Some foods may contain high amounts of essential amino acids while other foods may be low. Examples of high-quality proteins include eggs, beef, fish, and milk. Low-quality protein sources include beans, potatoes, and whole wheat bread. In general, plant sources of protein are not as rich in the amount or variety of amino acids as animal sources, but no food has all the elements you need. We know, now, that such assiduous food combining is not necessary.
All foods have other nutrients in addition to protein, of course.
Some also have qualities that are potentially harmful. Soy, for example, has ample protein, can provide fiber, and contains isoflavones, which help to protect against certain types of cancer and heart disease. Conventional corn fed beef, on the other hand, is a protein source containing high amounts of saturated fat, which can contribute to the development of elevated cholesterol and, over time, coronary artery disease. Whole milk, too, contains saturated fats suitable only for children. Bacon and salami contain protein, but their processing saddles them with carcinogens.