The fat that you need on a brisket is on the inside. That's why a choice or better is best. They will have a lot more fat on the outside. Some folks like to keep fat on the bottom so to have a barrier and not burn the bottom while cooking. By trimming it, it allows the penetration of the rub and helps to form more bark surface. The rub will not penetrate the fat. Everyone does their own thing. You can expirement and see what you like best. I was in Huntsville, TX and visited with the owner at the Church of the Holy Smoke (New Zion BBQ restaurant). He doesn't trim anything off of the brisket there. I have tried that here at home and didn't like it so much, it seemed more fatty than juicy. One of the other things I have tried is to trim all the fat and place the fat bove the brisket in the cooker and cover the fat with rub. That's supposed to baste the brisket with rub flavored fat and keep it moist. I don't think that helped any. I have done better by trimming the fat outside and in.
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