You really can write a book about just one of the many proteins found in milk. Like all things in nature, protein in milk is just plain complicated, and the closer you look into them the more complicated they get. There are two groups of proteins in milk, those that precipitate or have decreased solubility at pH = 4.6 and those that do not. In this article I will focus on the proteins that remain after the caseins have precipitated. Since the fluid part of milk after the casein curd has been removed in cheese making is called “whey” the proteins found in this portion of milk are called whey proteins. As you might expect (it is dairy science after all) the actual mix of protein in whey from different cheese making procedures and other processes such as Greek Yogurt, vary considerably.
Whey is a collective term referring to the milk serum, milk permeate or liquid part of milk that remains after milk has been curdled and the curd removed. It is a by-product of cheese making that has been referred to as liquid gold throughout history. (Source: http://www.omnomcheese.com/how-to-use-whey/)
|Classification of whey|
|Sweet whey||Obtained from classical renneted cheese; contains casein macro peptide|
|Acid whey||Obtained from acid curd cheeses like cottage cheese and cream cheese and from Greek yogurt|
|Salt whey||Made from Cheddar Cheese and other salted curd cheese higher in NaCl content|
|Rennet casein whey||Similar to sweet whey without starter bacteria and lower in fat|
|Acid casein whey||Similar to acid whey without starter bacteria and lower in fat with more Ca++ and PO4|
The most prevalent protein in whey is B-Lacto globulin. It represents a little more than half the protein in whey. Like most proteins, its properties depend on temperature, pH and ionic strength. B-Lacto globulin exists in three genetic variants in cow’s milk, though this does not seem to affect practical properties much. B-Lacto globulin has in its structure two disulfide linked amino acids and one free sulfhydryl group. In milk it is present mostly as a dimer held together due to the relatively hydrophobic nature of the molecule. At low pH (below 5.5) the protein aggregates into groups of up to 8 monomers. Below pH=3.5 the octamers disassociate and the protein exists as primarily monomers. At high temperatures (and neutral pH) the protein disassociates to monomers. When this disassociation (heat denaturation) happens and the protein structure unfolds, the sulfhydryl group can become exposed. This allows it to react covalently with a disulfide group. This group can be on the molecule or on another molecule with a disulfide group available. In this way dimers, trimers and tetramers etc. can be formed. These aggregations are quite stable and depending on temperature, pH, ionic strength and concentration may become insoluble particles. It is this reaction that is the reason that for acid whey from Greek yogurt having a relatively low protein content. If the concentration of protein is high, this aggregation can cause a gel to form. In neutral milk this reaction can associate the whey protein with the casein micelle. This leads to casein micelles achieving a larger volume than in unheated milk, the volume increase is responsible for a higher viscosity in dairy products thus treated.
Another protein, α-Lactalbumin is the second most prevalent of the whey proteins and comprises about 20% of the whey protein. It is a small compact molecule that does not associate except at low ionic strength. It does bind a calcium ion that at pH<4.0 or if the calcium is otherwise removed the protein will enter a less orderly state (molten). In this state the protein can be easily and irreversibly denatured. In the presence of calcium or other protein the denaturation is reversible. The biological function of α-Lactalbumin seems to be as a coenzyme involved in lactose synthesis.
About 6% of the whey protein is BSA – Blood Serum Albumin. It probably enters the milk by passing into the mammary gland from the blood. It has 17 –s-s- groups and one free sulfhydryl group. It is a large molecule with 3 globular domains. It resembles egg albumin in its properties.
Roughly 11% of the whey proteins in cow’s milk are immunoglobulins. This is a large group of polymers of protein molecules produced by the animal in response to foreign challenges from the environment like bacterial proteins. They are important in the immune defense of the young animal and can exert an antimicrobial action in raw milk. Immunoglobulins are very heat sensitive and their immune function does not survive even milk pasteurization. In cow’s milk (not sheep, goat or buffalo) there exists a set of immunoglobulins called cryoglobulins that are involved in cold agglutination of fat globules. In cold raw milk the cryoglobulins coat the fat globules doing so they cause the surface of the fat globules to become sticky. The sticky fat globules agglomerate into loose flocks. This causes the fat to rise quickly sweeping up more and more globules as the flocks rise until they form a layer at the top of the vessel. This is one of the reasons for the abnormally high speed for creaming in cow’s milk.
There are several minor proteins in the whey protein fraction of milk. B2-microglobulin, lactoferrin, transferrin, protease- peptone, various glycoproteins and lipoproteins are all found in the whey protein fraction. Whey proteins have many functions in dairy products. One of the major uses is as a protein source in foods. Whey protein is thought to be helpful in increasing lean body mass but the effect size in many studies is small (not much increase in lean body mass but the results were statistically significant). There have been several studies on weight loss; the results have been mixed with small effects. There is some research on whey protein and insulin resistance these studies have mixed results with small effects. Where there seems to be some evidence for whey protein’s effect on health is in muscle protein synthesis. Whey protein seems to stimulate muscle protein synthesis in the body. More studies will be needed to confirm these findings. There are many studies done and being done with whey protein and its effect on the body. Here is a link to a summary of some of the studies for those of you that want to read more about it.