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ELEMENTALLY
The very little fat that you did have, left the brisket, without binding to the collagen molecules.
I know nothing about your cut to start with or your smoker so fat up or fat down for how long I can't say.
HOW THEORIZED (Only My Opinion)
If you ever notice on a .... choosing word carefully... "traditionally" cooked brisket packer, it comes out more tender and is more forgiving. This is due to its mass and amount of fat it has. It also a packer, due to its mass and fat, takes higher temperatures for a period (only the end of the cook after the stall is the leastforgiving). Flats will climb quicker for this reason and therefore you nearly have to follow all the rules (even the typical low and slow under 230 rule) or have to foil it to get it to come close to a packer. Flats are very tempermental this way. I hate it when one of my suppliers (who owns a grocery store) donates flats or prices them at a nutty price I cannot resist for a few reasons. One it is harder to tell more about the cow it came from. Its trimmed, its harder to find if its a lefty or righty, and sometimes its maybe a 6-7 pound FLAT that was trimmed to 4 lbs to match the others (using the cut offs for hamburger). So you just don't know. You can even often cook those puppies perfect and still have 6 or 7 come our tough right next to 40 or 50 that came out tender.
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Also that Fat/collagen bond i was talking about happens easier in a flat when its covered by foil not so much for the steaming as from what I can tell sitting in its juices (which essentially is a bath of fat that will bond with those collagen molecules and let them let go of the meat fibers) How do I know this?????????????????????????????????????????????
I don't foil anything (cept in emergencies) because it is messy when doing 30-40 packers or 50-60 briskets. I PAN which is almost the same BUT I don't cover and flats (in the rare back against the wall case i do this) it still comes out tender and with better bark. The tough ones I will transfer into one or two pans and cover (steel cover) and steam for a few.
Remember, this is only my Opinion about why... listen to everyone and put some thought into it.
