Xjyuk

I have exactly the same procedure and ingredients for cooking vegetable and chicken stock - of course, the latter also contains chicken meat, which is the only difference. So things I put into my stock are - onion, few garlic cloves, two carrots, celeriac, parsley root and leek. Also 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.

So my question is, why there's a risk of vegetable stock becoming bitter, while it is not that much of a problem for a meat stock?

I notice that your recipe doesn't include any salt. That's important, because salt decreases the sensation of bitterness. Chicken contains a certain amount of salt, and I suspect that's making the difference. (The "umami" -- brothy -- taste of chicken may also decrease the sensation of bitterness, though as I understand it there's still some disagreement about that.)

Try mixing 1/8 tsp of salt into one cup of your vegetable stock as a test. I suspect that'll decrease the bitterness to a comparable level.

Oh, and if you want to make your stock less bitter without making it more salty, use parsley stems and leaves instead of parsley root, and celery (including leaves) instead of celeriac. Those two roots will be the primary sources of bitterness.

Incidentally, I very much approve of you not salting your original stock, and instead salting whatever you use it in. Unsalted stock is more flexible, and is more forgiving if you decide you need to concentrate it.

I make veg stock overnight in a slow cooker on high with similar ingredients to you: onion, garlic, carrot, bay, peppercorns. But: celery instead of celeriac (I grow celery and often have some old tough stems and leaves which are perfect for stock), rarely parsnip or leek, and often some other herbs or veg I've got to hand. I don't add salt, and my quantities are a bit random, but I don't have problems with bitterness. The slow cooker maintains a very gentle simmer.

I also don't brown the ingredients first, but the bits that stick out start to caramelise by the end. I've never had trouble with bitterness, and wonder if your garlic, onion or leek may be catching a little, if you fry them first or if they end up stuck to the bottom of the pan.