A slow cooker or Crock-Pot (a trademark often used generically) is a countertop electrical cooking appliance that maintains a relatively low temperature (compared to other cooking methods, eg, baking, broiling, or frying) for many hours, allowing unattended cooking of pot roast, stew, and other suitable dishes. They are not suited to some purposes, such as baking.
The Naxon Utilities Corporation of Chicago developed the Naxon Beanery All-Purpose Cooker. Rival Industries bought Naxon in 1970, and reintroduced it under the Crock-Pot name in 1971. In 1974, Rival introduced removable stoneware inserts. The brand now belongs to Sunbeam Products, a subsidiary of Jarden Corporation.
A slow cooker consists of a lidded round or oval cooking pot made of glazed ceramic or porcelain, surrounded by a housing, usually metal, containing a thermostatically controlled electric heating element. The lid is often transparent glass and is never hermetically sealed. The ceramic pot, or a 'crock', acts as both a cooking container and a heat reservoir. Slow cookers come in a variety of sizes, from 500 ml (16 oz) to 7 liters (quarts). Due to the placement of heating elements (generally at the bottom and often also partway up the sides), there is usually a minimum recommended liquid level to avoid uncontrolled local heating.
Many slow cookers have two or more temperature settings (e.g., low, medium, high, and sometimes a "keep warm" setting). A typical slow cooker operates at approximately 160°F on low, to perhaps 190-200°F on high, and 1 hour on high followed by an indefinite time at medium heat. In any case the cooking temperature is below the boiling point of water (212F / 100C). Some slow cookers sold in the US in the past several decades did not slow cook at all: all of the settings brought the contents to full heat, with the only difference in setting being the amount of time needed. This may have been due to concerns about product liability from unsafe food holding temperatures.
Raw food, and a liquid which is predominantly water, such as water, wine, stock, but not oil without water, are placed in the slow cooker. Some recipes call for pre-heated liquid. The cooker lid is put on and the cooker is switched on. Cookers often have high and low heat thermostat settings. Some cookers automatically switch from cooking to warming (maintaining the temperature at 160°F–165°F,) after a fixed time or after the internal temperature of the food, as determined by a probe, reaches a given goal.
The heating element heats the contents to a steady temperature in the 175–200°F range. The contents are enclosed by the crock and the lid, and attain an essentially constant temperature. The vapor that is produced at this temperature condenses on the lid and returns as liquid.
The liquid transfers heat from the pot walls to its contents, and also distributes flavours. A lid must be used to prevent warm vapor from escaping, taking heat with it, typically faster than it is replaced, thus cooling the contents, perhaps dangerously (see safety below).
Recipes intended for other cooking methods must be modified for slow cookers. Often water must be decreased, as ordinary cooking at higher temperatures requires enough liquid to allow for evaporation, whilst slow cookers prevent vapor loss. Some slow cookers are supplied with recipe booklets; many slow cooker recipes are to be found in cookbooks and on the internet. Some cookbooks provide recipes for making complete dishes in a slow cooker using fewer than five ingredients, while others treat the slow cooker as a serious piece of culinary equipment capable of producing gourmet meals. With some experience, timing and recipe adjustments can be successfully made for many recipes not originally intended for these cookers. The long, moist nature of the cooking method gives good results with cheaper (and tougher) cuts of meat. If excessive liquid is present at the end of cooking, it can be reduced and concentrated by rapid boiling in a saucepan. Even cakes can be prepared in a slow cooker and the internet offers a larger number of free recipes sites.
Some foods do not take well to boiling. In particular, cheaper cuts of meat with connective tissue and lean muscle fibre are suitable for stewing, and tastier than stews using expensive cuts; but if they are cooked for a short time the connective tissue and hard-worked muscles will make them tough and gristly while long boiling will dissolve the connective tissue, but the muscle will be dry and tough Long slow cooking will soften the connective tissue without toughening the muscle. Boiling dissolves the connective tissue and enriches and thickens the cooking liquid; slow cooking leaves the gelatinised tissue in the meat, so that it may be advantageous to start with a richer liquid.
Even with foods that do not suffer from boiling, the temperature in a slow cooker is low enough to avoid badly overcooking food even if cooked for far longer than necessary. However, success does depend to some extent on timing, as in all cooking. Meat will become nearly tasteless or "raggy" if overcooked.
The long cooking time can be an advantage: food can be set to slow-cook before leaving for a day's work, and will be ready on return; an inexpensive timer can also be used if necessary. If on a tariff with cheap night-time electricity, overnight cooking is cheaper.
Cooking the meal in a single pot reduces washing up, and the low cooking temperature and glazed pot make cleaning it very easy.
Vitamins and other trace nutrients are lost, particularly from vegetables, partially by enzyme action during cooking. When vegetables are cooked at higher temperatures these enzymes are rapidly denatured and have less time in which to act during cooking. Since slow cookers work at temperatures well below boiling point and do not rapidly denature enzymes, vegetables tend to lose trace nutrients. Blanched vegetables, having been exposed to very hot water, have already had these enzyme rendered largely ineffective, so a blanching or sauteing pre-cook stage will leave more vitamins intact. Green colors are retained better when vegetables are cooked quickly as plant cells are less likely to lose acids.
Raw kidney beans, and some other beans, contain a toxin, phytohaemagglutinin, which is destroyed by boiling for at least ten minutes, but not by the operating temperature of a slow cooker. Raw beans must be boiled prior to slow cooking to avoid food poisoning; canned beans do not require this, already having been so treated. Even a few beans can be toxic; and beans can be as much as five times more toxic if cooked at 80 °C than if eaten raw, so adequate pre-boiling is vital. Cases of poisoning by slow-cooked beans have been published in the UK; poisoning has occurred in the USA but has not been formally reported.
Slow cookers do not provide sufficient heat to compensate for loss of moisture and heat due to frequent removal of the lid, e.g. to add and remove food in perpetual stews, (pot au feu, olla podrida). Added ingredients must be given time to cook before the food can be eaten. If the food is allowed to cool below about 158°F and not reheated, toxic bacterial growth is possible; some toxins, once present, are not destroyed by later heating.
Slow cookers are less dangerous than ovens or stove tops due to the lower temperatures and closed lids. However, they still contain a considerable quantity of near boiling temperature food and liquid (several pounds or kilograms at minimum). In addition, since they contain heating elements, it is possible to cause a fire if slow cookers are misused. For instance, in slow cookers with two heating elements, one heating element can be out of contact with the liquid due to insufficient liquid or food and so cause local overheating, perhaps cracking the ceramic pot or melting or deforming a metal one.