To make a Brisket tender the internal temperature has to climb fairly high to 180° F to 205° F (82° C to 96° C). The trick is to use indirect heat to get the internal temperature up high enough to break down the Connective Tissue into soft Gelatin, without drying out the meat. The conversion of Collagen in connective tissue into gelatin does not even begin until an internal temperature of 140° F (60° C) is achieved and the conversion is most efficient as internal temperatures approach 212° F (100° C).
As a very rough rule of thumb, figure 1 1/2 hours per pound of Brisket for a whole, 8 to 12 pound (3.62 kg to 5.44 kg), Packer Brisket (a single, whole Brisket sold in its Cryovac “pack”) trimmed of excess fat. Be forewarned that this is a rule of thumb, only. Actual cook times will depend on Brisket thickness, density of connective tissue, actual oven, grill or smoker temperatures, weather and even how often you open the oven door to turn, baste or check the Brisket.
Authors Chris Schlesinger and John Willoughby say in their book How to Cook Meat, that “tougher cuts of meat should be ’Cooked Through’ doneness to tenderness.” What they mean is that traditional notions of “doneness” like Rare or Medium Well don’t apply to tougher cuts which just are not tender or palatable at 125° F (52° C).
"Low and slow" cooking at 225-270° F (107° C to 132° C ) over many hours is the preferred method to convert collagen in the connective tissue to gelatin. The long exposure to heat will dry out the Brisket some but the gelatin will compensate as will frequent Basting. The goal is to create a Brisket that is “Fork Tender,” without overcooking it. Fork Tender means that a fork, knife or Meat Thermometer will easily slide in and out of the meat with little resistance. The best way to avoid overcooking is to use the meat thermometer and pull the Brisket from the heat when it reaches your preferred internal temperature. Preference is personal but the range of temperatures is again, 180° F to 205° F (82° C to 96° C) with 188° F and 190° F (87° C to 88° C) being very popular Final Cooking Temperatures. Be aware that measuring internal temperatures in the Point of the Brisket may be inaccurate because of all of the internal fat and connective tissue. Measuring temperature inside the Flat of the Brisket is a better bet. Insert the meat thermometer from the side and not from the top to get the best reading.
If your Brisket appears to be stalling in its temperature climb, don’t worry. That is common and is called the Temperature Plateau. The lack of progress can last several hours but with some patience and good heat, it should begin climbing again eventually.
An overcooked slice of Brisket will fall apart when handled or moved. A properly cooked slice should pull apart easily while maintaining good texture and Mouth Feel. If the meat is too tender you can slice it thicker to help it hold together.
If you have a very large Brisket on the heat and are running out of time (or patience), you can turn up the cooker up to as high as 275° F (135° C) without hurting anything. Alternatively, if the Brisket has gotten up to at least 160° F to 175° F (71° C to 79° C) you can foil wrap it and finish it at 300° F (149° C ) to your desired Final Cooking Temperature. Once you have achieved your desired Final Cooking Temperature the Brisket should rest so that Carry Over Cooking can redistribute the juices and finish the meat.