Treatment Of Bovine Milk By Horseradish Peroxidase And Its Impact On The Quality Of Set-Yoghurt And Cheese Prepared

Bovine milk is one kind of natural foods close to a perfect food as it provides nutrients especially calcium needed by human body. With the enhancing of people's living strandards, the nutrition and health related functions of the milk were recognized and welcome by the customers. The production and sales of dairy products are increasing in the recent year. However, the texture of dairy products might be changeable due to the limitaion of production process, transportation and storage conditions, which finally impaires the acceptance of dairy products to the customers. To solve this problem, some modification methods were used to modify the functional properties of milk proteins, among which cross-linking was applied widely. Cross-linking of food proteins is carried out by formation of covalent bonds in polypeptide chains within a protein molecule (intra-molecular) or between two protein molecules (inter-molecular), to improve the functional properties of the proteins treated.In this reseach, bovine milk or skimmed milk was treated with horseradish peroxidase (HRP) in the presence or absence of ferulic acid (FA), in which cross-linking of milk proteins was demonstrated by SDS-PAGE. The rheological properties of the treated bovine milk or skimmed milk by HRP and HRP & FA were evaluated. The textural and rheological properties of the yoghurt prepared by HRP-treated and HRP & FA-treated milk were also evaluated. The texture of Cheddar cheese prepared by HRP-treated or HRP & FA-treated bovine milk was analysised. Results of these studies were listed as follow.(1) Bovine milk or skimmed milk was subjected to a treatment catalyzed by HRP or HRP & FA. HRP-treated and HRP & FA-treated milk was analyzed by SDS-PAGE. Compared to the original milk, there were some protein polymers with molecular weights larger than 97.4 kDa in the HRP-treated and HRP & FA-treated milk. The results showed that cross-linking of milk proteins occurred in these treated milk samples.(2) The rheological properties of the control milk, HRP-treated and HRP & FA-treated milk were evaluated. Compared to control milk, the apparent viscosity, storage modulus and viscous modulus of the HRP-treated milk increased, especially when FA was added.(3) The textural and rheological properties of the set-yoghurt prepared by HRP or HRP& FA-treated milk were evaluated. Compared to that prepared from control milk, the hardness of the yoghurt prepared by HRP & FA -treated or HRP-treated milk was enhenced about 34.16 and 19.69 %, the adhesiveness of them was increased about 40.93 and 28.47%, but the syneresis extent of them was decreased about 10.81 and 6.77%. The hardness of the yoghurt prepared by HRP & FA -treated and HRP-treated skimmed milk was enhenced about 29.87 and 28.55 %, the adhesiveness of them was increased about 56.56 and 46.86%; however, the syneresis extent of them decreased about 10.73 and 5.21%. The apparent viscosity, storage modulus, viscous modulus and thixotropy of the set-yoghurt prepared by HRP & FA -treated and HRP-treated milk were all increased. If FA was added, the increase level for these parameters became more obviously.(4) The microstructure of the set-yoghurt samples prepared from the treated milk was observed under scanning electron microscope. Thee results indicated that a more compact and small pores were formed in the yoghurt sample prepared with HRP-treated milk while yoghurt sample prepared with HRP & FA-treated milk had a densest, highest branched and uniform structure. The results indicated that HRP could catalyze the cross-linking of milk proteins and lead to an improved structure in the yoghurt samples by the formation of dityrosine. The cross-linking reaction was increased and more dityrosine was formed as a result of the addition FA, which resulted in the microstructure of yoghurt sample to be the compactest.(5) Cheddar cheese was prepared by HRP-treated or HRP & FA-treated milk, and evaluated for its texture during ripening. The evaluation results showed that the hardness, spring, cohesive and chewiness of the cheese was ordered as cheese prepared by HRP & FA treated milk, cheese prepared by HRP treated milk, control cheese.