The quality of protein is normally classified in one of two ways.The first method is called the protein efficiency ratio (PER). It is determined via evaluation of the weight gain of growing rats fed a specific protein in comparison to a standard protein (egg whites).The second method is called the protein digestibility-corrected amino acid score (PDCAAS).This method is acknowledged globally as the best method of comparing protein sources for humans.In both cases, the higher the value, the higher the quality of that protein. That being said, here are a couple of great food sources of protein!Protein from egg whites is considered the reference standard for comparing protein quality. Countless studies have evaluated the effects of egg protein on adaptations to training in comparison to other sources. Results of these studies by and large point out that egg protein is as effective as whey protein in rebuilding damaged tissue. A testament to this is that it’s PER is 2.8 (remarkably high) and its PDCAAS is 1.00 (which indicates that the protein exceeds the requirements of the body).Another great food source is soy. Despite lacking the amino acid methionine, soy is an excellent source of protein extracted from soybeans. Depending on the particular form of soy, the PER can range from 1.8 to 2.3, while it’s PDCAAS is 1.00.Another excellent food source of protein is milk. One cup of milk provides about 8 grams of protein. Of this protein, roughly 80% is casein and the remainder is whey protein. Milk serves as a legitimately ideal source of essential and conditionally essential amino acids. Therefore, milk has a comparatively high protein efficiency ratio of about 2.8 (whey protein is near 3.2). Milk, along with soy foodstuffs and eggs, are probably amongst the cheapest of foods with remarkably high levels of protein. Each of these sources has a PDCAAS of 1.00.