Characteristics And Forming Mechanism Of Chinese Buffalo Milk Acid Gel

The gel plays an important role in food protein making process. Yogurt gel as one of the nutritional and delicious dairy products is popular all over the world. Currently, most of the yogurt gel was fermented using Holstein milk by lactic acid bacteria fermentation. Buffalo milk as a high quality and nutritional value of milk resource, fermented to buffalo yogurt gel occupies an extremely important status and a huge potential development. Yogurt gel property is essential for yogurt quality; thus, it is necessary to understand the macromolecules interaction between the yogurt gel and the formation mechanism to improve the theoretical basis. Therefore, in our work, the physical and chemical characteristics of the three kinds of buffalo milk during milk making process was studied; the textural properties of buffalo milk acid gel with or without exopolysaccharides (EPS) were analyzed; meanwhile the structure changes of casein (CN) and the interaction between CN and whey protein (WP), EPS, oleic acid were also explored. The following are some main results from our work.Comparing the components among different types of buffalo milk, the protein, fat and total solids contents in hybrid buffalo milk were higher than those of Murrah and Holstein milks, so did the stability. The buffer capacity of buffalo milk was higher than that of Holstein milk, while Murrah milk was higher than hybrid offspring, but there were no differences between hybrid offspring milks. Viscosities of four kinds of milk decreased gradually with the increasing temperature and dilution factors. The surface tension of milk dispersion system was smaller under45℃, pH6.6, or6times dilution. Zeta potential, conductivity and stability of the milk were significantly affected by pH value. SDS-PAGE displayed slightly smaller molecular weight of αs-casein in the buffalo milk compared to the cow milk. On the contrast, the molecular weight of P-casein of the former was slightly larger than that of the latter. Furthermore, Guangxi buffalo milk casein was lack of one kind of as-casein.Three buffalo milk was used to ferment yogurt gel by lactic acid bacteria. It is found that the rheological and textural properties of different kinds of buffalo acid gel were different. The apparent viscosity of the acid gel was proportional to the total solids content of the milk. The gel hardness made from hybrids buffalo milk was lower than that made from the male parent (Murrah buffalo milk); however, the syneresis (STS) was higher than Murrah buffalo milk high. The simple correlation and canonical correlation analysis showed the significantly related between hardness, gumminess and milk protein content.The isolated exopolysaccharides (EPS) were added to the different types of buffalo milk and Holstein milk, fermented by lactic acid bacteria. The effect of EPS addition on textural properties of different types of milk acid gel was different. The impact of EPS on the properties of (hybrids) F1was remarkable. EPS had a certain role in promoting the formation of milk acid gel and the denser texture characteristics, its viscoelastic and water holding capacity (WHC) were proportional to the amount of EPS addition. Confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) were used to observe the microstructure of buffalo milk acid gel with or without EPS addition. The original structure of the micelles was damaged by the electrostatic interactions and steric barrier of EPS and protein with the increasing EPS addition. The dimensional network structure of casein micelles gathered and rearranged, which resulted in forming the new three-dimensional structure of acid gel system, showing the hexagonal porous state. Circular dichroism (CD), fluorescence spectroscopy and particle size analysis were used to explore the secondary, tertiary and quaternary structure of CN as functions of protein concentrations, pH and ionic strength. The results showed that the CN structure was significantly influenced by protein concentration, pH and ionic strength. Buffalo milk concentration significantly affected the secondary structure of the CN. When the protein concentration was low, the effect on tertiary structure was not obvious. However, the fluorescence intensity linear increased with the protein concentration, resulting casein rearrangement, hydrophobic groups exposed, and the particle size increased. Within the scope of this study, the secondary structure of buffalo milk CN was the most stable at pH6.0. In the higher ionic strength, lower pH value environment, the tertiary structure of buffalo CN was influenced evidently.CD, Trp and ANS fluorescence spectroscopy were used to characterize the interaction between CN and whey protein (WP). It is found that the protein concentration, ionic strength and pH affected the interaction between CN and WP. The secondary structure of CN/WP system was influenced by both the protein concentration ratio and the ionic strength, and strongly affected by the environmental pH value. When the ratio of CN/WP was5:3, the secondary and tertiary structure of CN/WP system changed visibly. The structure of the CN/WP system changed with the gradually increasing ionic strength, showing Trp intermolecular quenching increased, especially during0.01M-0.15M. A certain degree of interaction between NC and WP occurred in pH6.0-pH5.0, and significantly in pH6.0, the secondary structure of the CN/WP system expressed as the increase of the β-sheet; meanwhile, it affected the tertiary structure of the CN/WP system, resulting in Trp exposed in a hydrophobic environment, the hydrophobic groups exposed.CD, Trp and ANS fluorescence spectroscopy were used to characterize the interaction between CN and EPS. It is found that the protein concentration, ionic strength and pH affected the interaction between CN and EPS. The CN structure changed significantly accompanied by the emergence of the EPS, under different pH value. The lower concentration of EPS expressed the stronger impact on the secondary and tertiary structure of CN; furthermore, the marked impact on CN structure under the lower pH value, the low concentration of EPS increased the a-helix content of CN, while the high concentration of EPS resulted in the secondary structure of CN mainly in β-sheet. The secondary structure of CN was changed by the emergence of the EPS as a function of ionic strength, showing the a-helix was destroyed. However, compared with the pH, the higher ionic strength slightly affected the CN structure. EPS concentration was less impact on the tertiary structure of CN when the ionic strength was in0.15M-0.30M.CD, Trp and ANS fluorescence spectroscopy were used to characterize the interaction between CN and oleic acid. The interaction between CN and oleic acid was affected the protein concentration, ionic strength and pH. CN structure changed significantly with oleic acid added varied pH, and the impact was the greatest under the neutral condition. Nevertheless, high concentration of oleic acid influenced the secondary and tertiary structure of CN, showing the increased a-helix content of CN, the increased aggregation, and a large number of hydrophobic groups were masked and Trp was exposed to the hydrophobic environment. While the low concentration of oleic acid slightly impacted on the secondary structure of CN. The (3-sheet of CN increased with the emergence of the oleic acid under pH5.0. The secondary structure CN changed obviously with the amount of oleic acid added range from0.5mM to1.0mM; however,0.5mM oleic acid had a marked impact on the tertiary structure of CN. Oleic acid had notable effect on CN structure under the lower ionic strength, but slight effect on the secondary structure of CN and a marked impact on the tertiary structure of CN and under the higher ionic strength, the hydrophobic groups were hided.