Studies On The Stabilization Mechanism Of Casein In The Acidified Milk Drinks Induced By CMC And Soybean Polysaccharide

Casein precipitation under acidic conditions is a key issue on the production and development of acidified milk drinks (AMDs). Researches on the stabilizers/casein interaction and the stability of the final products have become hot in the Dairy Science. The studies on the CMC are limited to the product formulas and process optimization. The soybean polysaccharide (SSPS) products in China are still rare and the applications quite few. Studies on the stabilization mechanism of casein induced by CMC, SSPS and process parameters on the stability of AMDs can provide a theoretical reference for compound stabilizers development, and a practical guide for production optimization and the final products stability. The main contents and conclusions of this paper are listed as follows:For CMC/casein complex system and the SSPS/casein complex system, the potentiometric titration, turbidity,ζ-potential, particle size and fluorescence intensity had been studied during acidification. The initial pH (pHc) that CMC or SSPS interacted with casein was 5.3±0.1. After interaction, the turbidity of system increased. Casein isoelectric point is pH 4.53. The interactions between casein and CMC or SSPS were pH-related and occurred before casein precipitation by electrostatic adsorption. CMC adsorbed onto casein micelle at a relatively high speed, while SSPS gradually adsorbed. Electrostatic adsorption is reversible. When pH was higher than the casein isoelectric point, desorption occurred. Great changes had taken place to the casein micelle during electrostatic adsorption, for polarity of micro-environment where Trp residues located decreased. Na+ did not change the initial pH (pHc), but caused the decrescence of particle diameter. While the entry of Ca2+ led to Calcium Bridge between casein and CMC or SSPS, changed the pHc value and increased particle diameter.The studies, which focused on the relationship betweenζ-potential and particle diameter under acidic conditions and effects of rennet, cellulase, pectinase,1,4-β-D-galactosidase, as well as Na+, Ca2+on the stability of system, showed that there existed a criticalζ-potential value to ensure casein stable. For CMC/casein complex system, the criticalζ-potential value was -20mV and for SSPS/casein complex system, the criticalζ-potential value was-15mV. The hydrophilic peptide (106-169) of K-casein had an effect on casein stability. The greater hydrolysis degree it was, the higher pHc value that casein/CMC or casein/SSPS interaction occurred and the larger particle diameter would be. CMC stabilized casein mainly through electrostatic repulsion. The presence of adsorption layer can increase energy barrier for particles interaction. The stabilization mechanism of casein induced by SSPS is mainly steric repulsion and the electrostatic repulsion can also contribute to the stability. Ions changed the electric double layer's thickness and charge conditions of particles, but Na+ had a smaller effect on the stability of casein than Ca2+. In addition, the adsorption layer on the casein micelle can reduce hydrolysis efficiency of K-casein by rennet.The studies on phase states of system at different pH values and with different CMC, SSPS concentrations showed that phase state and pH had great relevance. At pH>5.2, the depletion flocculation occurred due to thermodynamic incompatibility; At pH5.2 or 5.0, phase separation occurred because of weak adsorption; At pH<5.0, the system exhibited uniform and steady state.Phase state was also related to CMC or SSPS concentrations. In the neutral condition, there was a single-phase system with low CMC or SSPS concentrations and phase separation occurred when concentration increased. In acidic condition with low CMC, SSPS concentrations, bridging flocculation precipitation could easily occur and increased concentration was beneficial to the stability. Too much SSPS, to the contrary, did harm to the stability. Water mobility increased as the CMC concentration decreased. For SSPS/casein complex system, water mobility was the worst with 0.4% SSPS. Compared to SSPS, CMC was more effective on the casein stability.Effects of different process conditions on the AMDs stability were studied. Results showed that the AMDs stability increased as the CMC concentration increased and viscosity can contribute to the stability. The AMDs was the most stable when 0.4% SSPS was added, but too much SSPS was not conducive to the stability. During acidification, Ca2+ in CCP (casein calcium phosphate) would dissolve, resulting in increased Ca2+ activity. Adding chelating agents can increase the AMDs stability while the adding orders affected the stability. At pH 3.6-4.4, adding more than 0.4%CMC can make AMDs stable and with lower pH, the sedimentation, particle diameter and viscosity all reduced. AMDs with SSPS were more stable at higher pH values in pH 3.6-4.4 range. When acidification temperature was 20℃, the sedimention of AMDs was the lowest. Accelerating stirring speed and acidification speed were both beneficial to the stability. Homogeneous can reduce sedimention and viscosity, and affect the particle diameter distribution. But homogenization pressure should be appropriate, or higher pressure may lead to instability.