Studies On Regulation And Mechanism Of Sodium Caseinate-based Foam Systems

Food components(proteins,polysaccharides,lipids,etc.)and their phase characteristics(aggregated state,micellar state,and micro/nano-particles,etc.)often adjust the physical and chemical properties(appearance,flavor,stability,rheology,and bioavailability,etc.)of the interface-oriented food system by changing the multiphase interface(liquid/liquid,air/liquid,solid/liquid,etc.)properties,controlling the formation of the interface film,etc.The seemingly simple food has the characteristics of multi-component,multi-phase and multi-scale,which makes the in-depth study of the real food system extremely difficult.Foamy foods such as whipped cream,mousse,beer,and cakes provide consumers with fine visual enjoyment and dense taste texture due to the existence of a large number of air/water interfaces.However,maintaining the stability of the foam system has always been a huge challenge,which is a difficult point in food science research.The formation and stability of protein foam is mainly determined by the molecular layer on the interface,and its properties can be adjusted by a variety of methods,so as to reasonably improve the desirable characteristics of the macro-foam.Therefore,the hierarchical study of the relationship between the structure,interaction,interface behavior and macroscopic properties of interface active molecules has become an important part of the current research in this field.Based on the above background,sodium caseinate(Na-Cas),tannic acid(TA)/gallic acid(GA),and octenyl succinic anhydride starch(OSA-starch)/Carboxymethyl cellulose(CMC)were used as the research objects to construct a multi-component system in this thesis,so as to(1)reveal the interaction mechanism between food components,(2)establish the relationship between the structural characteristics of the product and its air/water interface behavior and the properties of the macro-foam,(3)provide the guidance for research of food multi-component complex system and its processing and functional properties utilization.The main research results of this thesis are as follows:1.The changes in turbidity,particle size,secondary structure,and heat flow of binding caused by the combination of Na-Cas and TA were evaluated by spectroscopy and thermodynamic methods,respectively.The results show that the combination of Na-Cas and TA caused the particle size of the complex to decrease first and then increase.Excessive TA led to protein aggregation,which increased the turbidity of the system.TA led to a change of the microenvironment of amino acid residues in Na-Cas,and an increase of the number of?-sheets,thereby causing a change of the secondary structure of Na-Cas.Correspondingly,the interaction between Na-Cas and TA was a spontaneous exothermic process.It was measured that about 1.033 TA molecules bind to 1 Na-Cas molecule and the combination was dominated by a hydrogen bond.2.Dynamic light scattering(DLS),fluorescence probe(ANS),and interfacial rheological technology were used to systematically evaluate the relationship among the phase behavior,interface behavior of Na-Cas/TA complex and macro-foam properties,respectively.The results showed that with the increase of TA concentration,the particle size and surface hydrophobicity(H₀)of Na-Cas/TA complex decreased,while the number of negative charges increased.The surfaces activity of the Na-Cas/TA complex decreased gradually as the increase of TA concentration,which led to the decrease of foamability of the Na-Cas/TA complex.The interfacial rheology results showed that the interface layer stabilized by Na-Cas/TA complex was dominated by elastic behavior,and the elastic modulus(E)of the interface layer increases with the increase of TA concentration so that the foam system stabilized by Na-Cas/TA complex has excellent stability.3.The regulation mechanism of the air/water interface stabilized by Na-Cas/TA complex was analyzed at the molecular level by exploring the adsorption kinetics of Na-Cas/TA complex on the air/water interface and the interface dilatational rheological properties,as well as the quantitative evaluation of interface protein components.The results showed that the size of the bubbles in the foam system stabilized by the Na-Cas/TA complex was kept within a small range,and the complex fused at the air/liquid interface and tightly wrapped on the surface of the bubble during the storage of the foam,thus forming a huge space barrier to prevent the disproportionation and coalescence of bubbles.The particle size of the Na-Cas/TA complex formed by hydrogen bonding was smaller than that of a single Na-Cas,resulting in a greater diffusion rate of the Na-Cas/TA complex.However,the addition of TA reduced the surface hydrophobicity and increased the surface charge of the Na-Cas/TA complex,and the foamability of the complex system was lower than that of Na-Cas,indicating that the surface hydrophobicity and surface charge of the complex play a decisive role in the foamability in this study.As the concentration of TA increases,the interface layer formed by the Na-Cas/TA complex has a higher dilatational viscoelastic modulus(E),which was mainly due to the enhancement of the Na-Cas interaction at the interface caused by the TA;The interface layer formed by the complex has both rigidity and flexibility,which could quickly and accurately respond to external deformation and prevent the interface from being broken by force,thereby maintaining the long-term stability of the foam system.Through quantitative analysis of interface components,it is found that the addition of TA led to an increase in the adsorption of?-casein on the interface,which is conducive to the formation of a more elastic air/water interface film,thus helping to improve the stability of the foam system.4.According to the mechanism for the influence of TA on the functional properties of Na-Cas,the interactions between Na-Cas and GA and their interfacial behavior and foam properties were comparatively studied.The results showed that the binding process of Na-Cas and GA was mainly driven by hydrophobic interaction and hydrogen bond.GA binding to protein per mg increased with the increase of GA concentration.The surface hydrophobicity of Na-Cas was significantly reduced after combining with GA.Moderate concentration of GA could improve the surface activity of Na-Cas,enhance the foam stability of Na-Cas,and with the increase of GA content,the foam stability of the complex increases gradually.The results of interfacial rheology showed that the combination of GA and Na-Cas significantly improves the viscoelasticity of the interface layer.Compared with a single Na-Cas,the interface stabilized by Na-Cas/GA complex has higher interfacial viscoelasticity,showing better foam stability.The corresponding relationship between foam stability and interfacial viscoelasticity was verified by the Lissajous curve,that is,adding GA enhanced the interaction between proteins at the interface,increasing viscoelasticity of interfacial layer and the stability of foam system.Because of the complexity of interface layer structure,the macro properties and micro interface properties are not always one-to-one correspondence.The Na-Cas/GA complex with a mass ratio of 1:0.5 exhibited good foamability,but the existence of GA reduces the rate of Na-Cas/GA complex diffusion from solution to the interface.5.The regulation of OSA-starch on the interfacial behavior and foam properties of Na-Cas-based complex systems(Na-Cas and Na-Cas/TA)was evaluated.The interfacial behavior and macro-foam characteristics of the Na-Cas/TA/OSA-starch complex was characterized.The rheological properties of the interface layer were studied by linear and nonlinear rheological methods,and the nonlinear rheological behavior of the interface layer was quantitatively analyzed by the Lissajous curve.The results showed that OSA-starch had a significant effect on the foamability of the Na-Cas-based complex system.Compared Na-Cas and Na-Cas/OSA-starch,Na-Cas/TA and Na-Cas/TA/OSA-starch,their foamability increased significantly,while the complex system still maintained excellent foam stability.The addition of OSA-starch significantly reduced the bubble size in the foam system.Na-Cas/OSA-starch complex had the lowest initial adsorption value,which was mainly because OSA-starch with surface activity can also be adsorbed on the interface,thus reducing the surface tension.Compared with Na-Cas/TA,Na-Cas/TA/OSA-starch complex system had lower initial surface tension and final surface tension,which led to better foamability of Na-Cas/TA/OSA-starch.Compared with Na-Cas/OSA-starch complex,the stable interface layer of the Na-Cas/TA/OSA-starch complex had higher interfacial dilatational viscoelastic modulus(E),and Na-Cas/TA/OSA-starch foam had higher foam stability.Therefore,there is an important correspondence between interfacial viscoelasticity and macro-foam stability.The results of nonlinear interfacial dilatational rheology and Lissajous curves showed that the interface formed by the Na-Cas/TA complex had a high dilatational viscoelastic modulus(E),which exhibited a solid-like elastic behavior and strain hardening occured during the extension and compression process,indicating that the interface was a highly elastic two-dimensional gel structure.In the interface stabilized by Na-Cas/TA/OSA-starch complex,adding OSA-starch increased the strain softening of the interfacial film and the strain hardening during compression,revealing that the interface layer may be transformed from the gel structure formed by the Na-Cas/TA complex to the structure mixed by Na-Cas/TA and OSA-starch domains.6.The regulation mechanism of OSA-starch on the interface behavior and foam properties of the Na-Cas-based complex was evaluated by constructing a phase diagram,characterizing the properties of the air/water interface and the rheological properties of the foam.The results showed that OSA-starch could promote the interaction between Na-Cas and TA and produce soluble complexes at p H 6.By characterizing the diffusion,adsorption,and rearrangement process of the complex from the bulk to the interface,it was found that the presence of OSA-starch could promote the diffusion of the Na-Cas-based complex,thereby enhancing the foamability of the complex.OSA-starch promotes the adsorption rate of the Na-Cas-based complex on the interface.The limited thermodynamic compatibility between the Na-Cas-based complex and OSA-starch near the air/water interface leads to an increase in the adsorption concentration of the Na-Cas-based complex at the interface through a depletion mechanism.On the other hand,due to the binding effect,there was synergistic adsorption between the Na-Cas-based complex and OSA-starch,increasing surface pressure(decrease in interfacial tension).Due to the shear thickening properties of OSA-starch,the viscosity of the foam system stabilized by Na-Cas/OSA-Starch and Na-Cas/TA/OSA-Starch complex increases,thereby contributing to the stability of the foam.At the same time,the synergistic effect of OSA-starch and TA improved the overall yield stress of the foam system,thereby helping to improve the response of the foam to external forces and conducive to the stability of the foam.The results of micro-rheology further confirmed the above conclusions and revealed that TA bridges Na-Cas and OSA-starch to form a highly elastic interface film,which is beneficial to the stability of the foam system.7.The regulation mechanism of carboxymethyl cellulose(CMC)on the interface properties and foam properties of Na-Cas-based complex was comparatively evaluated by selecting carboxymethyl cellulose(CMC)of different viscosities.The results showed that the foamability and foam stability of the Na-Cas-based complex were improved in the presence of CMC,and the foam stability of the complex was significantly enhanced.By observing the change of bubble size with time,it was found that the bubble size in foam is significantly reduced after adding CMC,and without change over time.This is because CMC has a certain viscosity,which could hinder the discharge of liquid between the interface layers,keeping the bubbles moist.Observing the microstructure of the bubble interface,it was found that the Na-Cas/TA/CMC complex could adsorb and fuse on the air/water interface to form a tight interface film,which may be the main reason for the higher foam stability of the complex.Through the interface rheological analysis,we obtained the relevant information of the complex interface layer.In comparison,Na-Cas/TA and Na-Cas/TA/CMC complexes have a higher interface complex dilatational viscoelastic modulus than Na-Cas and Na-Cas/CMC,which meant that the interfacial film formed by Na-Cas/TA and Na-Cas/TA/CMC complexes was more viscoelastic,and the foam system exhibited better stability.