According to the Webster’s New Collegiate Dictionary; Whey is “the serum or watery part of milk that is separated from the coagulable part or curd especially in the process of making cheese and that is rich in lactose, minerals and vitamins and contains lactalbumin and traces of fat”.
This definition is a valuable starting point, however, it fails to provide adequate clarity to a segment of the dairy processing industry that continues to undergo evolution and growth.
For centuries whey was considered a waste product that needed to be disposed of in some fashion. Utilization of whey and whey fractions has been a relatively recent phenomenon with the majority of the development of whey processing technologies occurring in the past 40 years. Prior to 1980, there was a small amount of dry sweet whey manufactured along with some incidental quantities of lactose for pharmaceutical use. It was not until the first commercial-scale ultrafiltration membrane systems were installed very early in the 1980’s that it became practical to begin fractionating whey into its’ components and thereby increasing the utilization and value of these products. When examining the possibilities for the whey-based products the following order is generally considered as it relates to the potential value of the recovered product: Food, Feed, Fuel and Fertilizer. In other words if a fraction can be utilized in a food for human consumption it is ranked as the highest priority followed by use in animal feed, as a substrate for some type of fuel production and lastly as a material to be land applied as a fertilizer.
Whey, as outlined in the Webster’s definition, is primarily the serum obtained from rennet set cheese manufacture, this is referred to as ‘sweet whey’. However, whey may also be obtained during the manufacture of Greek-style yogurts, industrial grade acid casein, cream cheese, Ricotta cheese and cottage cheese which are examples of products where the production of large amounts of lactic acid, or the addition of some other acid source, causes the coagulation instead of the action of the enzyme rennet, this is referred to as ‘acid whey’. This distinction is important because the nutritional profile and processing characteristics of this substance are substantially different because of a change in the acidity of the solution.
Sweet whey obtained from the manufacture of most major cheeses such as Mozzarella and Cheddar has an average composition of 0.3 - 0.5% milk fat, 0.8 – 0.9% protein, 4.9 – 5.4% lactose, 0.5 – 0.6% mineral and 93.7 – 92.6% water. The pH of this solution is generally between 5.9 and 6.2.
Acid whey has a typical composition of 0.08 – 0.2% milk fat, 0.8 – 0.9% protein, 5.0 – 5.3% lactose, 0.6 – 0.8% mineral and 93.56 – 92.8% water. The pH of this solution is generally between 4.4 and 4.8. While these values do not appear dramatically different the greatest difference is in the mineral portion where there is over twice the amount of Calcium and at least a 50% increase in Phosphorus in acid whey when compared to sweet whey. This leads to several challenges in being able to process and dry acid whey directly related to the solubility of these minerals.
The protein fraction remaining in whey, both acid and sweet, is made up of α-lactalbumin, β-lactoglobulin, bovine serum albumin and the immunoglobulin fractions. There are also some non-protein nitrogen compounds found in the solution. What is not present in whey is the major protein fraction found in milk, αs-casein. The casein fraction is the protein that participates in the formation of the coagulum matrix when acted upon by rennet or when the pH of the solution is adjusted to 4.6. Casein proteins are stable to elevated heat treatments but are destabilized by changes in acidity. The whey or serum proteins are stable to changes in acidity of the solution but are not able to withstand excessive levels of heat treatment without undergoing irreversible denaturation. This denaturation is similar to that of an egg white during the frying of an egg; once it begins to coalesce the process cannot be reversed so that the egg becomes liquid again.