Sometimes it's easy to know when your food is done. A steak is ready when it's the shade of pink you prefer. An egg is ready when you like the look of the yolk, and a cake is done when the middle is set and the edges start to pull away from the pan. Other foods make it a bit more difficult to judge, and beef brisket is definitely one of those.
Brisket "Done" Temp Is Variable
The USDA has a pretty straightforward approach to doneness in meats. It's based on food safety, and it's conservative enough to allow for sloppiness on the consumer's part, but has the virtue of being simple and easy to understand. Non-ground red meats, like a big hunk of brisket, are considered to be done once they reach a temperature of 145 degrees Fahrenheit.
That's a functional definition of doneness, as far as it goes, but it doesn't take into account the meat itself. If you're cooking up a tender ribeye or T-bone, for example, you might prefer to cook it to rare or medium-rare. Those are below the USDA's standard of doneness for maximum food safety, but they're juicier and tastier that way. Tough cuts, like brisket, are the opposite. Because of how they're constructed, they need to cook longer and finish at a higher temperature.
A Bit of Basic Meat Science
One of the fundamental truths about cuts of meat is that the more used a muscle is, the tougher it gets. The loin muscle just generally holds the backbone in place, so it's very tender. The shoulder and pectoral muscles, on the other hand, get used a lot as the steer gets up, lies down and moves around while grazing. The brisket is what we call the pectoral muscles after the animal is butchered, and it's a prime example of a tough, well-used cut.
It's made up of very long, strong muscle fibers, tightly woven with connective tissue called collagen. To break down that slab of chewy, leathery beef, the usual technique is to cook it for a long, long time at a low temperature. This does two things. For one, it breaks down the proteins that bind those muscle fibers together and make them chewy. As the brisket's internal temperature climbs, the collagen also starts to break down. It gradually melts into gelatin, which moistens the meat and keeps it juicy and tasty when sliced.
Your Finished Temperature
The connective tissue in your piece of brisket doesn't start to dissolve until it reaches temperatures in the range of 180 F to 190 F and ideally, you need to keep it in that range for a couple of hours to get a seriously tender brisket. That's why most brisket-cooking methods call for low temperatures – to prolong that period when the collagen gets lush and melt-y.
The brisket is completely cooked when it reaches a temperature somewhere between 195 F and 205 F. After that point, it begins to get crumbly and to dry out. The exact temperature that gives a "perfect" brisket is hotly debated by cooks and barbecue enthusiasts everywhere, so you may want to try a few different temperatures and see which one you like best.
The Fork Test
If you don't have a meat thermometer or an instant-read thermometer, or if you want to learn to cook "by eye" instead of by instruments, there's a simple and lower-tech test. Pick a spot close to one edge of your brisket and stick a fork in it. If you can easily twist away a piece of beef, and it's tender and easy to chew when you pop it in your mouth, the brisket is done. If it fights back, let it go a while longer.
Brisket Cooking Methods
The total time you invest in cooking your brisket depends on your cooking method. For pot roast, or for corned beef – which is usually made by salting a brisket – that typically means braising or simmering. You'll cook the brisket in a slow cooker, roast pan or Dutch oven in some kind of liquid, usually broth or wine for a pot roast or well-spiced water for corned beef. In the oven, it'll take 2 to 4 hours for your brisket to become tender, depending on how large a piece you're working with. In a slow cooker, it might be 3 to 4 hours on high, or twice that on low.
If you're smoking your brisket, plan on it taking a lot longer. In a smoker, a large piece of brisket could easily take 8 to 10 hours and sometimes more.