The present invention relates to a new source of high quality plant protein, termed, "quinoa protein concentrate (QPC)", which contains at least about 50 wt % protein which is food-grade and/or pharmaceutical-grade and methods of preparing such protein concentrates as well as starch, oil, and fiber from quinoa grain. The quinoa protein concentrate of the invention is useful as food ingredients, infant formula ingredients, cosmetic ingredients, pet food ingredients, and animal feed supplements.