Marinades are used for a variety of reasons in cooking. Different ingredients accomplish different things when used as a marinade. The most common reasons to marinate something are to add flavor or make a piece of meat juicy. Marinades also might be used to preserve something, such as vegetables, or to add more flavor to a dish by marinating it after it has been cooked. There are many ways to marinate a food item, but a couple of common ones are with an acid-based marinade, such as vinegar, or with a dry marinade, made with a variety of spices.
The benefit of using a mixture of ingredients to marinate a protein is that it creates a balance of several flavors that work to enhance the flavor rather than cover it up. A mixture of red wine, spices, soy sauce, herbs and sugar might compliment a grilled steak wonderfully. Mixing soy sauce, lemon juice, vinegar, sugar, onions and garlic together to marinate chicken can enhance the flavor tremendously. Using a mixture of ingredients to marinate meat works in the much the same way as sprinkling something with salt and pepper, but it more evenly distributes the flavors.
Vegetables can be marinated as well. Many people do this to preserve the vegetables in their fridge for just a little longer. Vinegar and olive oil are great preservatives, and they work well to preserve the natural flavor and freshness of the vegetable when they are used as a marinade. The vegetables might then be steamed or even cooked in the marinade.
Another way to marinate is to use a dry marinade. One of the greatest benefits of using a dry marinade to marinate meat is that it allows an individual to flavor his or her meat without the use of the fat that normally is present in a wet marinade. A dry marinade is made using a variety of spices and herbs, and it is a great way to add some heat to a dish in a controlled way.
One common misconception is that marinating a piece of meat flavors the meat entirely or tenderizes it. Several food scientists tested this theory and found that using a marinade on meat did not flavor the entire piece of meat or tenderize it. The flavor does coat the outside surface of a piece of meat if you marinate it, which generally is much harder to accomplish if the meat is merely seasoned with a few spices.