Effects of transglutaminase-induced modification in the presence of oligochitosan of 1 kDa on the structure and gelling properties of caseinat

Abstract

Milk gels have both traditional and high commercial value within the dairy industry (Schorsch et al. 2000;Siamand et al. 2014). Besides renneting, acidification of milk is the most routine way to form milk protein gels. Acidifying milk using glucono-δ-lactone (GDL) can slowly form structured gel products (Lucey 2004). Two major steps are involved in the formation of the acidified milk gels. In the first step, a collapse of the hairy brush of the casein micelles occurs, and the colloidal calcium and phosphate are then dissolved out of the casein micelle as a result of the protonation of ionised phosphate groups (Gaygadzhiev et al. 2009). Micellar structure is thus profoundly altered but without dissociation. In the second step, at a pH value about 5.0, casein precipitates isoelectrically, or under quiescent conditions it forms rather fragile gels (Lucey 2004). The balance between attractive and repulsive forces within protein molecules results in gel formation (Havea 2006;Rocha et al. 2009). Both intrinsic (e.g. amino acid compositions, molecular weights, hydrophobicity, etc.) and extrinsic (protein concentrations, pH value, temperature, metal ions, and ionic strengths) factors all show impacts on gelation and physiochemical properties of protein gels.During the last years, transglutaminase (TGase, EC 2.3.2.13) has been used to modify proteins via inducing protein cross-linking (between glutamine and lysine residues of the proteins) (Anema & Kruif 2012;Domagała et al. 2015). TGase-induced modification is able to form a more homogeneous gel network (Faergemand & Qvist 1997;Ercili-Cura et al. 2013), and thus improve water retention (Schorsch et al. 2000;Ercili-Cura et al. 2013) and texture (Ercili-Cura et al. 2010) of protein gels. 564Food Technology and Economy, Engineering and Physical Properties Czech J. Food Sci., 34, 2016 (6) Caseinate, transglutaminase (TGase), and an oligochitosan of 1 kDa were used to prepare a glycated and cross-linked caseinate (GC-caseinate), aiming to assess potential changes in both the structure and gelling properties of such caseinate. The results of Fourier transform infrared analysis revealed that only GC-caseinate contained saccharide portions in its molecules, evidencing TGase-induced caseinate glycation. Circular dichroism results showed that GC-caseinate possessed a more ordered secondary structure than caseinate. Other results also demonstrated that TGase-induced modification resulted in a lower gelation temperature of GC-caseinate (59°C vs. 68°C) and increased the final tan δ value (0.30 vs. 0.15) compared to caseinate during the development of acidified gels. In addition, the acidified gels from GC-caseinate were detected to have lower water holding capacity (0.720 vs. 0.781 g/g gels), expanded gel network, and larger pore sizes than those from caseinate. It is thus evidenced that the used TGase-induced modification could confer caseinate with ordered secondary structure, expanded gel network but lower water holding capacity.