First things first: a sweetbread is neither a sweet nor a bread.
If, as a newbie, you order these bad boys expecting some sugary roll, you're going to be real disappointed. If, however, you have a little culinary spirit and are a lover of especially tender cuts of meat, you'll be over the moon.
Sweetbreads are offal meats. Offal refers to those parts of an animal that are used as food but are not skeletal muscle (so, the organs and entrails). The word literally means "off fall," or the parts that fall from a carcass when it is butchered. There are many offal meats that we enjoy here in the states — foie gras, chicken livers, trotters — and copious others with various preparations eaten all over the world.
Sweetbreads are the thymus and pancreatic glands of young veal, lamb, and piglet. These thyroid glands produce the hormone thyroxin, which regulates metabolism. The gland found in the pancreas is rounder and somewhat heart-shaped, so it also goes by the name heart sweetbread. The thymus is an elongated gland in the throat, so it's often called neck or gullet sweetbread. The heart sweetbread is larger, creamier, and more prized for its delicate flavor, so naturally, it costs more and is slightly harder to find on a menu.
Premium sweetbreads come from milk-fed veal or young lamb. You can eat sweetbreads from older beef or pork, and many cultures do, but they are tougher, with an intense musky flavor. They aren't a common menu item in the U.S., so if you're new to the sweetbread scene, don't start there — they aren't gateway offal.
Little gems of sweetbreads are basically a blank canvas, if that canvas were made of velvet. Ted Dombrowski of Ted's Butcherblock in downtown Charleston describes them as having a toothsome bounce, with a rich, creamy texture that takes on flavor easily. With a delicate meatiness and a forgiving nature, these babies are up for any preparation under the sun. In the '90s, it seemed like every fancy restaurant in America was featuring sweetbreads. They were a culinary craze for the affluent, nuggets of cultural capital for upscale eateries to offer guests. But much like chokers and Luke Perry, they lost their trendiness by the early 2000s. You'll still find them on menus from time to time, particularly in French restaurants, where sweetbreads are still reverentially esteemed. (Ris de veau, or veal sweetbreads, are a timeless national dish.) However, now that a growing number of restaurants pride themselves on nose-to-tail eating, sweetbreads are riding a wave of a resurgence.
If you're shopping for sweetbreads to make at home, choose sweetbreads that are plump, firm, and white, since they redden as they age. You probably won't find these at a regular grocery store, but a butcher or specialty store should be able to help you make a good selection. Don't buy sweetbreads unless you plan to cook them within one day — they're so perishable that they must be prepared within 24 hours of purchase.
Soaking sweetbreads in several rounds of milk, buttermilk, or vinegary water is a vital first step in making them; it removes impurities. Because they're so soft, sweetbreads usually need to be blanched for a moment then shocked in cold water to stop the cooking process. This step ensures that you'll have a firmer piece of meat to work with. This is also the time to peel away the thick membrane encasing and running through the sweetbreads, which may leave you with smaller nuggets, but it's worth it.
Sweetbreads can be cooked countless ways — poached, grilled, souffléd. They are so tender, it's nearly impossible to dry them out. The best way to eat them, though, in my opinion, is fried. They get crunchy on the outside but stay sweet and supple inside. The meat is so rich that it's often served with an acidic sauce to cut the creaminess, such as a grenobloise of lemons and capers.
Why are they named sweetbreads? Frustratingly, no one knows for sure. Food historians theorize that they are called "sweet" because they taste richer and sweeter compared to typical meat, and they are "bread" because one Old English synonym for morsel is "bræd." A much-quoted clue about sweetbread's etymology comes from the 1578 work The Historie of Man. Author John Barrister writes, "A certaine Glandulous part, called Thimus, which in Calues...is most pleasaunt to be eaten. I suppose we call it the sweete bread."
The mystery of the sweetbread moniker may be like that of the number of licks it takes to get to the Tootsie Roll center of a Tootsie Pop — the world may never know.