I have exactly the same procedure and ingredients for cooking vegetable and chicken stock - of course, the latter contains chicken meat, which is the only difference.
The ingredients I put into my stock are:
- a few garlic cloves,
- two carrots,
- parsley root,
- spices: bay leaf, allspice and peppercorns.
Simmer time - about 2 hours.
It quite often happens that my vegetable stock is bitter, but it never happened with a chicken stock. I read here and there that vegetable stock shouldn't be cooked for long - even 45 minutes should be enough, and if simmered for too long it may become bitter. However, chicken or any other meat stock recipes call for a much longer time, and bitterness should not be a problem.
Thus, why is there a risk of vegetable stock becoming bitter, while it is not that much of a problem for a meat stock?