BY-PRODUCTS TECHNOLOGY

Module 3. Processing and utilization of whey

Lesson 31
METHOD OF WHEY PROTEIN RECOVERY FROM WHEY

31.1 Introduction

Whey proteins are recovered from whey either in the form of whey protein concentrates (WPC) or whey protein isolates (WPI). WPC are mostly available with 35, 55, or 75– 80% protein on dry matter basis, while WPI contain >90% protein on dry matter basis. The principal manufacturing processes of whey protein products are based on known behaviour of whey components under defined conditions. Properties that have been exploited commercially include: molecular size differences (ultrafiltration, gel filtration), insolubility of protein at high temperature, charge characteristics (demineralization, protein removal by ion exchange), aggregation by polyphosphates and crystallization of lactose. Capital cost for most of these processes are high and product yields are characteristically low.

31.2 Heat Precipitation Process

Whey proteins may be precipitated (and thereby rendered insoluble in water) by heating whey at acid or near-neutral pH. Acid whey must be heated to at least 90°C and maintained at such temperature for at least 10 min to achieve maximum yields. For sweet wheys, good yields can be obtained by heating at pH between 6.0 and 6.5, although products so derived have higher mineral concentrations than those of acid whey unless pH is adjusted to 4.6 prior to protein removal. The precipitate so formed is firmer and more readily separated than that formed in unacidified whey. Precipitated proteins are removed by settling (static or accelerated), washed, reseparated, and dried. In modern plants, high speed centrifuges such as clarifiers and decanters are used to affect both primary and secondary (after washing) separations. Ring, fluid bed, roller, and spray driers have been used to obtain the finished dried product. Typical yields are 4.2 to 5.2 kg/m³.

Process refinements include demineralisation prior to heating, pre-concentration by reverse osmosis and ultrafiltration, and continuous, high temperature reaction (120°C for 8 min at pH 6). Most processes result in an insoluble product, but through heating whey to 95°C at pH 2.5 to 3.5, then adjusting to pH 4.5 prior to separation, it has been claimed that a product soluble at pH 5 can be produced.

31.3 Precipitation By Complexing Agents

Salts of heavy metals have long been used to precipitate proteins from solutions. This process is also called the “cold precipitation process”. Numerous complexing agents can be used to recover proteins from whey; of these, only polyphosphates appear to be used commercially for this purpose.

31.3.1 Polymeric phosphates

Long-chain polyphosphates precipitate protein from whey at low pH e.g., 2.5. Typically, potassium polymetaphosphate and sodium hexametaphosphate are used. Use of hexametaphosphate (up to 0.07 molar concentration of phosphorus) was found to be quite effective for the precipitation of nearly 75% of the proteins. A group of workers adjusted the pH of cheese whey to about 2.5 followed by addition of 0.5% sodium hexametaphosphate. The whey was not heated. The precipitates so formed are removed by centrifugation, and washed. The precipitated proteins gave electrophoretic patterns similar to the native whey proteins. Further processing by ion exchange or gel filtration was required to increase the protein content by removing the polyphosphate and lactose. The protein phosphate complex can be resolubilized by raising the pH to > 6 and metaphosphate can then separated by large scale sephadex column, dialysis or precipitation with added calcium ions. A treatment with Ca(OH)₂ at pH 8.9 was found to remove some of the complex hexametaphosphate from WPI. The typical process is given in Fig. 31.1.

Dialysis could reduce the ash content of WPC by only about 12%. Column hydrolysis treatment of hexametaphosphate WPC in conjunction with dialysis or desalting reduced the ash content by 35-91%. This latter process is costly and, therefore, not commonly used. As a result most metaphosphate complex WPC retain their metaphosphate and thus exhibit greatly impaired solubility at pH < 6. Removal of calcium prior to phosphate addition reduces the amount of phosphate required and results in recovery of up to 90% of the original whey proteins at pH 3. On dry basis, the products contained 70-85% protein, 10-20% sodium hexametaphosphate and 10-15% lactose. Further modification of this process is also possible. The dried protein concentrate contained upto 80% undenatured whey proteins.

31.3.2 Ferric salt precipitation

In this procedure, a chelated protein complex is prepared from whey by adding to it a dilute solution of a ferric salt (Fig. 31.2). The ferric ion may be later removed from the product by treating with SO₂. Sodium thionite treatment of ferric precipitated WPC in conjunction with dialysis or desalting reduced the ash content by 35-91%. The use of ferric chloride for precipitation of whey proteins has been described in a process patented. In this process, use of FeCl₃ at the rate of 0.93 g Fe per litre of sweet whey and 1.35 g Fe per litre of sour whey is recommended for maximum yields. A group of workers showed that ferric chloride could be effectively used for the precipitation of whey proteins at pH 4.5. The product prepared by them was soluble at neutral pH and free from the metal ion (dialyzable fraction). Nutritional studies carried out by them showed that the iron present with such protein isolates was readily available for regeneration of haemoglobin in rats and protein had PER value of 3.30 (casein 3.11).

31.3.3 Carboxy methyl cellulose (CMC) precipitation

A group of workers described the use of carboxymethyl cellulose (CMC) for the reclamation of whey proteins. At pH 3.2, more than 90% of the protein was recovered; the product contained 60% protein and 30% CMC. The product was solubilized by adjustment of the pH in the range 7-7.5. The interaction between whey protein and CMC is only at low ionic concentrations (maximum at ionic strength less than 0.1). Therefore, necessary dilution is attained by diluting the whey system with equal volume of water, in which CMC at 0.8% level has been dissolved at 60°C with rapid agitation (Fig. 31.3). After cooling, the pH is adjusted to 3.2 with (1:4) HCl. Then this mixture is left undisturbed overnight. Precipitated protein is recovered by centrifugation, and washed.

31.3.4 Polyacrylic acid precipitation

In one procedure, a group of workers adjusted pH of cottage or cheddar cheese whey to 4.0 with 2.5 M H₃PO₄ at a temperature of 18°C, mixed with 1.5 g of filter aid and 3 ml of a 25% polyacrylic acid solution. After 10 min stirring, the precipitate was allowed to settle for 1 h and the supernatant siphoned off. Solids were separated by vacuum filtration suspended in 100 ml water and mixed for 1 h with 0.6 g of magnesium carbonate, which increased the pH to 6.5. After filtration, the solution containing the protein was concentrated in vacuum at 40^oC up to 20% total solids and freeze dried.

31.4 Adsorption Process

Adsorption techniques, based on the ion exchange properties of whey protein, are used for the recovery of whey proteins. Whey proteins being amphoteric, solid phase charged adsorption media can be used to remove them from whey under appropriate conditions. At pH values lower than their isoelectric point, whey proteins have a net positive charge and behave as cations, which can be adsorbed on the cation exchangers. Conversely, at pH values above their isoelectric point, the proteins can be adsorbed on anion exchanger. Media suitable for this purpose include regenerated cellulose, titania plus alumina, and silica with pore sizes and surface characteristics specially designed for the recovery of proteins from whey. Of these, cellulose and silica-based systems have progressed to semi-commercial operation. Two major ion exchange fractionation processes have been introduced for the preparation of whey protein concentrates based on modified cellulose and silica.

31.4.1 Vistec process

Regenerated cellulose is used in the "Vistec" (BioIsolates) process. The development of sulfopropyl cellulose resins of high charge density (1.1 meq/g) for use in Vistec process has enhanced its commercial prospects. The pH of the whey is adjusted to < 4.5 and pumped into the stirred tank reactor and mixed with resin to allow protein adsorption onto the ion exchanger (resin). Lactose and non protein materials are eluted with water. The desorption of bound proteins is accomplished by changing the pH to 9. The dilute eluate containing the protein is separated from the resin by filtration in the tank reactor. Ultrafiltration is used to concentrate by vacuum evaporation (and demineralize) the protein solution, which is then spray dried. Protein yield is 85%, and the dried WPC may contain as much as 95% protein. Because lipid molecules are not adsorbed by the media, such products have low fat concentrations. For better efficiency, a newer process has designed using resin packed in a column in place of stirred tank reactors.

31.4.2 Spherosil processes

Silica-based adsorbents are used in the "Spherosil" process. The “Spherosil” processes use their cationic spherosil S or anionic spherosil QMA ion exchangers, and the adsorption is accomplished in fixed bed column reactors.

31.4.2.1 For acid whey

For acid whey, an adsorbent with strong cation exchange properties is used. Acid whey (pH ≤ 4.5) is adsorbed to the spherosil S reactor and after lactose and other solutes are eluted with water, the pH is raised by the addition of alkali (generally 0.1 N ammonium hydroxide) and adsorbed proteins are eluted from the reactor.

31.4.2.2 For sweet whey

For sweet whey, an anion exchange properties is used. Sweet whey (pH ≥ 5.5) is passed through a spherosil QMA reactor to permit negatively charged protein molecules to adsorb on to the ion exchanger. After the elution of non protein materials with water, the proteins are eluted with an acidic solution, such as 0.1 N Hydrochloric acid. As with the Vistec process, ultrafiltration and spray drying are then necessary. Protein yield is 90%.

Several following major problems are associated with the ion exchanger processes:

• Production of large volumes of rinse, chemical solutions and deprotenized whey that must be processed or disposed.

• The need to concentrate and purify the dilute protein fraction by ultrafiltration, evaporation and drying.

• An excessive time required to conduct each fractionation cycle.

• Control of microbiological contamination of the reactor.

31.5 Removal of Lactose and Minerals

Composition of whey can be modified by removal of lactose and minerals to give whey protein products of 15 to 40% protein on dry matter bases. For many years, the lactose industry has produced a protein concentrate in the form of mother liquor, the material that remains after lactose has been crystallized and separated from concentrated whey. The product, known as delactosed whey powder, contains about 25% protein. It has been used as a stock food because of the extensive protein denaturation in the process. In more modern plants, with lower temperature-shorter residence time evaporators, the product is more functional and, therefore, of greater value. Delactosed whey powder has a high mineral concentration (up to 25%). Processes have been described whereby pre-concentrated whey (up to 30% TS) is subjected to electrodialysis, the whey is concentrated to 60% TS, lactose is crystallized and removed, and the remaining liquid is concentrated and spray dried. The resultant product may contain up to 35% protein.

Selected references

Kilara, A. 2008. Whey & whey products. In: Dairy processing and quality assurance edited by Ramesh C. Chandan, Arun Kilara and Nagendra P. Shah., Wiley Blackwell Publishing: 337-355.
Matthews, M.E. 1983. Whey protein recovery processes and products. J Dairy Sci., 67: 2680-2692.