Effect of TGase and laccase on the quality of probiotics yogurt

In order to contrast the effect of adding TGase or laccase on the sensory quality,protein cross-linking and texture change of bifidobacteria-yoghurt. The rate of change of free amino groups,sensory scores,texture charateristics,apparent viscosity,protein bands and microstructure of the enzymatic cross-linking probiotic yogurt were measured and analyzed,and adding ferulic acid to improve the quality of lactase yogurt. The results showed that the rate of change of free amino groups,the sensory scores and hardness value of yoghurt treated with TGase were significantly increased(p<0.05). Gumminess,cohesiveness and apparent viscosity were increased and then decreased(p<0.05),and no significant difference(p>0.05)for adhesiveness was found with increasing addition of TGase. The optimal addition of TGase was 1.8 U/g. With elevating addition of laccase,increasing and then decreasing trends for the rate of change of free amino groups,sensory scores,hardness,adhesiveness,gumminess,cohesiveness and apparent viscosity were observed(p<0.05). The optimal addition of laccase was 0.3 U/g.The sensory and qualitative properties for laccase cross-linked yogurt were significantly improved(p<0.05)by adding 4.5 mmol/L ferulic acid. There was a shortage of β-lg in all yogurt. Protein bands of κ-CN and β-CN of yoghurt treated with TGase disappeared,and new proteins formed,while compared with the control and yogurt treated with TGase,protein bands of yogurt treated with laccase or with ferulic acid and laccase were significantly widened about 14 kDa of molecular weight. The three-dimensional network structure of yogurt by enzyme crosslinking became denser than control. Yogurt treated with TGase had more uniform distribution of particles and more compact network structure than laccase cross-linking yogurt. And the addition of ferulic acid was able to make protein micelles of laccase cross-linking yogurt more tightly packed together. Therefore,different types of enzymes had different catalysis on milk protein cross-linking,but all improved the sensory and edible quality of yogurt.