This is probably too obvious, but did you peel them? The skin seems to produce bitterness when cooked, although I've eaten carrots straight from the ground which tasted very good.
I've had bad luck cooking those tiny carrots that come in a bag. There's a mechanized way they are peeled, and they don't seem to get all the skin off, so you can get that tart carrotskin taste from cooked ones.
re: Bill F.
They had an unhappy childhood.
Actually, I'm not being flip. Underwatering when growing is said to cause bitter carrots.
Also, heard you should avoid carrots with green tops (not the leaves, the carrot itself). In this case the cause could be too much water, heavy rains that uncovered the top of the carrot exposing it to sunlight.
Large or older carrots can have a slightly bitter taste. Supposedly removing the bitter core after slicing lengthwise should help.
Some varieties are more bitter than others. Some high carotene carrots can have alot of turpenoids which can make some carrots strong tasting and bitter
Storage is a big problem too. Even companies that can carrots have this problem. Carrots absorb odors from apples, pears or any fruit that produces ethylene gas which makes carrots bitter.
Cut the tops off carrots, put them in a plastic bag in the vegetable crisper in your refrigerator away from apples and pears ... etc.
Although not fool proof because of growing or storage problems:
- Smaller carrots with the best color are usually the sweetest and most tender.
- Besides removing the top, eat bunch-type carrot as soon as possible The leaves suck water and sugars from the root.
Other than trying to remove a bitter core, I don't know if there's any cooking trick to sweeten a bitter carrot.
If you are interested in more scietific stuff (this is a big research topic), here's a link to more than you probably want to know