Some soup recipes want the soup to cook on the stove for a few hours. Most of the time it is not a problem for me to do that if I am home. However, sometimes I have to leave the house for a few hours and I do not like leaving with the stove on. To me, crock pots seem safer for that. I am not asking about the safety of leaving the house with the stove on though. I want to know what setting to put the crock pot on if the soup recipe says to simmer on low (referring to the stove).
It depends on what kind of settings your crock pot has. A low simmer on the stove is probably equivalent to something fairly low on your slow cooker, though. If all you have is warm, low, and high, you want low - in some cases it might be too hot, though. You're aiming for just short of boiling, so a setting that gets you a few bubbles now and then is good. More than that and you're cooking a bit vigorously, and on a "warm" setting you likely aren't heating it enough to cook properly and stay safe. If you have some kind of continuous settings, you can possibly adjust to get exactly where you want!
An alternative to the crock pot is the oven: preheat your oven to 200F/90C; meanwhile, assemble the soup in a heavy oven-proof pot (no plastic handles!), bring to a simmer on the stove, then cover and bake.
There are two reasons you may want to boil soup:
- To hold the ingredients at the liquid's boiling point for a period of time, so that they get cooked
- To reduce the soup - that is to thicken it by removing water in the form of steam
You're not likely to be able to reduce soup effectively in a crock pot. For one thing, they are only likely to bring the liquid to a very gentle boil -- usually to reduce soup we bring it to a full rolling boil, to lose as much water as possible. For another thing, crock pots are designed to be run with the lid on, so very little steam gets lost.
Crock pots are brilliant, however, for holding food at boiling point. Start it on high; as soon as you see bubbles, bump it down to low. If you're going out, just set it to low - it'll reach boiling point eventually.
If you usually add water to the soup, use slightly less when using a crock pot, because less of it will boil away.
The discussion about boiling and reduction is all perfectly true, but the OP asked about simmering, not boiling.– Cascabel ♦Dec 20, 2011 at 21:53
A simmer is a just a very gentle boil.– slimDec 21, 2011 at 10:31
1(1) Not exactly - a temperature just below boiling is a simmer, and while a few bubbles are fine, they're coming from the bottom of the pot, and the rest of the liquid isn't boiling - see for example the second sentence of the first paragraph of wikipedia on simmering, and (2) a gentle boil won't reduce anything, so the discussion about that and full rolling boils is extraneous either way.– Cascabel ♦Dec 21, 2011 at 13:22