OK, made another beef stew in the crock pot and again, it has a bitter flavor. No tomatoes, only 2 T canned tomato paste and 1 C cabernet. Thinking maybe a lighter wine to fix this and/or eliminate the tomato paste? Add brown sugar?
It's the tomato paste. Many brands of tomato paste can have a bitter, almost metallic flavor if it isn't fried off first. I don't use it in crock pot recipes for that very reason unless I saute it in some oil for a minute before adding it. You can add some sugar but that doesn't counteract the bitterness.
Also you are adding too much of it, 2 tbsp for 1 cup of wine is very, very tomato pastey for lack of a better term. You could try half that and see how you get.
So try a bit less paste, fried off first or use a good thick canned tomato sauce instead.
Hmm, what are other veggies you ad? Or only canned tomatoes? I usually add a spoonful (teaspoon) of sugar to around 1 cup of tomato sauce. Sometimes less when using sweet peppers. You can try to salvage with adding sugar (just slowly). If you oversweet use vinegar to balance the taste. Cabernet is definitely a wine you should balance with something sweet as it have a lot of tannins. If I use any wine I just macerate the meat in it beforehand and not add during process.
Totally agree with GdG about too much tomato paste. But I'd also say cabernet is too heavy a wine. And what else are you putting in there? A regular beef stew would have a lot of onions, which sweeten the pot a lot, especially if you pre-saute them as you really should to develop taste. Carrots are another traditional sweet vegetable. I think you may be tasting bitterness more because of a lack of other flavors?
If you like tomatoes and beef in the slow cooker use V8 juice. I think it is a little much for drinking straight, but it is awesome for slow cooking stew and chili.
Even if you use high end good paste, pure wine as the only cooking fluid is a lot and you might get bitterness just from that. Beef bourguignon is the classic and it uses half and half wine and beef stock. You could go half and half wine and V8 and then you will have some wine left to drink while you wait.