Effect of enzymatic cross-linking of β-casein on proteolysis by pepsin

Food texture has a significant influence on the sensation of satiety. The digestibility of a protein matrix can be decreased by e. g. disulfide cross-links during heating, but the structure and properties of a single protein molecule can also be modified by cross-linking enzymes. In this study the effects of cross-linking of beta-casein by fungal Trichoderma reesei tyrosinase (TrTyr) and bacterial Streptoverticillium mobaraense transglutaminase (Tgase) on digestibility by proteolytic pepsin were investigated by different methods. The enzymatic reaction conditions were selected in such a way that high and low molecular mass cross-linked beta-casein polymers were formed. SDS-PAGE (sodium dodecyl sulphate-polyacrylamide gel electrophoresis) was used to analyze the pH stability of the cross-linked beta-casein in acidic solution mimicking gastric conditions typically present during proteolytic digestion. In order to monitor the extent of pepsin digestion, the proteolytic process was halted at specific time points and aliquots of the reaction mixtures were subjected to SDS-PAGE, size-exclusion chromatography (SEC) and matrix-assisted laser desorption ionization-time of flight mass spectrometric (MALDI-TOF MS) analyses in order to evaluate sizes and quantities of the digested protein fragments. A pH-stat method was used to determine the degree of hydrolysis (DH) of the enzymatically cross-linked beta-casein. The results demonstrated that enzymatically cross-linked beta-casein was stable under acidic conditions and was more resistant to pepsin digestion when compared to non cross-linked beta-casein. The research results will have high impact on the development of novel food structures with improved properties such as good satiety, controlled energy intake and digestibility. (C) 2010 Elsevier Ltd. All rights reserved.