The Complete Book of Pressure Cooking. L.D. Michaels
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!
The other half of the cooking conundrum is the question of heat, and the way in which it is applied. Professional cooks and domestic science teachers tend to distinguish between two types of heat: dry and moist.
The “purest” form of dry heating is old-fashioned roasting on a spit in the open air. The heat is entirely radiant and progresses from the outside of the food to the middle relatively slowly; as each particle of food gets heated up, it passes on some of the heat to the next particle further inside. If you place your food too close to a strong source of heat, such as a grill, the outside will start to burn before the inside has started to cook at all. We do this to good effect when we toast bread, but if you place a 7.5-cm (3-in) steak under the same grill, then the outside will be charred and the inside raw.
With moist heat, the heat is transferred not only from particle to particle of food but is transported by molecules of liquid or steam. If you are stewing or boiling, then the heat is carried through by stock or water; if you are steaming or pressure cooking, then the heat is conducted through the food by molecules of steam. With pressure cooking it is possible to make the steam hotter than it normally would be and also (unlike other forms of cooking), because everything happens under pressure, the heated steam is forced into the heart of the food much more quickly.
Critical to both forms of heating is the size of the piece of food to be heated. Meat minced finely enough will cook almost instantly if put in fairly hot stock or fat. An enormous potato baking away in an oven for a long period may look gorgeous from the outside but may still be raw on the inside. Every cook with only a little experience knows this. Pressure cooking is no exception – food cut up suitably small will always cook more rapidly than great chunks.
However, cooking is much more than bringing the ingredients to the right temperature so that the appropriate chemical changes can take place. Reading through the previous passages as one does when one is writing, I fear I may have sounded too obsessed with cooking processes and biochemistry. I’m glad to be able to turn to more familiar material. Moving away from the lab and back into the kitchen, we ought to return to those things that we admire in food: taste, texture, appearance and nutrition.
Whatever the domestic science theorists say, it’s these that count and the clear-cut distinctions between dry and moist forms of heat are not always so easy to make when we look at what cooks actually do.
Microwave cooking can be thought of as using both dry and moist techniques – the microwaves agitate molecules within the food, generating heat. But some of the molecules in most pieces of food will be of water and the effect of heating turns them into steam.
One of the classic British ways of “roasting” a joint consists of setting the meat in a tray of fat and then placing it in a preheated oven. Periodically, the fat is basted over the joint to keep it moist. Then, at the end of cooking time, the joint is removed and put on a carving dish and the juices collected in the bottom of the pan are made into gravy by adding a little thickening, such as flour or cornflour, and heating fiercely. If the theorists were to break down this cooking process, they would say that this isn’t really roasting at all; the bottom of the joint is actually frying (a form of dry heat because moisture is absent), and after the first few minutes of heating, the water in the joint of meat will turn to steam. If the joint were truly being roasted, in the open air, the water vapour would disappear into the atmosphere but, in fact, since the whole process is taking place in an oven, the steam collects and becomes itself a vehicle of cooking in the form of moist heat that then reaches the centre of the joint rather more quickly than “pure” dry heating does. Add to this the spurting fat and the hot juices flowing over the meat as they bubble out that are basted over, and you get your classic dish – but with what technique?
Now with pressure cooking, there is nothing inherent in the process that would give us anything like that result. So if we want effects similar to the ones we are used to, we have to replace them in various ways.
If this sounds alarming, that is, in fact, what all the recipes in this book (and most of the ones that you are likely to find elsewhere) try to do.
A pressure cooker cannot:
Fry – but you can fry the outside of meat and then pressure steam the inside to give a similar yet juicier result.
Roast – but you can pot-roast, giving meat a delicious outside crust, either by searing or by using burnt sugars and then pressure steaming the inside far more rapidly than any traditional roasting method and with far less loss of intrinsic goodness.
Grill, bake, flambé, barbecue, smoke or deep-fry.
Slow-cook/casserole – but it can take the same ingredients and produce something equally interesting.
Neither will your pressure cooker play music, speak your weight or take off into orbit in outer space!
However, in addition to acting as a pressure cooker, your appliance has other culinary uses:
Quite obviously it is a superb saucepan with the ideal characteristic of spreading heat evenly round the base and sides due to the thickness of the metal. You could even deep-fry in it if you wanted to.
It is a well-designed steamer at ordinary temperatures and pressures (you just don’t use the weights or valve). Steaming is a marvellous way of cooking vegetables and fish. One way in which this book differs from any other you’re likely to come across is that I sometimes suggest it is silly to pressure steam foods that cook rapidly anyway – who needs to have spinach ready in one minute?
Most pressure cookers can be adapted to become double boilers. You rest a smaller pan or bowl on the trivet and then fill the outer pan with water that you heat in the normal way to 100°C (212°F). The contents of the inner bowl will keep just below that temperature – ideal, in fact, for making delicate sauces. The French name for this contraption is a bain-marie.
THE PHYSICS OF PRESSURE COOKING
Now more specifically on to the “science” bit. The temperature at which water becomes steam depends on the pressure of the surrounding air. The higher the pressure, the higher the temperature at which the water boils. Conversely, several hundred metres up a mountain, water will boil below 100°C (212°F).
Water in an open pan at sea level (15 psi or less familiarly 101.325kg/m2 or 1 bar) boils normally. Close the pan, let the air escape and the steam builds up pressure inside, just like a steam engine. By letting a little of the steam at a time out of the pan, you can control the pressure inside. Normally this is done by means of weights or a spring-loaded valve and usually the pressure