The appliance of science

Physicist Dr Peter Barham lectures on meat and two veg at Bristol University. He is the scientist's answer to Delia Smith or Victor Ceserani: infallible and reliable.

While most chefs can roast a joint or boil cabbage with their eyes shut, Dr Barham can explain why the beef is tough or the Brussels sprouts watery; why the Yorkshire pudding sinks and the roast potatoes don't crisp.

No one needs a PhD to cook a roast, but a dash of chemistry and a sprinkling of physics can enhance the skills of even the most brilliant chefs. A basic understanding of some scientific principles will explain why the tightest recipe can fail and why some techniques work better than others.

There are two basic ways of roasting potatoes. Method one: put potatoes in a roasting tray with hot dripping. Season and roast for about an hour. Method two: parboil the potatoes. Drain, turn into a roasting tin with hot fat, season and bake for one hour.

Which works best? If the aim is to achieve crispiness, method two works better. When the surface of the potato reaches boiling point it forms a translucent gel as it absorbs water from the pan.

Drain the potatoes, turn them into hot dripping and the water in the "gel" will evaporate, leaving a thick, porous layer which can absorb fat better.

• Fork them around in the boiling water before draining.
• Turn the potatoes into smoking, hot dripping and don't allow them to cool before putting them in the oven.
• After roasting, don't store them at low temperatures or they will go leathery. Keep them above 60ºC.
• Where possible leave them to stand in hot fat until ready to serve.
• You will find that more floury potatoes will absorb more dripping and therefore give fluffy insides.

The standard recipe for cabbage instructs chefs to quarter and boil the vegetable for 20 minutes in plenty of salted water. It should then be drained and any excess moisture pressed out. Alternatively, you can shred the cabbage, steam and spray with a salt solution.

The object in cooking greens is to weaken the cell walls and extract water from the cells. The longer you cook them, however, the more nutrients the cells lose and the less flavour the dish will have. Cooking times given in recipes for greens are notoriously too long and should be ignored.

If you are going to add salt to the boiling water, it must be more than the proverbial pinch. Unless the salt concentration is higher in the water than in the vegetable, the flavour won't be enhanced.

• Either add more than 10g salt per litre of water or let customers add the salt themselves.
• Five minutes is adequate cooking for most green vegetables.

Dr Barham's recipe for Yorkshire pudding uses 50g plain flour, one egg, about 50ml milk, salt to taste, and fat for baking.

To make a mean Yorkshire pud, sift the flour and salt into a bowl. Lightly beat egg with enough milk to make up to 100ml of liquid. Add a quarter of this liquid to the flour and mix to a stiff paste. Beat until smooth so it forms "thin sheets" when beating.

Add the remaining liquid a little at a time, beating throughout to keep the mixture smooth and lump-free. The batter should have the consistency of single cream. Set aside for two hours.

Pour the mixture into a preheated dish or tin containing smoking, hot fat (200ºC) and bake at 200-220ºC for 20-30 minutes until it has risen and is brown.

Good Yorkshire puddings have a crisp, risen surface yet are moist inside. Baking involves a series of chemical reactions. The hot fat cooks the bottom of the pudding and stops it sticking to the tin. The top of the pudding sets as the starch granules in the flour swell. Water evaporates from the pudding and the protein in the egg and flour changes its structure.

The outer crust, still soft, swells like a balloon as the centre of the batter reaches boiling point and generates steam. The surface stops expanding and browns.

Wild boar has dark red meat; pig's meat is pink, as is veal, whereas beef is red. Grouse have dark red breasts, chickens white. But chickens have brown legs. Why?

The red colouring comes not from blood, but from myoglobin molecules - the proteins that store oxygen in the muscle. The more active an animal is, the more myoglobin there is in themuscles.

Meat colour is only an indication of age and activity. It can suggest more flavour, but not what the flavour is, and is no pointer to how tender the meat will be.

Slaughter and hanging: After slaughter, chemical changes occur in both red and white meat. Muscles continue to use the oxygen stored in the myoglobin, but since the blood no longer flows waste products, especially lactic acid, accumulate. This inhibits bacterial growth, which lengthens potential shelf life (30 days-plus for beef). Combined with enzyme action, it increases tenderness and improves flavour.

Poor slaughterhouse practices such as stressing the animals and rapid chilling of the carcass will spoil the keeping quality of meat and prevent its maturation.

Scientifically, there is no foolproof way a chef can tell from just looking at a piece of meat whether it will be tough or tender, tasty or bland. Dr Barham's advice is: "Trust your butcher."

Flavour: Roast beef has more than 400 identified flavour ingredients. The fat has more taste than the flesh, but in cooking the most characteristic beefy "notes" are linked to sulphur compounds.

Gristle: There are three kinds of connective tissue in animals: collagen, elastin and reticulin. The first softens and turns to gelatin above 60ºC. Neither of the others will be structurally changed at normal cooking temperatures, but are chewy, contract when heated and toughen meat.

Connective tissue holds muscle fibres together in bundles and connects muscle to bone. On a fillet of beef, for example, fine muscle fibres are bound by collagen, but the outer "skin" which connects the fillet to the backbone is elastin and should be pared away.

"Cooking meat is always a compromise," Dr Barham says. At high temperatures, protein changes structure and toughens, but until the higher temperature threshold is reached (60ºC), the collagen won't soften. "The general rule is to cook meats with a low connective tissue content at a high temperature, and to cook those containing a lot of connective tissue for a long time at low temperatures," he says.

The larger the joint is, the lower the temperature at which you need to roast it.

Cooking times: These are not directly related to weight, but to the distance from the surface of the meat to the deepest point of the muscle. So, a 5kg joint of rolled topside will roast almost as quickly as two 2.5kg joints.

Instructions such as "15 minutes per pound plus 15 minutes" can be misleading. The conformation and weight of bones may alter roasting times, as will the temperature of the joint when it is put in the oven.

Core temperatures: Remember, a mis-placed probe when measuring core temperature can lead to undercooking, especially it if goes between seams of deep muscle and in joints between bones. Most roasting should take place between 60ºC for medium rare red meats, and 70ºCfor white meats and poultry.

Intramuscular fat acts as a lubricant and carries flavour.

Fat on the outside of a joint, however, does neither of these things. At best it protects the surface from drying out.

Dr Barham warns that self-basting poultry (where fat has been injected into the flesh), doesn't work. "Most of it just drains out during cooking," he says.

Most people imagine water to be perfectly fluid. In fact, it becomes more viscous as it cools. Some 60% of the weight of raw beef is water, and roasting can remove up to a third of it. The remainder is what makes a joint succulent.

Resting for up to a quarter of the cooking time prior to carving improves the texture of meat. It becomes firmer and the juices are more evenly distributed. With large joints, it is advisable to turn them during the resting period so the juices circulate in the meat.

During roasting, juices from the joint drip into the roasting tin and these form the basis of most gravies. If they burn, they will be useless. To protect against this, coat the roasting tray with hot dripping before placing the joint in the oven.

Dripping from a joint, especially when combined with a little alcohol, contributes extra zip to the flavour of gravy.

If the dripping is going to be used as the basis for a gravy thickened with a roux, it is better to make it with a little flour that has been roasted until brown, rather than use a raw flour which leaves a starchy taste.