A marinade is a sauce which is designed to flavor and tenderize meats. Marinades can also be used to flavor vegetables, especially harder vegetables like eggplant, zucchini, and mushrooms. Typically, food is soaked in a marinade for at least several hours and sometimes several days before it is cooked. As it soaks, the marinated food absorbs the flavors of the marinade, and when it is cooked, the food will be more flavorful and complex as a result. Many markets sell packaged marinades, and it is also easy to devise your own at home, especially with the assistance of a cookbook which provides options and suggestions.
The concept of marinating food is quite ancient, and many cultures have some tradition of marination. Meats have classically been marinated because they can be stringy and tough, and a marinade will tenderize the meat and improve the flavor. Many cultures have a tradition of eating meats from older animals, like mutton, which comes from adult sheep, and these meats would be rather unpleasant if they were not marinated and slowly cooked.
When a marinade is designed as a tenderizer, it includes an acid like vinegar or lemon juice. The acid attacks the fibrous connective tissue between the muscles, softening it so that the meat will be less stringy. As the acid penetrates the meat, it can also carry flavorings along with with it. Obviously, the longer the meat marinates, the softer it will be when cooked, although there is a point when the meat will turn to mush, making it useless.
Flavorings in marinades vary widely. They can be spicy, smoky, sweet, bitter, and sour. They can utilize herbs, spices, fruit juices, vegetables like garlic and onions, soy sauce, yogurt, wine, sugar, chocolate, and a wide assortment of other ingredients. A very basic marinade might include vinegar, olive oil, herbs, salt, and pepper, and this base can be endlessly revised and updated, replacing vinegar with orange juice, for example, and adding a hint of brown sugar or molasses for a sweet, dark flavor.
Once food has been marinated, it can be cooked in a variety of ways. Many people like to grill marinated foods, but they can also be baked, roasted, broiled, fried, or stewed. Older meats tend to benefit from a slow, gentle stewing which helps to break down the fibrous tissue of the meat even more, while cuts of chicken and steak can be quite excellent when they are cooked on a grill.