The "Beef well done" temps you are speaking of are "food safe" temps, and does not equate to tenderness. Tenderness is more a function of the piece of meat being cooked AND how it was cooked AND what temp it was cooked to.
What makes raw meat tough is connective tissue. The more used a piece of muscle is used by the cattle, then the tougher it is when it is raw because of all of the tough connective tissue. Little used muscles with very little connective tissue are tender even when raw.
Thin sliced beef tenderloin is tender even when raw, because it is jsut muscle with a little bit of fat marbling and very little connective tissue. When you cook it, you are actually making it tougher because as the proteins in the meat are being heated, they coil up and intertwine together. Since a tenderloin is moslty muscle with little fat and connective tissue, this makes for a big tangled mess of proteins that are tough and chewy if you overcook it. This is why these are cooked to lower temps like 165, where it is "food safe", but not overcooked to the point of being tough.
A brisket is tough as nails when raw however, because it is loaded with connective tissue. This is a heavily used muscle by cattle. Cooking it to food safe temps of 165 actually makes it even tougher than when it started because not only is it still tough from all the connective tissue, but you just bound up all those proteins making additonal toughness.
The thing about connective tissues is, when they are heated over an extended time in the poresence of moisture (the moisture in the meat in this case), then the collagen (which is what these connective tissues are made of) breaks down through a chemical reaction caused by the heat+water against this collagen, and it converts from collagen to gelatin. Gelatin as we all know is very, very soft and tender. So converting the tough collagen in a brisket to gelatin will make a much more tender product.
Collagen converts to gelatin at temps higher than 165, so you have to cook the meat past these temps for this process to occur. You will know when this process is occurring in your brisket because this is when you hit the "stall". The "stall" is when your brisket temps stop climbing from when you first started cooking, and stay at this temp for maybe an hour or so before starting to rise again. The reason for this stall is because the heat is being used to convert the collagen to gelatin instead of simply cooking the meat. This is why there is a stall.
Once you are out of the stall, this does not mean the collagen has fully converted yet. It just means that "enough" of the collagen has converted to cause temps to rise again. The brisket is not normally tender until it gets over 190 degrees, but temp is not the true guide to determine when a brisket is done. This is because no two briskets have the exact same amount of collagen. There is a sweet spot where the amount of collagen in the brisket has converted to cause it to be tender. this is not caused by a specific temp, but by the amount of collagen that has been converted. So the brisket might be tender at 195, or 205. Whichever temp it is at when that sweet spot is reached in terms of how much collagen is left. The best way to tell is to probe the brisket and see how tender it is.
I hope this helps.
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