This extremely important ingredient in Asian cooking is a dark, salty sauce made by fermenting boiled soybeans and roasted wheat or barley. Although there is essentially one main type of soy sauce widely made in the United States, China and Japan produce a number of varieties ranging in color from light to dark and in texture from thin to very thick. In general, light soy sauce is thinner and saltier than its dark counterpart. Its flavor and color is also lighter and it may be used in dishes without darkening them. Dark soy sauce is slightly thicker than light soy sauce but generally not as salty. It has a richer flavor and color. Soy Sauce often contains Monosodium Glutimate (MSG), to which many people have an adverse reaction to.