Sweetbreads (offal) are the thymus glands and/or pancreas of calves, lambs and piglets under one year old.
In some parts of the country, people call all "innards" such as chicken gizzards, giblets and livers "sweetbreads."
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They are the thymus or pancreas of a calf or lamb when used for food
Sweetbreads are the thymus and pancreas glands of animals. There are two kinds of sweetbreads: (1) stomach sweetbreads (also known as heart or belly sweetbreads), which are an animal's pancreas. (2) neck (throat or gullet) sweetbreads, an animal's thymus gland. Such foods, along with other internal organs are called "Offal," meaning, literally, the "off-fall" or off-cuts from the carcass; many call these items "variety meats. Up until the time that America starting enjoying the luxury of large supermarkets (mid-1940s), people would butcher their own cattle for consumption. As times were hard and money was scarce, nothing was wasted. This included all parts of the animal butchered. Everything was used and eaten by the family. Now days, these foods are considered a delicacy by the people who enjoy them.
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