Protein quality can be thought of in several aspects. It can measure the rate at which protein is absorbed into the body, or the ratio of the amounts of nitrogen utilized by the body to the amounts excreted. It could determine the amount of essential amino acids—those amino acids that the body cannot produce—in the food, or how easily the protein is digested. There are multiple methods to determine protein quality, all of them with pros and cons.
The biological value, or BV, test examines nitrogen balance. This relates to the body’s ability to digest, absorb and excrete given proteins, which are the body’s source of nitrogen. BV testing is a lengthy and somewhat expensive process that requires test subjects to fast for several days and then adhere to a strict diet containing protein in only the form being tested. Their urine and feces are tested for nitrogen levels after fasting and while on the diet. To determine the biological value, the nitrogen levels are compared to that of the whole food. However, both exercise and a protein-deficient diet, or fasting, will promote nitrogen retention, which could compromise the results.
Chemical Score compares essential amino acid, or EAA, levels to measure protein quality. The EAA profile of protein being tested is compared to a reference protein that has been assigned a score of one hundred. The EAA in lowest quantity relative to the reference protein is determined to be the limiting amino acid. This limiting amino acid generally signifies the ability of the protein to fulfill a human’s nutritional requirements, according to "The Journal of Nutrition." However, this method does not address digestibility.
Protein Efficiency Ratio
The protein efficiency ratio, or PER, is a somewhat outdated method, though it is used by governments around the world. Lab rats are fed set amounts of a protein and then measured as they grow. The amount of weight they gain in grams is divided by the amount of protein eaten in grams, giving a PER score. This method is beginning to come under criticism as it does not take into account that humans need a different amino acid profile than rats, and there are several unmeasured variables.
Protein Digestibility Corrected Amino Acid Score
The Protein Digestibility Corrected Amino Acid Score, or PDCAAS, is the current gold standard for determining protein quality. It is used by the Food and Agricultural Organization as well as the World Health Organization. It refers to the quality of a protein in terms of the amino acid requirements of two to five year old humans. Like the Chemical Score, the PDCAAS looks at the limiting amino acid, but it also takes into account digestive efficiency.