Slow cookers are cooking vessels that braise food in liquid at low heat. The basic design was a ceramic crock heated by an electric heating element. The ceramic slow cooker was inspired by the ceramic beanpot. This article on CNET has pictures and illustrations of old devices. Ceramic beanpots, like Dutch ovens and casseroles cook dry beans in water or broth. Beanpots involved long cooking times at low heat.
The constraints for dry beans are heat and time. The heat source had to heat the pot enough to start the beans cooking slowly, and provide steady low heat. The goal is to cook at a temperature below the boiling point of water (212 F.). The simmering point of water is a culinary term which refers to a range from 185 F. to 205 F. At that temperature, the water has thermal energy and evaporates and bubbles slowly. Only a small amount of water turns to gas, condenses and becomes visible as steam
The food should reach an equilibrium that is will be sustainable for a time. The situation will change when heat is added to the system, too much evaporation has occurred, or the food is cooked.
The basic ceramic crock device has a simple electric heating element surrounding the crock. Elements were like elements in electric ovens and toaster ovens: straight metal, shaped into a circle or oval to surround the crock. Elements in modern machines are ribbon or wire elements in a belt. In some modern machines the element may have insulation. In basic devices the power is turned on to allow a constant electric current produces constant heat stated as in watts. The element heats the crock which heats the food. The heat at the element will be greater than the temperature of the inside surface of the crock. The element may be contolled by a switch or a control panel.
After initial preparation, the device could be left for hours without more work, and the stove or oven were not pouring heat out into the air. The slow cookers made from the 1950s to the early 21st century were low powered and poorly insulated. They could do dry beans, but it took a long time, and might not do beans at all. The ceramic crock slow cooker would cook root vegetables in a few hours; less dense material more quickly. Rival (now a Jarden Brand) began to build and market the Crock-Pot in the 1970’s (by the 1990’s “the Original Slow Cooker”) as a device to cook stew and chili.
Rival and its competitors pushed the standard size of the crock from 5.5 quarts to 6 or 7 quarts. The manufacturers increased the wattage of elements to meet concerns that the device was not cooking the food well enough to be safe and palatable after 8 hours of cooking. Another innovation: the three and four position switch. With the latter the cook can select Off; Warm; Low; High. Warm is not a cooking setting. High means the element runs hotter than low. This article says that 7 hours on low is equivalent to 3 hours on high.
In June 2015, Rival published a statement about Crock-Pots that
- the simmering point of water is 209 F.;
- the contents of a crock should reach that point in 7 to 8 hours on low or 3 to 4 hours on high;
- Food doesn’t need to reach the boiling point for safe eating – the simmer point is acceptable;
- The safe to serve internal temperature is around 160 degrees, which your food may reach well before three hours. Just use your best discretion.
The statement has been removed, but can still be seen in the Wayback Machine archive here. It focuses on food safety – is the food being too hot for bacteria to survive and contaminate the food with bacterial toxins. It does not address the cooking time to make food digestible and palatable.
The ceramic crock pot would cook dry beans. If the heat is constantly applied, the beans will be heated, simmered and gently boiled. Writers (e.g. Anupy Singla, The Indian Slow Cooker; Rick Bayless, Mexican Every Day) suggested several hours on high. Some beans need a long time on high. e.g. chickpeas (garbanzo beans), black urad beans, or red kidney beans. Some recipes incorrectly suggest that dry chickpeas cook in 6 hours on low. There are recipes for pinto beans that work on low, in under 8 hours.
Innovations extended product lines and marketing opportunities; some innovations added some value for consumers. Timers give cooks an option to turn off or turn down the heat. Jarden/Rival had a line of Smart-Cookers with buttons that allowed the user to select 4 or 6 hours on high, or 8 or 10 on low. These are not what a user may want. The Crock-Pot Count-Down timer was more flexible and has been widely emulated.
Books and recipes before 2016/17 assume 5.5-6 quart ceramic crock slow cookers with high and low cooking settings. Culinary writers try to get a stew, chili or curry done in 6 hours or less. Many slow cooker recipes for recommend using canned or cooked beans, because beans take long than any other ingredient. Cook’s Illustrated/America’ Test Kitchen produced three America’s Test Kitchen Slow Cooker Revolution cookbooks 2011-2015. Each discussed the uses and some of limitations of the device, and provided workable techniques and recipes – addressing the ceramic crock slower. Each book had product reviews of a few products. The products tended to work the same way.
The limitations of ceramic crock slow cookers include:
- A 6 or 7 quart crock is heavy;
- The crock could not saute, fry, or roast food. Some ingredients have to be cooked in a skillet or other vessel first to ensure the dish would be fully cooked, or to enhance flavour (bloom spices, heat onions and garlic, brown some ingredients);
- Users must not to use the ceramic crock on stove elements, in hot ovens, or in microwave ovens;
- Manufacturers and culinary writers warn users
- to not lift the lid or stir the food;
- to not add cold ingredients into a hot crock;
- The food near the element gets hot first and is always hotter. Food touching the crock near the element may brown, stick or even burn;
- Manufacturers began to use hotter elements to avoid leaving the food in the temperature range that fostered bacterial growth;
- Ceramic crocks develop cracks and break down. The heat source is in a belt around the lower part of the crock; recipes place variable demand on the device. Manufacturers deflect by blaming users for ignoring warnings and limit their liability to short warranty periods.
- Replacement ceramic inserts are hard to find – out of production, or out of stock;
- The device draws power constantly. It is cheaper and more efficient than using an oven, but not necessarily as efficient as other appliances.
Devices sold as slow cookers or having a slow cooker function, in 2019:
- oval or round vessels with surrounding elements with ceramic cooking vessels or chambers;
- oval or round vessels with surrounding or bottom elements, with metal, coated metal cooking vessels or chambers
- round metal pots with bottom elements (electric pressure cookers and other multicookers).
Slow cooker sizes go up to 8 quarts. There are roasting pans/ovens in the shape and style of slow cookers – quite large compared to slow cookers.
Some modern slow cookers have metal pans, with non-stick or ceramic coatings. Metal pan slow cookers may have the heating element in an aluminum hotplate below the pan – like rice cookers and electric pressure cookers. A rice cooker heats a metal pot of rice and fluid to a boil, and uses automated controls to change the heat to low simmer. An electric pressure cooker brings the contents of the pot to a rolling boil with a hotplate element (e.g. Instant Pots: 1000 watts in 6 quart pots). A pressure cooker heats food and fluid to the boiling point; under pressure the temperature rises higher. The elements in these devices are below at the cooking vessel, and temperature and pressure sensors are outside the inner pot.
Machines with high wattage elements and/or metal pots rely on temperature sensors and programmed controls to prevent the food from overheating. Temperature sensors are typically outside the cooking vessel, and read a temperature at a point on the outside surface. The chip makes progammed calculations that control the current and the read out/display, if any. Usually, the control chip turns the element off when a set temperature is reached, and turns it for short periods on maintain temperature at the point calculated by the manufacturer’s team. The temperature of the contents of the vessel over time should rise and then graph as peaks and troughs along a mean.
Cook’s Illustrated/America’ Test Kitchen The Complete Slow Cooker (2017) recommended modern slow cookers with features including temperature sensors, countdown timers and electronic controls. CI/ATK tested heating performance by heating 4 quarts of water in 6 and 7 quart slow cookers Parts of the tests and results are in a YouTube video and a background story. There is a graph which shows that several devices in their tests will heat the water to 210 F. on high heat in about three hours; other devices take longer. CI/ATK pointed out that many newer machines run too hot to execute the CI/ATK library of slow cooker recipes. They like devices that heat the food to nearly the boiling point in a few hours and stabilize the heat. CI/ATK highly recommended a 6 quart KitchenAid ceramic crock model with a 350 watt belt element, and a Cuisinart model with a coated aluminum pan and a 250 watt hotplate element.
Wattage does not necessarily predict results. A 200-250 watt element is not hot enough to to fry in a metal pot, but heats the food more directly and faster in a metal pot than a ceramic. Ceramic crock machines with lower wattage elements will not heat water to 210 F. in 3 hours on high. Crock-Pot has 370 watts for an 8 quart crock, 240 watts for 6 quart models and 210 watts for 4 quart models. These machines would execute most recipes within the parameters of the recipe books, with a little variation depending on the crock and the contents of the crock. A few hours at low may be enough for soup, stew and chili. Several hours at high will do dry beans.
It is useful to know if device can heat water to 190-210 F., how fast, and on which setting. It is possible to use a waterproof thermometer and a timer. Water will lose heat fast when the lid is removed, so this requires a thermometer that takes a reading fast, planning, time, and care.