I love making soups in my crockpot. SO, I make it, starting with the crock on High. After about an hour, I drop it to low (middle setting), and let it cook. (I always cook my meats outside the crock, usually in a wok or skillet, to make sure it is cooked thoroughly.)
Once I am confident that it is fully cooked, I turn the soup down to Keep Warm (the lowest setting), and we enjoy a hearty bowl of soup, and have soup ready for tomorrow. I believe it keeps the temp above 100(edit: actually 165F), and probably more like 120F. Well, after two days, it becomes a burnt broth. The top layer of solids forms a crust on top of the broth, and everything under this crust is still good. Is there anything I can do to preserve it, because we don't eat enough to use it up in two days?
Edit: 2016/11/21 Found out the "Keep Warm" setting holds the temp to 165F. Also, the other two settings, "Low" and "High" actually bring the temp of the crockpot to the same temp, but the "Low" just slows the time it takes to reach the "High" setting temp.
BTW, I made a chili I call "9 Furz" in the crockpot yesterday. It won third place in a contest, because the judges thought my pork loin was white meat chicken. (The judges are firm believers that chicken in chili is blasphemous.) But, the non-judges loved it, because it was the first crock to empty out. (Next year, I will write on the card that it is "PORK NOT CHICKEN!"
Best Answer
A crockpot's keep warm setting is not designed to keep food safe and fresh for days, it's really only good for an hour or two at most. What's happening in your case is that the water is evaporating from your food and then drying out inside the pot. Adding water periodically is not an answer as the temperature of the food is not high enough to prevent foodborne illnesses, you don't want to keep it on warm for long periods of time even if you can keep it from burning.
If you want to save your soup for later put it in containers and store it in the refrigerator or freezer, re-heating when you want to eat it.