Walnut Protein-polysaccharide Interactions At The Oil/Water Interface:Effect On The Properties Of Emulsion

China is the largest producer and consumer of walnut in the world. Walnuts oil is major product of walnut production and walnut oil production produces a lot of defatted walnut flour. The complex components of defatted walnut flour and scant information in walnut proein have hampered its potential widespread utilization, accounting for waste of this protein resources. Walnut milk beverage is well-received by consumers for its unique flavor and high nutrient. However, complexity in components as well as high content of fat in these protein drinks tend to make the system instable.Based on the deficiency of the study of walnut protein’s structure and functional properties and the disability of during the process of production of walnut milk beverage, in the present work, structural and functional properties of walnut and its fractions was firstly studied. Then the interactions between walnut protein(WP) and xanthan gum(XG) or guar gum(GG) in aqueous solution, at the oil/water interface and in emulsion system were investigated.Walnut protein and its components(glutelin and globulin) were extracted from defatted walnut flour. Then the physicochemical and functional difference and similarities of walnut protein, glutelin and globulin were studied. Walnut protein had higher denaturation temperature Td(104.42 °C) and higher enthalpy ΔH(12.93 J/g) than glutelin and globulin, with higher cooperativity of the thermal transition. For the sulfhydryl contents, walnut protein was the lowest while globulin was the highest. Most sulfhydryl groups of walnut protein were buried inside while those of glutelin and globulin were at the surface. The surface hydrophobicity index of walnut protein, glutelin and globulin were 963, 1230 and 634 respectively. The results showed that walnut protein has a more compact tertiary structure, but globulin structure is relatively loose. The three proteins mainly in β-folded and random coil states. The emulsifying stability of walnut protein was the strongest among the three due to its compact structure and high disulfide content, whereas glutelin exhibited the worst emulsifying stability. Globulin had a loose structure with more active sites, exhibiting greater gel properties than walnut protein and glutelin.Effect of polysaccharide concentrations on the interactions between WP and polysaccharides at neutral pH were studied. In the range of 0-0.3 wt%, the addition of gums promoted the adsorption and cross-linking of WP at the oil/water interface by forming complex through mainly hydrophobic interactions or hydrogen bonding. The higher the concentration of gum, the higher was the interfacial activity of WP, so did the interfacial dilatational modulus. The addition of polysaccharides increased the droplet size, but decreased the stability of emulsion. The 0.05 wt% XG create the worst stability via a bridging flocculation mechanism. When the XG and GG concentration reached 0.1 and 0.2 wt%, depletion flocculation occurred, but obvious creaming was suppressed by the high viscosity of the aqueous phase at 0.3 wt%. At the same concentration, XG had a bigger effect than GG.The effect of ionic strength on the interactions of WP and polysaccharides were studied. In the range of 0-500 mM, the presence of NaCl increased the hydrodynamic diameter and interfacial activity of complxe, changed the interfacial adsorption characteristics of protein, but weakened the viscoelasticity of interfacial protein film. In the emulsion system. NaCl reduced the apparent viscosity and stability of the system while increased the degree of shear thinning and the protein adsorption capacity. Compared to the xanthan gum, NaCl had little effect on the characteristic parameters of emulsion containing GG.The effect of pH on the interactions of WP and polysaccharides were studied. At pH 5.0, the hydrodynamic diameter and ζ-potential of the complex solutions were the highest, and diffusion rates(Kdiff), permeation rate(KP) and rearrangement rate(KR) of protein were largely limited in the vicinity of isoelectric point. The interfacial activity of two complexes and the mechanical strength of interfacial film of WP-GG system increased with an increasing p H from 5.0 to 8.0. Under neutral and slightly alkaline conditions, complex systems showed good thermodynamic compatibility, the movement of protein at the interface was restricted by interacting with XG through hydrophobic interactions or hydrogen bonds. thus the viscoelasticity of the interfacial film decreased at pH 8.0. The two kinds of emulsion system displayed the worst stability with the highest adsorption of proteins. WP-XG and WP-GG emulsion system displayed the highest stabilityat pH 6.0 and pH 8.0 respectively.