Improving milk protein functionality by treatment with transglutaminase.
The main fraction of milk proteins, the caseins, occurs in milk as sterically-stabilized association colloids, called casein micelles. They have a diameter of about 200 nm. It is the purposely induced destabilization of casein micelles that leads to the formation of a yogurt gel or a cheese curd or the first step in the isolation of the casein fraction for production of caseins or caseinates.
Recent research has shown that treating caseins with the enzyme transglutaminase, which catalyses the formation of covalent cross-links between proteins, can considerably improve the functional properties of milk proteins. Transglutaminase cross-links both caseins on the micelle surface as well as those in the core of the micelles. As a result of these cross-linking reactions, the stability of casein micelles against acid-induced coagulation is reduced, leading to firmer yogurt gels.
Transglutaminase treatment of milk prior to yogurt manufacture can be used to improve the texture and reduce syneresis of particularly reduced-fat yogurt products. In addition, treatment with transglutaminase can also help in saving ingredient costs by reducing the amount of protein required to create a desirable texture, firmness and stability of yogurt. Current work at NIZO food research is aimed at the creating novel milk protein ingredients for such tailored uses.
Furthermore, transglutaminase-treated casein micelles are less susceptible to enzyme-induced coagulation than their untreated counterparts, and extensive cross-linking even makes them unrennetable. This aspect is naturally undesirable for cheese-making; however, it can be beneficial in ultra-high temperature-treated milk products, in which the proteolytic degradation of caseins contributes to age-gelation during storage. For other types of cheese, e.g., cream cheese, treatment with transglutaminase can be used to increase product yield and increase viscosity and reduce syneresis of the product. Opportunities for using transglutaminase to design ingredients for reduced-fat and/or reduced-protein processed cheese products also exist.
Caseins in transglutaminase-treated micelles are less susceptible to dissociation. Hence, for products in which casein dissociation has been implicated as a major culprit in inducing instability, such as in concentrated or evaporated milk proteins, treatment with transglutaminase can improve product stability. Also, cross-linking milk proteins on the milk fat globule membrane can increase the stability of these globules against coalescence when cream-based products are in storage.
Besides using transglutaminase in traditional dairy products, efforts to prepare novel dairy ingredients, such as caseins or caseinates, with tailor-made stability and functionality through the enzymatic cross-linking of milk proteins, are currently underway at NIZO food research. These novel ingredients can potentially open new areas for the application of milk proteins.
Further information. Thom Huppertz, NIZO Food Research BV, P.O. Box 20, 6710 BA Ede, The Netherlands; phone: +31(0)318 659 600; fax: +31(0)318 650 400; email: Thom.Huppertz@nizo.nl.