Syneresis in rennet gels.

Researchers at the Royal Veterinary and Agricultural University (Department of Dairy and Food Science, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark) have been working on a number of projects in dairy science. They are developing a new technique for measuring syneresis in rennet gels.

They simulate the cutting of the gel during cheesemaking by wetting the gel surface, which avoids any mechanical handling of the gel. The amounts of water inside and outside of the gel respectively are measured by nuclear magnetic resonance (NMR). This is a simple measurement procedure that doesn't require separating the two phases. This research effort is still ongoing.

On another front, scientists have been investigating the effect of transglutaminase catalyzed cross-linking on the rheological properties of milk proteins in dairy foods. The work has involved studying the functional properties of acid milk gels made from skim milk cross-linked with transglutaminase. Rheological measurements show that cross-linking significantly increases the stiffness of the acid milk gels.

Permeability-a measure of the expected whey drainage of the milk gel-is at the same time reduced by the cross-linking process. Two different microbial transglutaminases were applied in acid milk gels. Researchers found that one gave a higher gel strength but lower permeability than did the other at the same enzyme dosage level.

Investigators used confocal laser scanning microscopy and image analysis to explain the effect of cross-linking on permeability. These techniques showed that cross-linked milk gels are composed of a denser network structure with smaller pores. The differences in microstructure were quantified by digital image analysis. A close connection between pore size and gel permeability was determined by image analysis. The rheological properties did not correlate directly with the parameters determined by image analysis. Investigators believe the strength of the individual structural components is important, as well as their spatial arrangement.

Other scientists at the university are investigating the enzymatically-induced cross-linking of milk proteins. The aim of this project is to develop methods for the enzymatic cross-linking of proteins. Scientists have investigated the effect of cross-linking on the functional properties of the proteins. They've examined the cross-linking of whey proteins with transglutaminase.

Whey proteins, especially b-lactoglobulin, are not readily cross-linked by transglutaminase. Reaction conditions have been optimized to cross-link whey protein isolate and purified b-lactoglobulin. Both a-lactalbumin and b-lactoglobulin can be extensively polymerized by transglutaminase when they are partly unfolded by the chemical reduction of the disulfide bridges stabilizing their tertiary structure, or by performing the reaction at alkaline pH. Scientists indicate that a-lactalbumin is more susceptible to cross-linking than b-lactoglobulin. B-lactoglobulin forms a very fine-stranded gel structure when polymerized with transglutaminase.

Researchers also found that the surface shear viscosity of two-dimensional protein films increased significantly at the oil-water interface when the proteins were incubated with transglutaminase. Further information. Jeanette Otte; phone: [phone]; fax: + 45 35 28 31 90; email: [e-mail].