Soyabean is the richest plant source of protein. It contains 43 percent protein as compared to other legumes which contain 20 to 25 percent protein. Soy protein is also of the highest quality amongst all legumes. Under guidelines adopted by the Food and Drug Administration and the World Health Organization for evaluating protein quality for children and adults, soy protein isolate receives a rating of 1, which is the highest possible score. This means that the quality of soy protein is equal to that of meat and milk proteins.