A By-Product of the Dairy Industry

Whey

A by-product of the Dairy Industry

Aidan Doyle,

Hugh Holland,

And Mark Naughton

Introduction to Whey

Whey or milk plasma is the liquid remaining after milk has been curdled and strained; it is a by-product of the manufacture of cheese or casein and has several commercial uses. Whey is used to produce ricotta and gjetost cheeses and is used to make many other products for human consumption and as an animal feed. The whey protein separated from this mixture is often sold as a nutritional supplement. In addition, liquid whey contains lactose, vitamins, and minerals along with traces of fat.

Because there are many types of cheese, there are many types of whey; but they fall into two major categories: sour/acid whey and sweet whey.

Sweet whey. This is also termed cheese whey and is produced during cheese-making, when rennet is used. Sweet whey forms a very large family of products. Their compositions may vary only slightly but their properties are very different. The pH value of sweet whey can range between 5.2 and 6.7

Sour whey. This can be acid whey, quark or cottage-cheese whey and sour sweet whey. Acid whey, also known as casein whey, originates from the manufacture of casein by means of lactic acid and hydrochloric acid. The origin of quark or cottage-cheese whey is self-explanatory. Lactic acid created through natural fermentation gives the whey a high acidity. The pH values of these types of whey range from 3.8 to 4.6. If insufficient care is given to the cheese whey, it becomes sourer by continued natural fermentation. Such a process is of course undesirable so that soured (not sour) whey cannot be considered a natural product

Individual Whey Proteins

Whey does not have a constant composition. The concentrations of individual proteins are affected by breed of cow, climate and weather, and stage of lactation. The whey composition is somewhat influenced by the type of cheese being made, as well. Carbery Group Ltd Whey Protein

• b -lactoglobulin 50%
• a -lactalbumin 20%
• Immunoglobulins 20%
• Serum Albumin 5%
• Minor Proteins 5%

b -Lactoglobulin. This predominant whey protein naturally exists as a dimer consisting of two identical monomer units, each having a molecular weight of 18,000. It thus behaves like a protein with a MW of 36,000 and retains this form throughout most milk processing and food applications. Most of the functionality of whey products can be accurately predicted from studies of the pure b -lactoglobulin. Until recently its purpose was completely unknown, and the protein is absent from the milk of some species, including humans. Dairy scientists have now discovered that it binds very strongly to retinol, the most important form of vitamin A in food and feeds, and it is thought to play a significant role in the transport and absorption of vitamin A in the intestinal tract.

a -Lactalbumin. This second most abundant whey protein has a molecular weight of about 14,400 and interacts with b -lactoglobulin on denaturation to account for virtually all of the whey protein functionality. By itself, a -lactalbumin is much more readily heat-denatured than b -lactoglobulin, but it readily reverts to its native form when the heat is removed. This led earlier researchers to report that it is very heat-stable, based on their comparisons of its structure before and after heating. When both a -lactalbumin and b -lactoglobulin are present, as they are in whey protein concentrates and whey protein isolates, they form intermolecular crosslinks on denaturation which stabilize the unfolded form of a -lactalbumin after the mixture is cooled. It has been known for some time that a -lactalbumin plays an important part in the enzymic process for synthesizing lactose in mammary tissue. One should not confuse the specific protein, a -lactalbumin, with the commercial protein preparation known as "lactalbumin" which is a heat-precipitated mixture of whey proteins.

Immunoglobulins. The immunoglobulins in whey are antibodies, of the types IgA, IgG, IgE and IgM, with molecular weights of 150,000 to 450,000 or greater. Some of them represent the normal antibody components of the cow and are the same as those found in the bloodstream; however, there is a secretory form of IgA which is produced in the mammary gland in response to infectious organisms. From a physiological standpoint, the immunoglobulins are important in establishing the infection-fighting capacity of the newborn calf, and they are present in very high concentrations in the first milk of lactation, called colostrum. They are of high nutritional value but do not appear to play an essential physiological role in the adult animal or in the use of bovine milk or whey for human consumption.

Serum Albumin. As the name implies, serum albumin (also known as BSA) comes from the cow’s bloodstream where it is the carrier of free fatty acids and is important in maintaining the physical properties of the blood serum. It has a molecular weight of 68,000 and is quite high in the amino acid cystine. Serum albumin is present in the milks of all known species and apparently comes directly from the bloodstream.

Minor Proteins. The least numerous 5% of the whey proteins are of many types, which were often simply assigned to the category "proteose peptones" before the individual components had been identified. Some of them represent breakdown products of the caseins which were formed either before or during cheese making. Others are specific enzymes or other proteins which have known physiological functions. A few of these minor proteins are worthy of more detailed discussion here.

Glycomacropeptide is not normally tallied as part of the milk serum proteins because it is not present until cheese is made, but it can potentially account for more than 16% of the protein in commercial whey products. The cheese making process involves treating milk with a proteolytic enzyme of the class known as rennet’s. These enzymes break a specific peptide bond in k -casein between the phenylalanine at position 105 and the methionine at position 106, which destabilizes the casein micelle and allows the cheese curd to form. The part of k -casein consisting of amino acid residues 1-105 is known as para-k -casein and stays with the cheese. The portion consisting of residues 106-169 is released into the whey and is known as glycomacropeptide (GMP) because it usually contains a number of carbohydrate attachments (hence the "glyco-" term). Its molecular weight, based on its amino acid content, is about 7,000; however, the carbohydrate units make it behave as if it were somewhat larger. This is important because the ultrafiltration membranes used to prepare whey protein concentrates and whey protein isolates perform their function based on molecular size.

GMP was not recognized as a major whey protein for many years because of two unusual characteristics. It contains no aromatic amino acids (phenylalanine, tyrosine and tryptophan) so it does not absorb ultraviolet light at 280 nm, which is the most common means of detecting proteins which are being analyzed by high-performance liquid chromatography (HPLC). Furthermore, it does not readily bind Coomassie Blue, the stain that is usually employed to detect individual protein bands on polyacrylamide electrophoresis gels. Thus, GMP is rendered invisible in the most powerful methods for studying protein mixtures unless special procedures are used to detect it.

PROCESSING OF WHEY

Membrane Filtration

Membrane processing is a technique that permits concentration and separation without the use of heat. Particles are separated on the basis of their molecular size and shape with the use of pressure and specially designed semi-permeable membranes. There are some fairly new developments in terms of commercial reality and is gaining readily in its applications: There two main forms of membrane filtration involved in the whey processing industry Ultrafiltration (UF) and Microfiltration (MF).

Ultrafilteration

Ultrafiltration (UF) designates a membrane separation process, driven by a pressure gradient, in which the membrane fractionates components of a liquid as a function of their solvated size and structure. The membrane configuration is usually cross-flow. In UF, the membrane pore size is larger allowing some components to pass through the pores with the water. It is a separation/ fractionation process using a 10,000 MW cutoff, 40 psig, and temperatures of 50-60°C with polysulfone membranes. In UF milk, lactose and minerals pass in a 50% separation ratio; for example, in the retentate would be 100% of fat, 100% of protein, 50% of lactose, and 50% of free minerals.

Diafiltration is a specialized type of ultrafiltration process in which the retentate is diluted with water and re-ultrafiltered, to reduce the concentration of soluble permeate components and increase further the concentration of retained components.

Microfiltration

Microfiltration (MF) designates a membrane separation process similar to UF but with even larger membrane pore size allowing particles in the range of 0.2 to 2 micrometers to pass through. The pressure used is generally lower than that of UF process. The membrane configuration is usually cross-flow. MF is used in the dairy industry for making low-heat sterile milk as proteins may pass through but bacteria do not.

The most valuable component of whey is the whey proteins. Ultrafiltration and diafiltration membrane technology have provided the means to further concentrate and separate whey components. The protein and fat (retentate) in the whey are separated from the lactose and minerals (permeate) by these processes. Once whey is dried, a producer can provide "whey protein concentrates," or WPCs, with protein levels from 34% to 90%.

Figure 1. The Whey filtration process

WPCs have been around for years, but product developers can expect better performance today.

A higher quality product means improved functionality for the product developer. Some of the basic properties that a WPC can provide in a food application are whipping/foaming, emulsification, high solubility, gelation, water-binding, and viscosity development. Generally, WPCs with higher protein content have improved functionality over those with lower protein content.

Some very exciting work in progress involves the use of 34% WPC in various food applications. This demonstrates their ability to work well in some applications, but not in conflict with the applications designed for the higher protein ingredients. High solubility over a wide pH range makes WPCs a good candidate for a sports beverage or meal-replacement beverage. Their water-binding capabilities also make them suitable for processed meats, cakes or breads.

Gelation characteristics will increase WPC benefits in some of the same products that profit from water-binding. Salad dressings, coffee whiteners, soups, cakes, infant formulas and sausages all can utilize the emulsification abilities of WPC. They also address the functional needs of viscosity in products such as soups and gravies. Cakes, desserts and whipped toppings can always use the added foam stability of a WPC.

Whey protein conformation and functionality are interrelated and dictated by changes in their globular folded structure. Their functional properties are affected by several factors within a food application, including concentration, state of the whey proteins, pH, ionic environment, (pre-) heat treatment and the presence of lipids.

In the native state, whey proteins are highly soluble and adeptly perform emulsification and whipping functions in a food application. However, heating whey proteins can result in a loss of solubility due to denaturation of the proteins, especially in the pH range of 4.0 to 6.5. While solubility is adversely affected by heat, emulsification can be improved through controlled heat denaturation of the protein. As the whey protein unfolds, hydrophobic amino acid residues are exposed, which enhance the ability of the protein to orient at the oil/water interface. The presence of salts during the emulsification process influences whey protein conformation and solubility. In their undenatured form, whey proteins can form rigid gels that hold water and fat, and provide structural support. The formation of disulfide bonds and ionic bonding controlled by calcium ions appears to determine gel structure.

Water-binding capacity is a measure of the amount of water held in a gel under a specific set of conditions. The water-binding abilities of whey proteins can help reduce formula costs due to the added water being held by the proteins. Other properties associated with water-binding - swelling, gelation and viscosity - are primary determinants of texture in processed cheese, yogurt and reduced-fat foods.

Whey proteins also contribute to browning by reacting with lactose and other reducing sugars present in a formulation, providing color to baked goods and sauces. Not only are WPCs functional, they also are bland-tasting and contribute no foreign or off-flavors to foods when used as an ingredient.

The category of specialty WPCs includes demineralized and hydrolyzed versions. The demineralized products are often used in infant formula, while lactose hydrolyzed products work well in cheese-type spreads and yogurt. In this case, the hydrolysis of the lactose into glucose and galactose allows for the addition of dairy solids with some added sweetness to the product.

Research involving whey protein isolates, alpha-lactalbumin, beta-lactoglobulin, lactoferrin, bovine serum albumin, lactoperoxidase, peptides and immunoglobulins has gained recent press attention. Research is currently in progress on the use of whey protein concentrate in the diet and its anti-tumor effects for head- and neck-cancer patients. Whey protein has been shown to stimulate cell-mediated and humoral immunity; to have an antioxidant role by increasing tissue glutathione; and to improve the body's nutritional status in stressed individuals, thereby inhibiting growth of several tumor types.

Whey protein isolates (WPI) are those products with a protein content of over 90%. Various processes remove the nonprotein components. These include precipitation, microfiltration and ion exchange. The composition (protein, lactose and mineral content) and product characteristics can vary with the process used.

Whey protein isolates can also be manufactured by selective ion exchange processes to select the primary functional proteins, beta-lactoglobulin and alpha-lactalbumin. They provide high gel strength, viscosity, aeration, water binding, and high solubility to an application. In their pure form, they can be used to replace other ingredients, such as soy protein, egg whites, or gelling agents. These isolates consist of completely undenatured protein and also contain up to 10% to 12% biologically active immunoglobulins.

Several whey protein fractions have been considered as dietary ingredients, with potentially greater activity against the development of colon cancer than the total whey protein product. Lactoferrin-supplemented diets have enhanced the protective effect of total whey protein in animal studies relating to colon cancer. Clinical trials with whey protein isolate have been conducted on children with AIDS. It has been demonstrated that the ability of lymphocytes to offset oxidative damage is measured by determining the capacity of these cells to regenerate glutathione. Patients in this study who started with low blood-lymphocyte glutathione exhibited a substantial increase in glutathione content after including whey protein isolate in the diet.

Several protein fractions display antimicrobial, as well as antiviral, activity. These fractions include lactoferrin, lactoperoxidase, lysozyme and immunoglobulins. Technology has been developed on a commercial scale to produce these proteins for use as biopreservatives and as natural anti-infectious components for prophylactic and therapeutic treatment of bacterial and viral diseases.

One fraction receiving a high level of interest for its nutraceutical properties is lactoferrin. This iron-binding protein, in addition to its bacteriostatic properties, is also associated with enhanced iron absorption, stimulation of bacterial gut organisms, such as bifidobacteria, as well as a potential immune-stimulating role. It is one of the principal whey proteins in human milk, present at a level of about 30% of total whey proteins.

"Lactoferrin, as well as being an antioxidant, binds certain gastric pathogens and is bactericidal against a variety of pathogenic bacteria and yeasts," he adds. "This could lead to its use for treatment of intestinal ulcers and other gastric disorders.

Not all research has focused on fractions for purely physiological functions. Currently, research is under way on the use of beta-lactoglobulin as a fat replacer in meats. Licensed technology exists for producing heat-induced, beta-lactoglobulin gels that resemble animal fat. Use of alpha-lactalbumin in infant formulas is becoming more common as research attempts to get closer to the composition of human breast milk. Alpha-lactalbumin also has been modified in attempts to perform some of the same functionalities found in egg white and its application in angel food cake.

Lactose or milk sugar is in abundant supply due to increasing cheese production. Lactose can be produced from whey or whey permeate. Its primary use is in bakery and confectionery products. Similar to other components of milk or whey, interest also has focused on the development and production of new lactose derivatives. The current news concerning lactose focuses on its derivatives, the galacto-oligosaccharides. Commercially successful examples are lactitol and lactulose.

Lactitol is produced by the chemical hydrogenation of lactose. Lactulose is produced by chemical isomerization of lactose. Galacto-oligosaccharides are an important part of the functional foods trend as a factor in the well-being of the digestive tract. Galacto-oligosaccharides are not digested or absorbed in the small intestine, so they reach the colon where they are fermented by beneficial colon bacteria, bifidobacteria.

Lactulose has a considerable influence in the medical treatment of some forms of liver disease, and it aids and/or restores health after bouts of salmonellosis. When used as a food ingredient to improve nutritional value, lactulose can be added to many applications, without loss of functional or nutritional properties due to processing. Some applications include: infant formulas or baby food; yogurt; diabetic sweetener; sugar substitute in confectionery products; and dried and liquid nutritional supplements for seniors.