Research On The Mechanism Of Stabilisation Of Acid Casein Gels By High Methoxyl Pectin In Yogurt Drinks

Yogurt drinks are prepared by fragmentation and further dilution of yogurt, with addition of sweetners and food acid, but are prone to textural instability during storage and the main defect are fat floating and whey separation as the result of network structure of casein and aggregation of fragment acid-casein gels after fermentation by lactic acid bacteria. Hydrocolloids have been widely used in stabilization of fermented milk products to solve this problem traditionally. However, experience in manufactory showed that acid-casein gels could be stabilized after denaturation and sedimentation by hydrocolloids. High methoxyl pectin (HMP) is preferred in acidic milk beverages in western countries and the mechanism of interactions between casein and HMP is not clear yet. Therefore, the research on the mechanism of stabilization of fragment acid-casein gels by HMP can provide a theoretical reference and practical guide for optimization of production and stability of products. The main conclusions of this paper are listed as follows:Acid casein gels occurred to be the phase transition at63℃around, and value of thermodynamic parameters exhibiting reduced compared to the casein without fermentation led to instability. Moreover, the higher temperature of phase transition was attributed to addition of HMP. And the system showed existence of a gel-like network with0.4%and0.6%concentration of HMP.The simulated system incorporating of HMP showed typical shear-thinning behavior, and an increase in concentration of HMP was accompanied by increases in consistency coefficient K and pseudo-plasticity shown by a decrease in the flow behavior index n. And increases of apparent viscosity and consistency coefficient indicated a strong thixotropy. Moreover, a threshold of HMP concentration might exist, the system presented a relatively liquid-like behavior in terms of concentration above the sufficient value, when the addition amount reached value, the system exhibited dependence and hysteresis of frequency. And the presense of a gel-like network was confirmed.Viscoelasticity appeared to a slight increase after the tendency of decrease with the drop of pH. The negative charge of polymer chains of HMP reduced and the effect of electrostatic adsorption weakened shown by collapse of the expanded layer due to the lower pH, when it fell to3.4, side branches of neutral sugar in HMP polymer chains tended to spread outward and provided function of stability. Besides, addition of a certain concentration of salt solution to simulated system could effectively reduce the degree of aggregation, particle size and apparent viscosity of complexes.Interactions between acid casein-gels and HMP were attributed to van der Waals force, hydrogen bonding, hydrophobic interaction and electrostatic interaction, etc. And electrostatic interaction gave priority to all kind of roles, while hydrogen bonding and hydrophobic interaction is the supporting role, and van der Waals force could be neglected.The turbidity, particle size and Zeta potential of simulated system were demonstrated relevant to HMP concentration and pH. In case of low HMP concentration, system at lower pH might result in flocculation phenomenon shown by larger turbidity, particle size and lower zeta potential with the decrease in steric stabilization; When the concentration reached a critical value, HMP adsorbing sufficiently led to smaller particle size and higher zeta potential; The adsorption reduced gradually after pH reached pKa value of carboxyl of HMP chains; HMP/casein gel complexes with pectinase could cause instability, presented by a decrease in zeta potential gradually, and a slight increase in particle size after a decrease.Stability of yogurt drinks was determined with HMP concentration and the final pH. The result indicated that serum separation of samples with HMP was dramatically reduced compared to samples without HMP; Lower HMP concentration (<0.2%) and final pH (<4.0) might easily cause instability between particles inducing bridging flocculation; The raise of HMP concentration gave rise to increases in viscosity, particle size and zeta potential; Further increase in HMP concentration could promote the influence of pH on particle size, while the zeta potential almost remained; Besides, it turned out that in yogurt drinks of practical concentration most part of HMP was not absorbing onto casein gels and was involved in a network with HMP/casein complexes.