Thawing food before you cook it is time consuming, and it requires a degree of planning ahead that isn't always going to happen. Often, you can just skip that part of the process and cook your food from a frozen state. It increases your cooking time by about 50 percent, but it's convenient. It works with small items, such as cut-up chicken pieces, and quick cooking methods, such as roasting. You should never cook from frozen in your Crock-Pot.
Don't Put Frozen Meat In a Crock-Pot
Your slow cooker may be your favorite appliance ever and the most-used thing on your kitchen counter, but it's not good at everything. If you were putting together a list of things it's not good at and should never be used for, defrosting meats should be right at the top of that list. The slow, gentle heat that makes your Crock-Pot such a wonderful cooking tool is exactly the same feature that makes it dangerous for thawing.
A Question of Time and Temperature
A large part of basic food safety boils down to questions of time and temperature. Potentially harmful bacteria, or pathogens, are pretty much like any other living things: They're not very concerned about making babies when they're struggling to survive. They only really flourish at temperatures between 40 degrees Fahrenheit and 140 degrees. Below 40 degrees they're cold and sluggish, and above 140 degrees they begin to die off. The trick is to keep them out of that range of temperatures as much as possible.
The longer they're between those temperatures, the more they reproduce and the more bacteria build happy lives and large families in your food. The frozen chicken in your slow cooker probably has some bacteria present already, and the longer they stay in the danger zone, the more bacteria you'll eat. That's important because for every kind of bacteria there's a certain population — called the infective dose — needed to make you sick. It can be as few as 10 bacteria or several millions, but the more time you give them, the higher the risk of illness.
Frozen Chicken in a Crock-Pot
When you start frozen chicken in your slow cooker, the outer surfaces of those pieces will reach a pleasantly warm temperature long before the inside begins to thaw. The outer surfaces are exactly where most bacteria are found, so at this point, you're actually giving the undesirable bacteria ideal growing conditions. It's like giving your dandelions fertilizer and a grow light. Some bacteria are more heat tolerant than others, and some produce heat-stable toxins, so your chicken can make you sick even if it reaches what's normally considered a food-safe temperature.
Doing it the Right Way
The best way to cook chicken legs in a slow cooker, or any other chicken pieces for that matter, is to thaw them first using a food-safe method. This could be overnight in the fridge, for an hour or two in cold water or just a few minutes in the microwave. The chicken pieces can go into your pot once they're fully thawed, or you can take the extra step of browning them first in a hot skillet. That shortens the cooking time, and browning makes your chicken taste better as well. Most recipes suggest starting your Crock-Pot on its high setting and then switching to low, which also helps the chicken reach a food-safe temperature quickly.