Pressure cooking always requires liquid. Pressure cooking cannot be used for cooking methods that produce little steam such as roasting, pan frying, or deep frying. However, Kentucky Fried Chicken restaurants use a combination of pressure cooking and frying, with special pressure fryers in which the chicken’s own juices supply the water. Cooking time is reduced substantially, to approximately 12 minutes per amount cooked, but the breading texture is much softer (less crispy) than deep-fried chicken since moisture remains in the breading. Thick sauces do not contain enough liquid to vaporize and create pressure, so they usually burn onto the interior base of the pressure cooker after prolonged heating. Sauces should be thickened after pressure cooking.At higher altitudes, the boiling point of liquid in the pressure cooker will be slightly lower than it would be at sea level. When pressure cooking at high altitudes, cooking times need to be increased by approximately 5% for every 300 m (980 ft) above 610 m (2,000 ft) elevation. The absolute pressure in a pressure cooker will always be lower at higher altitudes, since the differential pressure remains the same (if one were to travel high enough the pressure within the cooker would drop below sea-level pressure). Since weight is one of the major concerns, mountaineering pressure cookers may be designed to operate at a much lower differential pressure than regular units so that thinner, lighter construction can be used. Generally, the objective is raising the cooking temperature to make cooking possible where it would otherwise be completely impractical and to conserve fuel by reducing heat lost through boiling.Pressure cooking is the process of cooking food, using water or other cooking liquid, in a sealed vessel, known as a pressure cooker. As pressure cooking cooks food faster than conventional cooking methods, it saves energy. Pressure is created by boiling a liquid, such as water or broth, inside the closed pressure cooker. The trapped steam increases the internal pressure and allows the temperature to rise. After use, the pressure is slowly released so that the vessel can be safely opened.Foods cook much faster with pressure cooking than with other methods (except for small quantities in microwave ovens). Pressure cooking requires much less water than conventional boiling, so food can be ready sooner. Less energy is required than that of boiling, steaming, or oven cooking. Since less water or liquid has to be heated, the food reaches its cooking temperature faster. Using more liquid than necessary wastes energy because it takes longer to heat up; the liquid quantity is stated in the recipe. Pressure cookers can use much less liquid than the amount required for boiling or steaming in an ordinary saucepan. It is not necessary to immerse food in water. The minimum quantity of water or liquid used in the recipe to keep the pressure cooker filled with steam is sufficient. Because of this, vitamins and minerals are not leached (dissolved) away by water, as they would be if food were boiled in large amounts of water. Due to the shorter cooking time, vitamins are preserved relatively well during pressure cooking.The natural release method allows the pressure to drop slowly; this is achieved by removing the pressure cooker from the heat source and allowing the pressure to lower without action. It takes approximately 10 to 15 minutes (possibly longer) for the pressure to disappear before the lid can be opened. On many pressure cookers, a coloured indicator pin will drop when the pressure has gone. This natural release method is recommended for foods that foam and froth during cooking, such as rice, legumes, or recipes with raising agents such as steamed puddings. The texture and tenderness of meat cooked in a pressure cooker can be improved by using the natural release method. The natural release method finishes cooking foods or recipes that have longer cooking times because the inside of the pressure cooker stays hot. This method is not recommended for foods that require very short cooking times, otherwise the food overcooks.Cooking under pressure requires less liquid than conventional cooking methods since there is less evaporation. You can always use more liquid than recommended in a pressure cooker recipe, but never use less.Remember that pressure cookers cook quickly since they use the pressure created from the built-up, hot, trapped steam in the pot. Therefore:• Always use at least 1 cup of cooking liquid. • Never fill the pressure cooker more than ½ way with liquid.• If you end up with too much cooking liquid after cooking under pressure, simply reduce the liquid by bringing it to a boil uncovered and reduce until the desired consistency.For first generation pressure cookers with a weighted valve or “jiggler”, the primary safety valve or regulator is usually a weighted stopper, commonly called “the rocker” or “vent weight”. This weighted stopper is lifted by the steam pressure, allowing excess pressure to be released. There is a backup pressure release mechanism that releases pressure quickly if the primary pressure release mechanism fails (e.g., food jams the steam discharge path). One such method is a hole in the lid that is blocked by a low melting point alloy plug and another is a rubber grommet with a metal insert at the center. At a sufficiently high pressure, the grommet will distort and the insert will blow out of its mounting hole to release pressure. If the pressure continues to increase, the grommet itself will blow out to release pressure. These safety devices usually require replacement when activated by excess pressure. Newer pressure cookers may have a self-resettable spring device, fixed onto the lid, that releases excess pressure.The flavor of some foods, such as meat and onions, can be improved by gently cooking with a little pre-heated cooking oil, butter or other fat in the open pressure cooker over medium heat (unless the manufacturer advises against this) before pressure cooking. It is important both not to overheat the empty pressure cooker and never to heat the empty cooker with the lid and gasket in place. Overheating can cause warping and other damage. The pressure cooker needs to cool briefly before adding liquid; otherwise some of the liquid will evaporate instantly, possibly leaving insufficient liquid for the entire pressure cooking time; if deglazing the pan, this has to be taken into account.Small containers such as plastic pudding containers, can be used in a pressure cooker, provided that the containers (and any covering used) can withstand temperatures of 130 °C (266 °F) and are not placed directly on the interior base. The containers can be used for cooking foods that are prone to burning on the base of the pressure cooker. A lid for the container may be used, provided that the lid allows some steam to come into contact with the food and the lid is securely fitted; an example is foil or greaseproof paper, pleated in the center and tied securely with string. Containers that are cracked or have otherwise sustained damage are not suitable. Cooking time is longer when using covered containers because the food is not in direct contact with the steam. Since non-metal containers are poorer heat conductors, the type of container material stated in the recipe cannot be substituted without affecting the outcome. For example, if the recipe time is calculated using a stainless steel container and a plastic container is used instead, the recipe will be undercooked, unless the cooking time is increased. Containers with thicker sides, e.g., oven-proof glass or ceramic containers, which are slower to conduct heat, will add about 10 minutes to the cooking time. Liquid can be added inside the container when pressure cooking foods such as rice, which need to absorb liquid in order to cook properly.What’s tricky about cooking in a pressure cooker? It’s a whole new way of cooking, with its own language and processes. You usually need to wait for a pressure cooker to heat up, then you add the food and the lid, and let it cook for a certain amount of time, at a certain pressure level. (How long? There are many pressure cooking charts that show you how long certain foods should cook — I use the one that came with my electric pressure cooker.) Then you let the pressure release (sometimes fast, sometimes slow — depends on the recipe).Before the pressure cooker lid is sealed airtight, the internal air has to be mostly replaced by steam. Steam has a much higher specific heat than air, and the presence of steam rather than air inside the pressure cooker is how it’s able to transfer sufficient heat into the parts of the food that aren’t submerged in liquid, such as a pot roast. If the lid is sealed before enough air has been removed, not enough heat can be transferred to the food, and food may be undercooked; the presence of air would make the food cook more like it’s in an oven than a pressure cooker. To remove the air, steam is vented for several minutes to replace the volume of air inside the cooker. This is why a pressure cooker takes about 10 minutes to reach pressure. For pressure cookers with a weight, the weight is placed over the steam vent pipe while steam is being emitted, to ensure the air inside has escaped. The newer generation pressure cookers, which have no weights, automatically expel air from inside for several minutes before a coloured pop-up indicator pin rises to seal the lid airtight; pressure then builds in the now airtight cooker. If the pressure cooker is already hot or a stovetop pressure cooker is placed on a very strong heat source – such as induction on too high a setting, the lid can seal airtight too quickly before the air inside has been removed. In these situations, a slightly lower heat setting can be used to allow the water to boil slower in order to vent the air.