Читать книгу The Complete Book of Pressure Cooking - L.D. Michaels - Страница 10

Yes, I realize many of you picking up this book will already have a pressure cooker, but please stick with me, as the writer’s device of considering the various options in pressure cooker purchase enables me to get over some important messages. And certainly read on if you are thinking of buying one or giving advice to someone about to.

The most expensive pressure cookers cost ten times as much as the least expensive, but they all work. The differences are to do with the metal and other materials from which they are made, the pressure at which they operate, the quality of accessories, whether they were manufactured in a low-labour-cost country and external appearance. And if the cooker has been endorsed by a celebrity chef, you are also contributing to their fee or commission.

All pressure cookers have certain features in common:

1 A PAN that is usually much thicker than an average saucepan. It will resist high pressures and withstand the effects of sudden cooling.

2 A LID that is similarly thick and which can be fastened down tightly. The lid will contain …

3 A PRESSURE VALVE OR WEIGHTS that control the pressure inside the pan. Some models have a spring-loaded flip-up valve, some have a spring-loaded indicator and some have a series of weights that fit over a small escape vent. The escape vent must be kept clear of food particles at all times; if it becomes clogged, even slightly, pressure will build up inside the cooker and at the very least you will find that the food overcooks. If your cooker has a valve there may be more than one “position” – typically for “full” pressure and for a “lower” pressure. If your pressure cooker has weights, then don’t be surprised if they are considerably lighter than the markings on them. The 15lb weight with my old Prestige cooker weighs 126g (about 5 oz) – it’s the combination of the weight sitting over a vent and having steam forced through it from underneath that causes the pressure build-up inside. Most “weight” systems consist of three sections: an inner (L) weight, a sleeve making the pressure up to (M) and then an outer sleeve to make up the normal operating pressure (H) or 15 psi. Another device is the valve that revolves slowly when the cooker is at the right pressure. This gives you a visual check on what you are doing. Such a device is found on one of Europe’s best selling models. The lid will also have …

4 A SAFETY VALVE. This is to prevent the inside from becoming overpressurized or overheated. Most valves have two components: one that is sensitive to pressure and which blows out at around 3½–4 atmospheres, and another that melts suddenly and rapidly if the heat becomes excessive. The valves are usually of rubber composition with a metal core, or of one sort of rubber on the outside with an inner plug of a different type of rubber composition. Under normal circumstances and with proper use, the safety valve should never “blow” unless after a time it becomes worn.“Blowing” occurs, if:(a) The pressure valve has become blocked and steam can’t escape at the rate at which it should. This will happen if the pressure valve isn’t cleaned regularly or if you are cooking foods that froth up and your saucepan is overfilled (to more than two-thirds).(b) You have let all the moisture leave the cooker, either by not putting in enough to begin with or by overcooking. In these circumstances, the pan ceases to be a pressure cooker and starts acting like an oven. The food will be spoilt and the safety plug will melt to prevent too much damage.You hear dreadful stories about safety valves blowing and directing a stream of extremely hot mashed food at the kitchen ceiling. Most of the stories are apocryphal. The more recently designed models tend to “blow” at a lower pressure than the earlier ones so that the cook has enough warning and the worst that happens is a messy dribble.

5 A TRIVET. This is a small inverted tray with holes in it that almost exactly fits the bottom of the pan. The water that makes the pressurized steam is placed underneath the trivet and food can be placed on top of it so that at no stage does the food touch the water. In some lower-cost models, the trivet is simply a bent piece of thick metal wire which does the same job. The food is then properly steamed and the valuable nutrients that could dissolve in the water are retained in the food. The trivet is not used for all recipes, of course.If you use the pan with the trivet and with another pan resting inside, but with the lid off, you then have a double boiler, which is suitable for making delicate sauces. Some makes offer you a wire basket, or pannier, that does the same job but is easier to lift out.

6 DIVIDERS AND SEPARATORS. These are containers, some of them solid, some punctured with small holes, that rest on top of the trivet. You can put different sorts of food in them and, because the cooking is done by steam, the flavours don’t mix. You can easily cook three sorts of vegetable together and not have them all taste like one another.The separators with holes in them should be used wherever possible so that the steam can penetrate freely all around the food. It is only food that is liquid or in very small particles (like rice) that should be cooked in a solid separator. Since the steam can’t get at the food that easily, the cooking is done largely by the heat of the pressure cooker, and not the pressure, so cooking takes a little longer.

7 A GASKET. This is a silicone or rubber-composition seal that fits between the lid and the pan, depending on the precise mechanical means of closure. The gasket ensures that the cooker is sealed tight. If steam is seen escaping from beneath the lid, then the gasket is faulty and will need replacing. Almost certainly you will find that you have been cooking at a lower temperature and pressure than you intended and, as a result, your food will be undercooked.

8 A MEANS OF CLOSURE. A pressure cooker needs to be mechanically secure. There are a number of different mechanical devices used. Some require you to twist the lid carefully into a series of locking lugs on the top of the pan, some have massive handles and springs, some depend on an eccentric oval fitting lid slotting into place and with some there are twisting handles over the lid that are then screwed up to tighten into lugs on the pan.Most pressure cookers are designed so that you can’t open them under pressure, though a few are.Most early mistakes in pressure cooking come from incorrect closure of the appliances. Read your instruction manual carefully!

When you are making a purchase, there are some things you need to check:

1 The first is the METAL from which the cooker is made. There are two choices – aluminium and stainless steel.Aluminium is significantly cheaper and models tend to weigh less than stainless steel versions of the same capacity. There are several grades of aluminium both in terms of thickness and finish. Its main weakness is that lower grades may tend to interact with food, either causing discolouration or leaching into items that will later be ingested. Foods most likely to interact are those that are quite acidic, for example tomato sauce. But even non-acidic food may cause a grey discolouration that is, in fact, oxidization. Higher grades of aluminium are anodized – put through an electrochemical process that hardens the exterior of the metal so that interaction with food is less likely to happen. There are also degrees of quality for anodizing – “hard” anodized is better. Some forms of anodization allow the metal to have a coloured exterior. Thicker grades of aluminium, particularly at the base, will have better and more even heat distribution – this is important, for example, when you sear meat in fat prior to the actual pressure cooking.Stainless steel does not suffer from the discolouration problem. Again, there are several grades and in fact all or nearly all stainless steel pressure cookers have bases that also contain an enclosed layer of some other metal such as aluminium or copper. That’s because stainless steel is a relatively poor conductor of heat, while the aluminium or copper is far better. Stainless steel is itself an alloy of iron and chromium or iron chromium and nickel. Stainless steel 18/10 consists of 18% chromium, 10% nickel and 72% iron.Internal coating A few manufacturers offer models with an internal non-stick protective coating such as Teflon. On the whole, such coatings should be avoided, unless of extremely high quality, and are not really necessary. Non-stick is useful when frying, but most pressure-cooking recipes consist either of steaming, when there is water or other fluid in the base, or simmering, when there is also fluid in the base. In time, all but the very best non-stick surfaces will degrade, the more so if they come into contact with metal tools and the like, which can cause scratches. In pressure cookers, the problems are the trivet and separators.

2 Then we must consider SIZE. A good average “family” size is about 6lt (10½pts or about 6 US quarts). This will cope with most meals. Larger pressure cookers are available, up to 12lt (21pts), but in addition to their increased cost and weight they also take longer to come up to pressure, so that overall cooking times are increased – and along with them associated heating costs. Very large pressure cookers are sometimes referred to as canning cookers, as they can be used to preserve food by canning – this used to be popular in some countries such as the USA, though most people would now go for freezing as an easier and better form of food preservation. A large pressure cooker usually makes sense for a large family or in a catering situation. Smaller pressure cookers are also available, some with capacities as low as 1.5lt (2¾pts). These are mostly aimed at people going camping where the advantages of small size and rapid cooking but with restricted forms of heat make this a good choice. Otherwise, 4-lt (7-pt) capacities can make sense for 1- or 2-person households, but often the smaller-capacity cookers are not significantly lower in cost than the “standard” 6-lt models.

3 What is the PRESSURE at which the cooker operates? Not all pressure cookers operate at the same pressure. Until relatively recently there was a generally agreed standard of 15 psi, which is twice the pressure of the atmosphere. This requires a little explanation: ambient atmospheric pressure at sea level is 15 psi, so that “15 psi” is relative to ambient pressure (that is, normal air pressure). Most of the general recipes you will find in books and on websites assume 15 psi. But many pressure cookers on the market operate at less than this, often 12 psi or as low as 8 psi. The lower psi figure means longer cooking times and hence less benefit from the method. Manufacturers go for these low pressures because they can save money and also lower the weight of the cooker. A pressure cooker needs to be engineered to be able to cope with accidental overpressures of 6 times normal operation; 90 psi for a 15-psi model, and only 72 psi for a 12-psi version. The lower requirement means savings in the fabric and engineering quality of the cooker. Some manufacturers refer to the pressure by other measurement units – 15 psi is 1 atmosphere (again, this is relative to ambient pressure), so that 12 psi is 0.8 atmosphere; 15 psi is also 103 kPA (kilopascal) and approximately 1 bar. The recipes and tables in this book are based on 15 psi, but with reminders about how to extend for 12 psi and below – this usually means having to add 20% to the cooking time.All but the lowest-priced pressure cookers also allow for cooking at lower pressures – useful for delicate ingredients. A typical lower pressure is 8 psi or 0.533 atmospheres. Some models offer three levels of pressure: 15 psi, 10 psi and 5 psi.

4 SPARES. The main fabric of a pressure cooker will last for years, provided you take reasonable care. But every now and then you will find yourself needing to buy spares. The most frequent requirement will be for the silicone/rubber seal that seals the lid to the body of the pan and makes the pressurizing possible. A faulty or weakened seal means that proper pressure is no longer possible. Other spares you may need include the spring in the valve (or weights) and overpressure blow-out devices. The reason for putting availability of spares so high up the features list is this: if you are tempted to buy a no-name cooker, or acquire one from abroad – by mail order or on holiday – you may find that spares are difficult or very expensive to obtain. Before making a purchase, carry out an online search on your preferred cooker and then add the word “spares” to check ease of availability.

5 ACCESSORIES. The manufacturers of the least-expensive pressure cookers sometimes omit essential accessories such as a trivet and basket. Check carefully what is included and the prices of any additional items.