Critical temperatures

Ask a baker or a pastry chef whether temperature matters, and he or she will throw a bewildering set of figures at you. Proving, baking, sugar boiling, chocolate work, ice-creams and doughs turn out better for working to precise temperatures. In the kitchen, though, chefs are more cavalier. They trust their senses more than the thermostats. At least, that used to be the case.

Now that "molecular gastronomy" (a term coined by Oxford physicist Nicholas Kurti) is more than a fringe fashion, cooks who treated their vocation as a craft are getting into the science behind it. When they do, they may disagree as to whether beef should be cooked to 55°C or 59°C, but they share the factual information on which they base their subjective opinions.

The science they rely on isn't hot off the press. Almost 50 years ago, US chef Lendl Kotschevar applied basic food technology to cost-sector catering in the book Standards, Principles and Techniques in Quantity Food Production. As part of his pioneering work, he analysed the benefits and limitations of slow-roasting at 100°C. His impact on creative cooking, though, was negligible.

The introduction of sous-vide, around 1980, alerted a handful of chefs to the benefits of low-temperature cooking. When its inventor, Georges Pralus, persuaded Jean and Pierre Troisgros to poach foie gras at 65°C in a plastic bag, he showed that quality, consistency and yield improve by working to precisely regulated formulae.

The third stage in this slow-moving evolution of chef trends was the publication of Harold McGee's two classic books, On Food and Cooking and The Curious Cook. He explained microbiology in a way that lay people could relate to and understand. When he said, as Kotschevar already had, that "sealing in the juices" was a culinary fallacy, he explained why in a way that made those who studied him trust what he was saying.

Over the past decade, chefs and scientists have continued the dialogue. The Adrià brothers in Spain, chemist Hervé This in France, and chefs Heston Blumenthal and John Campbell in the UK all probe for ways of applying controlled heat creatively. They adapt the theoretical information gleaned from universities or research laboratories to practical ends.

Low and slow

Cooking meat is not like boiling pasta (drop it into water and drain when ready). Shape, weight, tenderness and intra-muscular or surface fat are variables. Only the obviously tender cuts respond to grilling, pan-frying or roasting. The rest, in traditional cookery, are stewed, simmered or braised in liquids at temperatures between 80°C and something below boiling point. Pressure cooking with superheated steam is an alternative, though it's more common in domestic or manufacturing contexts.

A grilled "blue" steak has a core temperature of about 45°C. It's tepid and virtually raw in the middle, though the outside may be carbonised. At 55°C it's still rare; at 60°C it's tipping over into medium; and by 70°C the protein will have coagulated - that's why the juices are no longer pink.

Armed with this basic knowledge, chefs can approach cooking from fresh angles.

\* How do you take a tough piece of meat and make it as tender and juicy as fillet steak?

\* How do you take a tender cut like fillet steak and keep it equally moist and juicy from the surface to the core?

The solution in both cases is slow cooking at low temperatures. With an intrinsically tough joint or cut, the temperature has to be set lower than 60°C to tenderise the collagen, and the length of cooking time - anything up to 60 hours - will depend on the piece of meat and its weight. For example, at the Fat Duck in Bray, Berkshire, Blumenthal cooks vacuumed pork belly at 64°C and lamb at 60°C.

With prime beef cuts, the range will be from the low 40°Cs to 59°C (preferred by Campbell at the Vineyard at Stockcross, Berkshire, because it gives a hot finish and appeals to the majority of his customers). The same low heats work with even-sized portions of fish. To obtain mi-cuit (half-cooked) salmon, Campbell takes it to 42°C, and takes bass to 49°C.

To achieve an optimum result, moisture loss (loss of juiciness) must be kept to a minimum. Larger joints can be baked in convection or conventional ovens; smaller pieces may be vacuum-packed. Immersion in a flavoured liquid, fat or oil bath held at a set temperature (a process that probably developed from confit) is ideal for fish.

It's also helpful to combine cooking processes. Blumenthal serves pork belly, cooked for more than two days, along with the loin, which has been slow-cooked as a joint, portioned, vacuum-packed and cooked again.

Low-temperature cooking requires the most stringent hygiene standards, but high-risk products such as poultry would not normally be used, because it's cooked through to 70°C or higher.

Browning with slow cooking

Chefs use the terms "browning", "caramelisation" and, more recently, the "Maillard reaction" interchangeably, but they aren't exactly the same thing when applied to protein cookery.

Browning is the safest non-specific description, though the colour on the outside of cooked meat or fish may be anything from rust to charcoal. Caramelisation implies the presence of sugar; actually, it's the combination of fat with protein that causes the colour change.

The Maillard reaction causing browning starts at relatively low temperatures, when sugar/fat and protein are heated together. It doesn't describe a kind of caramel. The change of colour, though, doesn't account fully for the flavour of roast or grilled meat that is so desirable. This develops after 140°C and is a complex reaction, not yet fully understood by science, that involves the many hundreds of taste components in the meat or fish.

Chefs who want the true taste of "roast" while sticking with slow-cooking methods have to engineer new solutions. According to Blumenthal: "If you blow-torch a joint all over for two minutes and then ‘roast' [at a low temperature] for five or six hours, you get the right effect because you've triggered the Maillard reaction."

Liquids and simmering

Stocks, broths and jus also benefit from strict temperature control. If the liquid becomes agitated, as it will if it's boiled, the final liquor will be cloudy. The trick is to put the meat, bones, etc, into the pan, pour cold water over them and heat gently almost to boiling point, in order to bring the scum to the surface. Then take the pan off the heat and add as much as one-fifth the same amount of extra cold water to bring down the temperature. The bones, meat and vegetables then sink to the bottom and the impurities can be skimmed off the top. Finally, the stock is ready for simmering at a constant 80-82°C that extracts the taste and gelatine from the solids.

When the meat industry cooks hams and gammons in water or steam, or with humid air, it keeps the cooking medium between 77°C and 85°C, not because this is the most efficient temperature range, but because it wants to be quite sure that hams meet bacterial controls. To keep shrinkage to a minimum, joints are cooked to a 69°C core temperature.

Note on vegetables: According to Dr Keith Waldron of the Norwich Food Research Institute, root vegetables and potatoes won't cook at low temperatures. In fact, if you heat a carrot to 50°C and then cool it, an enzyme action takes place that prevents it from softening even when it's later boiled for an hour. As a rule, the most effective liquid or steam technique for vegetables is to boil them.

Oven temperatures

A century ago, cookery books didn't give oven temperatures, because the equipment wasn't thermostatically controlled. By the 1950s, a gas regulo number or Fahrenheit temperature had become the norm. Typical of the period's roast meat recipes was the instruction to start the process in a hot oven (say 220°C, Gas Mark 7) and then to reduce it to 160°C (Gas Mark 3). Shrinkage on rolled joints of beef prepared in this way can be as much as 45%. Slow-roasting between 100°C and
110°C reduces weight loss in joints of beef (4-5kg) to less than 20% and, under test conditions, to less than 10%.

What's lost in this process is the crisp, dried-out surface (the rissolage) that many people enjoy. If the current fashion is for very succulent textures, it's certain that there'll be a swing of the pendulum where chefs will make a virtue of dry but meaty crusts. Searing joints in smoking-hot dripping before roasting at high heat could make a comeback.

Low temperatures do nothing for roasted potatoes. There's not an optimum temperature. Something in the 180°C to 200°C range is ideal. What matters is that the starch reaches at least 85°C; otherwise the centre will be hard and gummy.

Frying

Deep-frying takes place in a band between 165°C and 190°C. Battered items do best somewhere between the extremes. The oil for whitebait and crisps should be as hot as possible because the aim is to extract all the moisture as fast as possible. Chips are blanched at the lower end of the scale, then crisped up in the hottest fat. When raw food goes into a fryer, the temperature drops according to the proportion of food to the amount of oil. The faster its temperature recovers, the less greasy the cooked food will be.

Perhaps surprisingly, most fried food is actually steamed. The outer surface provides a barrier against the oil while the water in, for example, the battered fillet of cod heats above 70°C or, more commonly, boiling point

A rough guide to cooking temperatures