Yoghurt gel formation by means of enzymatic protein cross-linking during microbial fermentation

A novel yoghurt fermentation process was investigated where the gel formation by isoelectric coagulation is enhanced by enzymatic protein cross-linking by transglutaminase (TG). In contrast to most previous studies the TG was applied during fermentation so that no additional process time is required. The challenge was whether the microbial acidification leaves enough reaction time for the enzyme, which gradually looses its activity upon pH drop and whether the enzyme is completely inactive when the pH has reached values below 4.5. Residual enzyme activities were measured in model experiments as a function of the pH in the range of 3.0-7.0. No residual activity was found below a pH of 4.0 at 40 °C for 120 min exposure time. During yoghurt fermentation, there was no effect of the enzyme on the fermentation time. It was found that 80% of the cross-linking effect is obtained within the pH range of 6.6-5.7. A higher viscosity and degree of protein polymerisation of TG-treated stirred yoghurt was found in comparison to non-cross-linked controls. Over storage time, however, the apparent viscosity of cross-linked yoghurt increased and a visibly coarse gel structure was formed which appears to be related to the high casein content. However, no formation of TG-induced covalently cross-linked protein polymers was measurable while storage. Hence, these structural changes are to be explained by a secondary effect of TG treatment, i. e. by other non-covalent bonds influencing the protein network.