Whey is the portion of milk that remains dissolved in the aqueous portion after coagulation of casein into curd during the manufacture of cheese. Before whey protein became popular as a dietary supplement and before its nutritional significance was understood, it was considered a waste product of the manufacture of cheese. The production of acid-coagulated cheeses produces acid whey protein that has a 2-fold higher calcium content compared to the sweet whey produced from the rennet-coagulated hard cheese. Both sweet and acid whey have a relatively high mineral and lactose content. After the whey is collected, it is freeze dried and then processed to produce a protein content of around 10 to 15% and a lactose content of around 65 to 75%. Further processing can increase the protein content to around 35 to 80% in the case of whey protein concentrate, or to 90% in the case of whey protein isolate.
Traditionally, the whey protein collected from the cheese industry was used as animal feed because it was inexpensive and nutritious. Processing the whey protein to create concentrates or isolates increases cost considerably. However, supplements of whey protein isolates have reduced in price in recent years because of increased popularity (economy of scale) and more efficient manufacturing processes. Whey protein isolate is used by athletes as a supplement because whey possesses unique nutritional characteristics, particularly with regard its high concentration of branched chain amino acids (BCAA) and sulphur containing amino acids. The BCAAs in whey can be metabolised for energy and may decrease oxidation of muscle tissue if present in the plasma following exercise. In addition, they may stimulate protein synthesis via similar mechanisms to insulin, something that is useful during times of low insulin secretion. The sulphur amino acids stimulate immunity via conversion to taurine and glutathione.
The protein digestibility-corrected amino acid score (PDCAAS) is the preferred method of evaluating protein quality in human nutrition. The PDCAAS is a measure of the amino acids available following digestion and is calculated by dividing the content of the limiting essential amino acid in the test protein by the content of the same amino acid in a reference pattern of essential amino acids. The test takes account of the digestibility of the protein based on findings from rat assays. The maximum PDCAAS is 1.0, which means that 100% of the essential amino acid requirement is achieved. Animal proteins such as casein, whey, egg and meat protein have a PDCAAS of 1.0 because they supply all the essential amino acids in the correct ratios for human growth. Plant proteins such as corn, wheat and soy have lower scores (0.35, 0.37 and 0.93 respectively), so adding whey to plant foods is effective at raising their protein quality.
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