Читать книгу The Complete Book of Pressure Cooking - L.D. Michaels - Страница 7

Our diet consists of five components: carbohydrates, fats, proteins, minerals and vitamins.

No one eats these pure ingredients by themselves, so any application of heat to a given piece of raw food has a variety of effects. We have to trade off the loss of certain nutrients in return for releasing others and making the entire result more palatable. This is what happens when heat is applied to the various basic components listed above:

Carbohydrates – the sugars, if they are solid, melt very easily and become sweeter to the taste. If they are heated above a certain point, they start to turn yellow and caramelize. Sugar eventually turns a light brown and then becomes black and burnt. You can watch the process happen if you fry carrots or onions (in fact, onions make a good colouring agent for stocks and soups, as well as a flavour, if used in this way – fry an onion clear for a white well-flavoured stock, and fry it dark brown for a dark stock).

The other edible carbohydrate – starch – is inedible unless cooked. The starch in uncooked flour, potato, rice and so on is locked up in a series of packages that resist the stomach juices. Cooking breaks down the packages so that the digestive system can reach the edible starch. Overcooking results in a breakdown of the shape of the food itself, and while that isn’t bad nutritionally, the texture is mushy and that offends us.

Fats – heating fat melts it. Whether one has to heat it at all depends on the type of fat and is really a matter of what you happen to like; most people will cheerfully spread butter on bread, but not lard. If overheating occurs, the fat burns and produces fatty acids. Butter burns at 137°C (278°F), beef suet at 180°C (356°F), lard at 200°C (392°F), vegetable oil at about 260°C (500°F) and olive oil at 290°C (554°F).

Proteins – on heating, protein coagulates and hardens; watch what happens to egg white. We can digest cooked protein rather more easily than uncooked, so raw eggs are not always as health-giving as is sometimes claimed – a raw egg can slip through your digestive system without having very much of its protein absorbed. Protein is located in the fibres of meat, which is why meat can taste tough or stringy if not carefully treated. The only meats that can put up with violent exposure to heat are the very soft-textured ones like steak. Otherwise, they have to be physically cut up (as in hamburgers and meat loaf), marinated in wine or vinegar to soften the fibres chemically or cooked slowly at a low heat. Rare beef is “done” at 65°C (150°F) and fresh pork and poultry at 87°C (190°F), those being the final temperatures of the cooked foods at their centre.

The hardening of the protein fibres also causes shrinking of the meat and a squeezing out of the juices that contain other forms of goodness. Cooking of protein in general – and meat in particular – is quite possible in a pressure cooker, but obviously needs a bit of thought. A lot of people waste meat using perfectly “normal” techniques. Excessive heat breaks down protein.

Minerals – there are no general rules here. The calcium in milk becomes a little less available if it’s heated, but if greens are boiled in hard water (not normally a good idea), then more calcium becomes available to be digested. Iron is normally more easily picked up after cooking (a lot comes from cast-iron cooking pots!), and salt is unaffected by heat. However, a lot of minerals are lost if they are allowed to soak or leach away into cooking fluid (water or gravy) that is then thrown away. For this reason, boiled vegetables are in general not a good idea, unless use is made of the vegetable water. Steaming is far better, as the vegetables retain all their fluids and very little water is involved in the process. Pressure cooking is high-pressure steaming, of course.

Vitamins – vitamins A and D are unaffected by baking and boiling, though vitamin A is destroyed at the high temperatures used for frying. Vitamin B, or rather the various sorts of vitamin B, can be destroyed by high heat but, more importantly, they are soluble in water, which means that losses can take place in the same way as for minerals.

The real problem vitamin is C. Vitamin C can be destroyed in a number of ways: by prolonged cooking; by heating in the presence of air (and that can include keeping meals hot); by dissolving in water (so keep those cooking fluids!); by plant enzymes (the living plant protects the vitamin C from its own enzymes, but ceases to do so when it is cut, which is why shredding lettuce for salad too long before eating is a bad thing); and finally by the presence of copper.

But really, all cooking results in pluses and minuses in nutritional value. Potatoes, which in their raw state contain vitamin C, have to be cooked if the starch in them is to become edible, by which time the vitamin C is destroyed. The plant enzyme that breaks down the vitamin C in a dead plant is killed off by temperatures in excess of 60°C (140°F), so that in certain circumstances cooking green vegetables is definitely a good thing.

In practice, most of the techniques we use are reasonably sane and safe. Pressure cooking, whatever the critics may tell you, in fact preserves far more in the way of essential nutrients than most of the traditional techniques. Further, because the techniques are a little different and unusual, it forces us from time to time to think carefully about our diet – are we getting enough of the right sort of ingredients, or too many of others!