The Interaction Between Carboxymethylcellulose (CMC) And Casein Micelle And The Stabilization Mechanism Of Acidified Milk Drinks Induced By CMC

Dairy industry in china has made significant development in recent years. The increase of dairy production is the highest among all kinds of agriculture products and lasts for a long period of time. Dairy industry has been widely regarded as a prosperous industry. One prominent feature of dairy industry development is the large increase in production and consumption in liquid milk. Acidified milk drinks are one kind of liquid milk. They are popular worldwide for its good feeling and health function. However, the flocculation and sedimentation of milk protein in low pH is a key factor on production and research in acidified milk drinks. Therefore, there are many works focused on the interaction between food hydrocolloids and milk protein and the stability of acidified milk drinks induced by them all over the world.CMC is widely used as a stabilizing agent in acidified milk drinks in Asia, especially in China. The addition of CMC could effectively avoid the flocculation of milk proteins and subsequent macroscopic whey separation. The production of acidified milk drinks keeps fast development in recent years for its good flavor. It is important to avoid the flocculation and separation of casein micelles in low pH during the investigation and production of acidified milk drinks. However, the investigation on the stability of acidified milk drinks induced by CMC only limited on the use of complex polysaccharides and optimizing the process of production. Little work has done on understanding the stabilization mechanism of acidified milk drinks induced by CMC. In the dissertation, the interaction between casein micelle and CMC and the influence on the interaction were systematically investigated. The stabilization mechanism of acidified milk drinks induced by CMC was studied. In addition, the influence on the stability of acidified milk drinks was also investigated. The main contents and conclusions are listed as follows.CMC in aqueous solution is mainly used in application that exploits its rheological property. The rheology of CMC aqueous solution and the influence on it during the production of CMC were studied firstly. CMC aqueous solution shows pseudoplastic behavior. The shear viscosity of CMC solution decreases with increasing the shear rate. Both the increase in Mw of CMC and an increase in segment density with concentration lead to a rise in the zero-shear viscosity via an increase in the intermolecular interactions. The zero-shear viscosity rises with increasing DS of CMC because the proportion of solvated molecules rise lead to the expansion of CMC coil and therefore the increase in the hydrodynamic volume of the CMC molecule. The viscosity of CMC solution decreases with decreasing pH and then increases a little when the pH lowers than pKa. Further decreasing pH leads to sedimentation of CMC from solution. CMC is a polyanionic polysaccharides. The viscosity of CMC solution decreases by adding Na+, K+ electrolytes for screening the like charges. Bivalent electrolytes also hider the volume expansion of a polyelectrolyte coils. An increase in the salt concentration first causes a fall in viscosity due to compensation for the polyelectrolyte effect. The viscosity minimum is reached at a ratio of calcium ions to charged side groups of approximately 0.5. This corresponds to complete compensation for the charge of the side chains by the bivalent cations and a maximum intramolecular compensation of the polyelectrolyte. If the salt concentration is further increased, the viscosity rises again because the formation of superstructures via bridging calcium ions. Similar behavior is caused by adding another bivalent electrolyte Mg2+. The concentration of Mg2+ which leads to flocculation increases due to the smaller radium of Mg2+. The addition of sugar will adsorb water which equal to increase the concentration of CMC and, therefore, lead to the increase in viscosity of CMC. CMC is water-soluble cellulose. The method to dissolving it has no influence on the rheology property of CMC solution.The electrosorption of CMC onto casein micelles takes place when the pH of CMC/casein micelle mixture at and below 5.2. The stability of casein micelles at low pH induced by CMC could be explained by steric stabilization caused by the anchor of CMC onto casein micelles' surface rather than electrostatic repulsion. The molecular properties of CMC have influence on the diameter of casein micelles during acidification. The results suggested that the CMC with high Mw or low DS will result in thick adsorbed layer onto casein micelles. Above pH 5.2 phase separation of the CMC/casein mixture takes place corresponding to thermodynamic incompatibility. The upper phase concentrated in polysaccharides and the lower concentrated in casein micelles.The stability of acidified milk drinks depended on the pH and CMC concentration added. The stability of acidified milk drinks composed of 4%MSNF/0.4%CMC was investigated. The results showed that above pH 5.2 phase separation took place corresponding to thermodynamic incompatibility because there was no interaction between CMC and casein micelles, while below pH 5.2 the adsorption of CMC leads to either stabilization by full coverage or bridging among two or more casein micelles with lower concentration of CMC.The stabilization mechanism of acidified milk drinks induced by CMC was further clarified on investigation the interaction between CMC and casein micelles. It can be understood that: one part of CMC added adsorbed onto casein micelles by electrosorption at low pH and the adsorbed CMC layer on the surface of casein micelles could prevent flocculation of casein micelles by steric repulsion. While, there was no network composed of casein micelles and non-adsorbed CMC in acidified milk drinks. The non-adsorbed CMC only increased the viscosity of serum and slowed down the sedimentation velocity of casein micelles and thus contribute to the stability of acidified milk drinks.The interaction between CMC and protein particles was different in direct acidified milk drinks and yoghurt drinks. Therefore, the stability of direct acidified milk drinks induced by CMC was different from yoghurt drinks. The adsorbed amount of CMC onto casein micelles did not increase when the full coverage taking place in direct acidified milk drinks. However in yoghurt drinks the adsorbed amount of CMC increased with increasing the CMC concentration. The excess fraction is crucial in effecting sufficient CMC adsorption during the mixing process of yoghurt and CMC solution. Pasteurism had no influence on the CMC adsorbed amount in direct acidified milk drinks; however, it led to the decrease of CMC adsorbed amount in yoghurt drinks.The stability of acidified milk drinks can be influenced by many factors, such as the molecular properties of CMC, homogenization pressure, different casein micelle's conformation and so on. The results showed that in direct acidified milk drinks the particle size decreased and the viscosity increased with high Mw CMC. The stability of direct acidified milk drinks had been improved a little with high DS of CMC which corresponded to the favor of interaction between CMC and casein micelles. The increase of CMC Mw will play a major role on the stability of direct acidified milk drinks, while the DS of CMC has little influence. The addition of sucrose will increase the viscosity of acidified milk drinks and improve the stability of acidified milk drinks. The protein particle size will obviously decrease, however, the sedimentation and serum fraction increases with increasing the homogenization pressure. This result was related to the decrease on the viscosity of acidified milk drinks because the Mw of CMC decreased with improving the homogenization. The Brookfield viscosity of directly acidified milk drinks and yoghurt drinks both increased with increasing the CMC concentration and decreased with lowing pH. The Brookfield viscosity of yoghurt drinks is always larger than that of directly acidified milk drinks in the same condition. In stable sample, the particle size in yoghurt drinks was 3-6μm, which was much larger than that in directly acidified milk drinks (1μm). The sedimentation of yoghurt drinks (1-5%) was more than that in directly acidified milk drinks (<1%). It is that the stability of acidified milk drinks was better than that of yoghurt drinks induced by the same concentration of CMC and at the same pH.The innovations of this dissertation are listed as follows:(1) The interaction between CMC and casein micelle and the influence of CMC molecular properties on the interaction were investigated on molecular level. The results showed that electrosorption of CMC onto casein micelles took place when the pH at and below 5.2. The stability of casein micelles at low pH induced by CMC could be explained by steric stabilization caused by the anchor of CMC onto casein micelles'surface. The molecular properties of CMC have influence on the diameter of casein micelles during acidification. CMC with high Mw or low DS will result in thick adsorbed layer onto casein micelles.(2) The stabilization mechanism of acidified milk drinks induced by CMC was systemically investigated firstly. One part of CMC added adsorbed onto casein micelles by electrosorption at low pH and the adsorbed CMC layer on the surface of casein micelles could prevent flocculation of casein micelles by steric repulsion. While, there was no network composed of casein micelles and non-adsorbed CMC in acidified milk drinks. The non-adsorbed CMC only increased the viscosity of serum and slowed down the sedimentation velocity of casein micelles and thus contribute to the stability of acidified milk drinks.(3) The different stable behavior between direct acidified milk drinks and yoghurt drinks was compared. The adsorbed amount of CMC onto casein micelles did not increase when the full coverage taking place in direct acidified milk drinks. However in yoghurt drinks the adsorbed amount of CMC increased with increasing the CMC concentration. The excess fraction is crucial in effecting sufficient CMC adsorption during the mixing process of yoghurt and CMC solution. Pasteurism had no influence on the CMC adsorbed amount in direct acidified milk drinks; however, it led to the decrease of CMC adsorbed amount in yoghurt drinks. The stability of direct acidified milk drinks was better than that of yoghurt drinks when containing the same MSNF and CMC concentration at the same pH.