Influence Of Charge Density On Chitosan-casein Complexation

It is a critical point for controlling and designing food of predicable and functionally-specific to establish the relationship between electric properties(for example charge density)of macromolecular and the structural characteristics of complex system.Chitosan as the only one positively charged polysaccharide in the nature so far,has amino and hydroxyl with the lively chemical properties,its electric properties are easy to change.So charge density of chitosan were adjusted through different condition and casein as a model protein complex with chitosan in this paper,then the stability and structural characteristics of complex and combination mechanism between chitosan with different charge density and casein were studied,the goal is to build the relationship between electric properties of chitosan and the structural characteristics and functional properties of complex system.In addition,it can provide more basis data for protein-polysaccharide complex system and direct applications for designing and controlling complex food.The main research contents and results were as follows:(1)Charge density of the chitosan were controlled through different pH,different molecular weight and different degree of the deacelation,and the results were obtained by Ohshima soft particle theory fitting.Results showed that according the increase of pH value and the increase degree of the deacelation degree,as well as lower molecular weight,charge density of the chitosan were lower.In addition,surface potential value by using the soft particles theory were lower than ?-potential value using Smoluchowski formula.(2)Charge density of the chitosan were controlled through different pH,different molecular weight and different degree of the deacelation.Turbidity analysis,?-potential,particle size,rheological steady-state viscosity and Differential Scanning Calorimetry(DSC)were systematically used to determine the stability and structural characteristics between chitosan with different charge density and casein.The effect of charge density of chitosan by different pH value to control on the formation of the complex were larger.With the increase of pH,complex formation was divided into three stages:soluble complex formation area(pH was around 3.50),insoluble complex formation area(pH was 6.50);and pH was around 7.50,the complex turbidity reached the maximum.The average particle size of chitosan-casein complex solution decreased slightly with the increase of pH and then increased drastically.In addition,The rheology steady-state viscosity of the complex results showed that viscosity of solution firstly decreased and then increased with the reduction of the charge density of chitosan.The decrease of the molecular weight of chitosan led to the shift of the pH critical point(the pH at which the soluble complex is formed is comfined pHc,the pH at which the insoluble complex is formed is comfined pHc,the pH at which the formation of the largest number and the largest particles of insoluble complexes is called pHopt)to the lower pH,the absorbance value of pHopt was greater than chitosan with higher molecular weight.DSC spectrum showed that thermal stability of the complex of chitosan with higher molecular weight and casein,chitosan with lower molecular weight and casein complex were higher than raw casein,thermal stability of chitosan with lower molecular weight and casein complex was lower than chitosan with higher molecular weight and casein.In addition,with the decrease of charge density of chitosan,the viscosity of complex solution was first increased then decreased.The influence of different charge density of chitosan through different deacelation degree to control on the pH critical points and ?-potential of chitosan-casein complex solution was not obvious.The viscosity of the complex solution reduced with decrease of charge density of chitosan.(3)The fluorescence spectroscopy,ultraviolet-visible spectroscopy and fourier infrared spectrum methods were used to analyze the interaction mechanism of chitosan with different charge density and casein.The results showed that the fluorescence quenching methods that chitosan with different charge density to casein were static quenching rather than dynamic quenching,there was one binding site between chitosan with different charge density and casein.Different charge density by different pH to control had no effect on chitosan-casein interaction types,their main force were electrostatic force.When charge density were controlled by the deacelation degree and molecular weight,the main force that chitosan with higher charge density and casein were electrostatic force,and chitosan with lower charge density and casein were mainly hydrophobic force.Chitosan with different charge density can change the hydrophobicity around casein,but the change extent were different.When charge density were controlled by different pH,chitosan with higher charge density had a greater degree of change in casein hydrophobicity,When charge density was controlled by deacelation degree and molecular weight,chitosan with lower charge density had a greater degree of change in casein hydrophobicity.Fourier Transform Infrared Spectroscopy(FTIR)spectrum showed-NH3+ group in casein and COO-group in chitosan experienced interaction.