I recently read that cooking stock for more than about 2 hours negatively affects the flavor, and eventually vitiates the vitamins due to heat degradation.
This seems to conflict with advice I've read previously, which states that you need to do a "shimmering only" simmer (no bubbles) for 24-36 hours in order to extract/denature all of the collagen.
My questions are:
Is it true that extended simmering destroys flavor and nutrients, and if so, is it due to the time, temperature, or some combination of the two?
Which advice is better to follow? What are the optimal simmering times for the meat and vegetables?
If the simmering time for vegetables is shorter - at what stages should I add (and remove) them?