Modify whey protein's rheological properties using immobilized enzyme.

There's obviously an advantage to converting waste products into value-added, high-priced commodities. Interest in achieving this goal has led to the development of processes that enhance whey protein functionality.

Whey has found use as a protein supplement, thickening agent, emulsifying agent, gelling agent and foam stabilizer. Modifying whey proteins to improve their functional properties in foods has become a focus of research efforts.

On this front, scientists have chemically and physically treated whey proteins by a variety of processes that include acidification, heating, enzymatic cleavage and cross-linking. All of this has been done both in the presence and absence of different salts. Treatments can enhance the ability of proteins to form gels or give them improved water-holding or foaming properties.

Now scientists at North Carolina State University have looked at transglutaminase to do the job. As you may know, transglutaminase is a transferase that forms isopeptide bonds between lysyl and glutaminyl residues. Microbially produced transglutaminase is stable and calcium-independent. For these and other reasons, its use in food systems has garnered interest. It has been used to modify a number of proteins to improve their functionality.

The North Carolina researchers have developed a method for modifying the viscous and elastic properties of whey protein isolate using an immobilized form of the enzyme. The process made it possible to easily separate the catalyst and substrate and eliminated any requirements for a downstream inactivation step.

Immobilized enzymes offer a variety of advantages over the use of free enzymes for treating process streams or in a batch format. For example, you can better control the extent of the reactions, and the enzyme may be reused indefinitely when properly stored.

In their process, the investigators immobilized microbial transglutaminase to controlled-pore glass. Avidin was adsorbed to glass beads that had been derivatized and biotinylated. The enzyme was biotinylated and adsorbed to the avidin affinity matrix. The researchers then incubated solutions of 8% whey protein isolate with the beads. This resulted in limited cross-linking of whey proteins. As incubation time increased, the intrinsic viscosity increased as well. Gelation temperature decreased, and stronger, more brittle gels formed upon heating.

This process enabled researchers to control the extent of the cross-linking and allowed them to recycle the enzyme. The scientists were able to modify the functional properties of several batches of whey protein isolate using less than 10 mg of the same enzyme. This is indicative of the capacity of immobilized enzymes to find frequent use in these types of applications.

Further information. Harold Swaisgood, North Carolina State University, Department of Food Science, Box 7624, Raleigh, NC 27695; phone: [phone]; fax: [phone]; email: [e-mail].