Cooking is one of those topics that you can spend years learning about, becoming ever more skilled, experienced and knowledgeable about what works, why it works, and so on.
You can also become quite a good cook just by following recipes and not thinking too much about what's going on. It's true!
But if you want to move beyond following recipes so you can confidently "wing it" in the kitchen, there are certain concepts that you will want to be familiar with, and one of them is the panade.
What Is a Panade?
A panade is a mixture of bread and milk, combined to form a paste, that is incorporated into ground meats so that they don't get too hard or dry when you cook them.
When you cook ground meats, the proteins contract and squeeze out moisture. You've likely noticed this when you've seen a burger on the grill shrink and change its shape during cooking.
Starches, on the other hand, absorb liquid when they're cooked, so they expand, and undergo a change called gelatinization. Think about what a bowl of oatmeal does if you let it sit for too long—it goes from being a thick but still pourable liquid into a jiggly, semi-solid, bowl-shaped mass. That's gelatinization of starches.
So when you combine a panade with ground meats, the starches interfere with the tendency of the proteins in the meat to curl up when cooked. Meanwhile, the gelatinization, which is caused by the starches absorbing moisture, helps ensure that the final product isn't too dry. It's basically like adding little milk-soaked sponges to your ground meat.
Making a Panade
Making a panade involves chopping (or food-processing) your bread into crumbs, adding milk, and letting it sit for 10 minutes before mashing the mixture into a paste.
The ratio is about 1/2 cup of bread crumbs (the equivalent of one slice of bread) to two tablespoons of milk. This amount of panade is enough for 1/2 pound of ground meat. Some cooks add an egg yolk—this is often the case with meatloaf recipes. It helps with the overall bonding of the starches with the protein.
Any type of bread will work to make a panade, from plain supermarket white to a whole grain artisanal loaf. Leftover garlic bread would be wonderful and flavorful. You can leave the crusts on or cut them off. Remember, you're mashing it all up, so the crusts will end up absorbing milk, too.
You could use panko breadcrumbs, but the whole point of panko is to make something crunchy, whereas bread is already soft and absorbent. So any type of bread is probably better than panko.
And yes, the protein in the milk helps to bond the starches to the proteins in the meat, so don't substitute water. On the other hand, using buttermilk will add a lovely flavor. You could also use plain yogurt (in which case you might want to thin it out with a bit of water), half and half, or heavy cream.
Using a Panade
To use a panade, gently fold the mixture into your ground meat before shaping it into meatballs or transferring it to your loaf pan if you're making meatloaf.
However, using a panade will not automatically prevent your meatballs and loaves from turning out hard. It is still extremely important that when you form your meatballs or loaves, you do so gently. Don't pack the balls too tightly or press the meat too tightly into the loaf pan, or it will still end up rock hard.
When Not to Use a Panade
One case where you could, but probably should not use a panade, is when making burgers. The reason for using a panade with meatballs and meatloaf is that meatloaf is cooked for a long time, and meatballs are often cooked in the oven and then simmered in sauce. Both of these cooking methods are long, so the meat will end up hard and dry without a panade.
A burger, on the other hand, is cooked quickly—that is, as quickly as humanly possible, while still achieving proper doneness. But even if you cook them welldone (and you probably should), your burgers will cook for about 10 minutes, not an hour as meatloaf does.
The key with burgers, then, is to treat them gently. Again, don't squeeze the burgers into tight balls or compress them into hockey pucks. Gently shape them into rounds and cook them quickly over high heat. You won't need a panade. In fact, adding a panade to hamburger meat will only contribute to a mushy mouthfeel, which is the opposite from the juicy, meaty bite most people expect from a burger.