Effect Of Milk Protein On Rheological Properties And Microstructure Of Set Yoghurt

The whey-off is One of the biggest physical defects in yoghurt manufacture. The influences ofincubation temperature, homogenization pressure, strains, stabilizers on the formation of yoghurtgels had been studied by many researchers, however the internal factors including milk protein,milk fat and ratio of casein and whey protein were seldom investigated. The content of eachcomponent of raw milk was affected by many factors, including season, temperature, cow breedand time and frequency of milking, and according to which the content of each component wouldchange to some extent. So in this way quality of yoghurt was influenced, especially the milk protein,which was the primary substance forming the yoghurt gels, have the most directly impact on theyoghurt gels forming and the physical chemistry properties and microstructure of yoghurt gels.In this study, the content of total milk solid of raw milk served as material was adjusted to 9%,the acceptable minimum, then the milk protein was reinforced to investigate the influences ofvariation of milk protein, ration of whey protein to casein on the quality of yoghurt gel. And thephysical chemistry index, rheological properties and microstructure of yoghurt and the change ofeach index of yoghurt gel during storage were compared; moreover, the relationship of physicalchemistry index, rheological properties and microstructure of yoghurt was discussed.1. The milk protein content: the hardness, viscosity and water-retaining capability of yoghurt gelincreased with the improving of milk protein content. When the addition amount of SMP wasincreased from 1% to 6%, the protein content increased from 2.93% to 4.7%, the hardness,viscosity and water-retaining capacity of yoghurt also increased from 132.305g to 209.799g, from944.972gsec to 1503.834gsec and from 28.85% to 46.37% respectively. When the additionamount of WPC was increased from 1% to 4%, the protein content increased from 3.32% to 5.55%,the hardness, viscosity and water-retaining capacity of yoghurt also increased from 144.094g to460.342g, from l139.684gsec to 3580.578gsec and from 39.63 % to 73.84% respectively.2. The effect of milk protein content on the quality of yoghurt gel during storage: when the milkprotein content was improved, the hardness, viscosity and water-retaining capability of yoghurt gelwould increase. And with the extension of storage time, the hardness, viscosity and water-retainingcapability of yoghurt gel all increase at first, then they all decreased. After 21 days storage, theascending order of diminished extent was WPC80Ⅰ, SMP, control 2 and control 1. In other words,while the content of total milk solid was the same, each physical index of the higher content of milk protein changed less.3. The ratio of casein to whey: when the milk protein content was consistent, the hardness, viscosityand water-retaining capability of yoghurt gel would increase with the decreasing of the ratio ofcasein to whey. And while the milk protein content was fixed, decreasing the ratio of casein towhey could improve the quality of yoghurt gel significantly. When the protein content was 4%,while decreasing the ratio of casein to whey, the hardness, viscosity and water-retaining capabilityof yoghurt gel increased 41.35%, 19.01% and 45.35% respectively. When the protein content was5%, while decreasing the ratio of casein to whey, the hardness, viscosity and water-retainingcapability of yoghurt gel increased 29.65%, 41.93% and 53.72% respectively.4. When the milk protein content was 4% and the ratio of casein to whey was 1:1, the hardness,viscosity and water-retaining capability of yoghurt gel were better than those of yoghurt gel whenthe milk protein content was 5% and the ratio of casein to whey was 4:1. And the hardness,viscosity and water-retaining capability of yoghurt gel were 230.63g to 210.99g, 1517.6 gsec to1493.4gsec and 59.74% to 45.38% respectively. So when the milk protein content was low,physical properties of yoghurt gel the same as or better than that of yoghurt gel with high milkprotein content could be acquired by decreasing the ratio of casein to whey.5. The rheological analysis showed that yoghurt was non-Newtonian fluid, with time dependence,thixotropy and yield stress. With the improving of milk protein content, the anti-shearing capacityof yoghurt increases significantly. And the decreasing order of different treatment was as follows:treatment with 2% WPC80, treatment with 2% SMP, control 2 and control 1. The yield stressincreased with the improving of milk protein content, and the decreasing order of differenttreatment was as follows: treatment with 2% WPC80, treatment with 2% SMP, control 2 andcontrol 1. And the decreasing order of different treatments showed that the increasing of milkprotein content could improve the hardness, viscosity and water-retaining capability of yoghurt gel.6. Microstructure: the void in network structure of yoghurt gel shrinked gradually along with theincreasing of milk protein content, and when the milk protein content was 4% and 5%, the diameterof void of yoghurt gel was 0.81μm-5.35μm and 0.635μm-4.78μm respectively. The morearborizing, the denser the three dimensions network structure. Decreasing the ratio of casein towhey, the degree of cross linking and arborization of the gel network structure were increased, thevoids became smaller and were well-distributed. When milk protein content with the ratio of caseinto whey was 4:1 was 4% and 5%, the diameter of void of yoghurt gel was 1.40μm-5.35μm and1.20-4.78μm respectively. When milk protein content with the ratio of casein to whey was 1:1 was4% and 5%, the diameter of void of yoghurt gel was 0.81μm-1.19μm and 0.635μm-1.090μmrespectively. 7. The relationship between physical properties and microstructure of yoghurt gel: the physicalproperties and microstructure of yoghurt gel influence each other, along with the increasing of milkprotein content, the quantity and intensity of linkage maintaining the network structure of yoghurtgel increased a few, so its microstructure was more well-distributed and denser, and it appeared thatthe hardness and network structure of yoghurt gel were reinforced. With the improving of waterretaining and bounding capacity, the water in network structure of yoghurt gel can't separate withcurd easily, in this way the water-retaining capacity was improved.