Effect of transglutaminase on rheological properties and microstructure of chemically acidified sodium caseinate gels

The mechanical properties and microstructure of 2.7% and 4.5% sodium caseinate gels chemically acidified by glucono-delta-lactone (GDL) and cross-linked by microbial transglutaminase (TG) were studied. The acidification was performed at different temperatures. According to SDS-PAGE TG clearly caused polymerisation of caseinate irrespective of the treatment temperature (4-50 degrees C), The cross-linking of the proteins was more extensive at temperatures 22-50 degrees C. Low amplitude viscoelastic measurements showed that 4.5% caseinate gels acidified at 50 degrees C were formed much faster than gels acidified at 22 degrees C. TG only slightly increased the time of gelling. Control gets prepared without TG at temperatures of 4, 22, 37 and 50 degrees C were mechanically weak. Examination of the control gels with a confocal laser scanning microscope showed that gels formed at 37 and 50 degrees C were coarse and porous with large cavities between particle aggregates, whereas those formed at 22 degrees C were much more homogeneous. The TG-treated and acidified sodium caseinate dispersions formed firm gets, indicating cross-linking of casein proteins. Interestingly, the strongest gels were formed at 22 and 37 degrees C. TG treatment improved the homogeneity of the gel structure at temperatures of 37 and 50 degrees C. The hardness of TG-treated gels acidified at 4 degrees C increased during I week of storage. (c) 2006 Elsevier Ltd. All rights reserved.