particularly appetising. It will not have the brown colour we expect and it will have no roasted flavours. This is because the reactions that make the brown colours and the rich flavours of a roasted joint happen at much higher temperatures.

When we fry or roast meat, the surface layers dry out and their temperature rises above 100C, the temperature at which water boils. Some very complicated chemistry now starts to happen because the sugars present in the cells of the meat, particularly a sugar called ribose, start to combine with the amino acids. (Basically, amino acids are the building blocks of protein.) When proteins are cooked, they break down and release the individual amino acids. It is these that react with the sugars in the meat.

Scientists have been studying this process for more than 100 years, but they still don't fully understand everything that happens. What they do know is that these reactions make brown colours and many, many very odorous molecules - we have learned to like these smells and associate them with roasted meat. Scientists also know that sulphur atoms are very important in helping give the aromas we like; this is why we often add garlic or onions to meat dishes, because these vegetables are relatively high in sulphur.

The reactions between sugars and proteins is important to many cooking procedures. It is what gives the colour and flavour to, for example, bread crust, toast, biscuits and caramel toffee, as well as to meat. Collectively they are known as the Maillard reaction although they aren't just one reaction.

By downloading the experiment below we can show how important this reaction is by using a sugar called glucose. (There are many different types of sugar and what we put in our sugar bowls - sucrose - is just one of them.)

Download an experiment

Cooking meat safely

Besides cooking meat to make it easier to eat and to give it a delicious flavour, we also cook it to kill any harmful bacteria that it might contain. Large joints of meat are relatively free from bacteria on the inside, but often have them on the surface. This is why we should always wash our hands after we touch raw meat.

Many bacteria are relatively harmless although they can cause the meat to spoil. However, some bacteria, such as salmonella and Eschericia coli can also cause food poisoning. These bacteria are killed at temperatures above 68C, so during cooking any part of the meat that might contain these bacteria must be heated above this temperature. This is why ground and minced meat, where surface bacteria can be mixed throughout the mass, should be cooked in a way that ensures it is all adequately heated.

Two-speed cooking

We can now understand the slow cooking procedure used by Heston to cook meat on Full on Food. Slow cooking causes the collagen in the meat to break down into gelatin and the meat fibres to shrink and release the meat juice. This makes the meat tender. The fibres separate easily and they are deliciously succulent from all the gelatin released from the collagen.

The high-temperature surface searing of the meat following the slow cook creates lots of aroma molecules when the ribose and the amino acids released from the meat react together, and these add to the rich flavour. This final heating also kills off any bacteria that may have survived the slow cooking.

  • 55 - 60C Shrinking of meat fibres and release of juices from them
  • 60 - 65C Slow breakdown of collagen to make gelatin which dissolves in water.
  • 65 - 70C Killing of any harmful bacteria.
  • 100 - 120C Formation of roast and fried flavours when ribose and amino acids from the meat react together.

Now try one of our slow-cooked meat recipes:

Discuss the results of your experiments, and any food combinations you've tried, with fellow foodies on The Food Lab message board.

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