There's several major reasons why meat and poultry can be tough:
- Collagen: muscles exist to impart a force between bones, when they contract there must be a connection between that muscle and the bone or the muscle would not be able to do work. Collagen is a strong protein that is distributed throughout a muscle and connects to a tendon, allowing the muscle to distribute its' force to the skeletal structure of the animal. The more load a muscle takes, the more collagen is needed to distribute the force, and the tougher the meat is.
- The animal was stressed before slaughter: if the animal was stressed before slaughter it will have lactic acid buildup resulting in tougher meat
- Poor treatment after slaughter: Once an animal dies chemical changes happen that toughen the meat, and the correct treatment is needed to allow the meat to relax again. If that isn't done right you'll get tough meat
- Freezing and thawing: freezing makes meat and poultry tougher
Collagen breaks down in the presence of heat and moisture, so a long stewing at low temperature could make your chicken tender. The collagen helps make your broth thicker and taster too! That would take a maximum of 2-3 hours on a low simmer. If it isn't tender by then it isn't tough because of collagen and there isn't much you can do. If you've got a chicken that was stressed, not processed correctly, and then frozen there's a limit to how tender you'll be able to get it. You can try cooking it a couple more hours, but if it isn't tender by then it never will be.
It's down to what the chicken ate while it was alive. Saturated fat sets, olive oil sets if you chill it, but not otherwise, and a number of seed oils do not set (rapeseed for instance). When you make a stock which has solidified fat on top, that's saturated fat, so I'd hazard a guess that the stock where the fat doesn't set means a healthier eating chicken, because it contained less saturated fat to start with.
UPDATE: Thank you to the person who bothered to do the research and said my answer 'might have some merit'. Chickens are no different from human beings - the fats you put in are the fats floating round your bloodstream and depositing in various places; think about corn fed chickens, where the fat composition is slightly different, not to mention the colour of the flesh itself. That will be a partial explanation; when taken together with the fact that not all chickens, even in the same flock or brood, get to eat the same diet, because the pecking order dictates that some free range birds don't always get the pick of the food, explains differences in chickens from the same supplier. Of course, if you can come up with another explanation, I'd be delighted to hear it...
UPDATE 2: Perhaps I should have been clearer. I am not for a moment suggesting that the fats eaten are deposited in their original form, but if you know anything about biology (chickens or otherwise) then you'll know that certain synergies occur, depending what's put in, which change the composition of any fats deposited within the body system. Hence the connection between eating lots of saturated fat and having high cholesterol in humans, for example.
UPDATE 3: Rumtscho: Can't find any scientific evidence so far to prove this theory regarding chickens, but, for interest's sake, and to prove how much of a difference it can make, farmed salmon in Britain no longer has a balanced omega 3/6/9 ratio, as it should do, and still does in the wild. It's because the feed had to be changed, and the consequence of that has been a much higher level of omega 6 in particular. I'm still looking for something on chicken.
UPDATE 4: Now I've had time to look properly, it's not at all difficult to find scientific evidence, there's plenty of it. There's a study carried out by The American Society for Nutritional Science in 2000 comparing the fat deposition (and other metabolic processes) between chickens fed the same diet, but one lot with saturated fat included in the form of tallow, and the other lot with polyunsaturated fats. The fat deposition in the birds fed tallow was greater, and the composition of the fat contained more saturates compared to the polyunsaturated group. These results reflect previous studies (Sanz et al 1999 and 2000).
Effectively, it's like everything else - you get back what you put in.
Best Answer
To your specific questions, bone in thighs would work well and you would not have to add extra broth/stock to compensate for not using the breast. In fact, I would prefer the dark meat of the thigh in crock pot cooking as dark meat is very forgiving if over cooked or in long slow cooking applications. I would also wait until the end to cook the noodles so as to avoid overcooking them. If you add them to the crock pot at the end, I would add extra broth/stock at the beginning because the dried noodles will soak up some liquid, maybe an extra cup or so. You can also cook them on the side until just al dente and let them finish in your soup which would require almost no added broth/stock.