Slow cookers are designed to cook for several hours at a time and heat food properly, but you should still practice some safety precautions.
Never fill the stoneware container more than two-thirds full, and keep the lid on throughout the cooking process to maintain ideal cooking conditions inside the container. Periodically, you should test the cooker to make sure the unit heats correctly and is able to cook food to a proper serving temperature. Food should cook to at least 140 degrees Fahrenheit (60 degrees Celsius) within four hours to avoid harboring bacteria.
To test your slow cooker, fill it one-half to two-thirds with water, cover it with the lid, and cook on the low setting for eight hours. Then, use a food thermometer to monitor the water temperature before it cools. If the temperature is 185 degrees Fahrenheit (85 degrees Celsius) or higher, the slow cooker is safe to use. A lower temperature may indicate that the heating element isn't functioning well enough to cook food thoroughly [source: Peterson-Vangsness, Glenyce].
Slow cookers run off a low wattage, so it's safe to leave the house while it's on. Although the base does heat up, the product is designed to not get so hot that it'll set a countertop on fire.
Never immerse the base of a slow cooker in water. If your slow cooker doesn't have a removable insert, use a soapy sponge to clean the inside and wipe away any spills.
Temperature extremes can cause the stoneware insert to crack, so you shouldn't freeze it or use it over direct heat, like a stovetop. Also, never put a hot crock on a cold counter -- use a hot pad to protect the ceramic. Likewise, if the pot is still hot, don't pour cold water into it.
Thaw meat and poultry thoroughly before adding them to the cooker. Before serving, use a thermometer to make sure they're hot enough.
When using beans in recipes, remember that you can't use dry beans in a slow cooker. Dry beans, especially dry red kidney beans, contain a toxin called Phytohaemagglutinin, which can cause nausea, vomiting, diarrhea and abdominal pain. Soaking and boiling dry beans helps remove this toxin to make beans safe to eat -- in a slow cooker, the temperature never gets hot enough to heat up dry beans sufficiently [source: U.S. Food and Drug Administration]. If you're using dry beans for your recipe, first soak them for a minimum of 12 hours, rinse them, and then boil them for 10 minutes. Another safe option is to use canned beans, which have been boiled prior to the canning process.
You should use a slow cooker only for cooking -- never for reheating. Any leftovers should be stored in shallow containers and refrigerated within two hours of the meal. To reheat, use another cooking method, such as a stovetop or microwave, to reheat the food to a temperature of 165 degrees Fahrenheit (74 degrees Celsius).