| As the by-product of peanut oil production,peanut protein isolate(PPI)has a biological value(BV)of 58 and a digestibility of 90%,and due to rich in essential amino acids and easily digested and absorbed by humans,PPI is commonly considered as a high-quality protein resource.Meanwhile,the large amount composition and staggered arrangement of amino acid give PPI natural amphiphilic nature.However,during commercial production process,the globulins in peanut protein are very susceptible to denaturation and aggregation,which makes it difficult for PPI to rapidly adsorb to the oil-water interface and thereby form stable emulsions.The use of plant proteins in combination with polysaccharides is an effective way to improve protein interfacial adsorption ability and increase emulsion viscosity.At the same time,the occurrence state of protein-polysaccharide in the mixture shows different depending on p H,which in turn leads to differences in the phase behaviour of PPI and HMP in the system.In addition to changing the composition of the emulsion materials,the use of different homogenisation methods also has a positive impact on the stability of the emulsion system.Ultrasound is a common way applied in O/W emulsions production,which can effectively degrade large droplets,change the macromolecular structure,and enhance emulsion stability.High methoxyl pectin(HMP),which can be extracted from fruits such as apples and citrus,is an economical and readily available natural plant polysaccharide that is often added to food systems to maintain stability and improve texture.Therefore,in this study,based on the differences in the occurrence states of PPI and HMP at different p H,the suitable p H was selected and the ultrasound was introduced to construct composite emulsions stabilized by PPI-HMP with different occurrence state.The stabilization mechanism of these emulsions was investigated in depth,with a view to providing theoretical references for the development of PPI emulsions.(1)The occurrence state of PPI-HMP at different p H and the effect of ultrasound on the macroscopic stability of PPI-HMP composite emulsions were investigated.The results showed that at p H 5.0,7.0 and 9.0,PPI-HMP existed in the system in the form of soluble complex,non-interactive and co-soluble states,respectively.The emulsification activity and emulsion stability of the PPI-HMP mixtures were significantly improved by the introduction of ultrasound at all three p H.The best emulsification activity and stability of the PPI-HMP mixtures were achieved at p H 5.0,7.0 and 9.0 with an ultrasound power density of 6.0 W/cm~3and ultrasound time for 4,8 and 10 min,respectively.Compared with coarse emulsions,the droplet aggregation or growth of the PPI-HMP composite emulsions prepared by ultrasound was inhibited after storing for 30 d,and the effect was more obvious under the optimum ultrasound conditions.The ultrasound temperature(10–50°C)had no significant effect on the emulsification characteristics of the PPI-HMP mixtures and the storage stability of the composite emulsions.(2)The effects of ultrasound on the droplet size,zeta potential,droplet breakup kinetic model and mesoscopic properties of PPI-HMP composite emulsions were investigated.The results showed that ultrasound treatment significantly reduced the droplet size of PPI-HMP composite emulsions.Under the optimum ultrasound conditions,droplet sizes reached the smallest as 931,697 and 765 nm at p H 5.0,7.0 and 9.0,respectively.Compared to the coarse emulsions,the absolute droplet zeta potential decreased significantly to 29.75 m V at p H 5.0,while it increased significantly to 41.70 and 38.33 m V at p H 7.0 and 9.0.The droplet breakup kinetic model of PPI-HMP composite emulsions in the ultrasound field at all three p H satisfied the equation:(?);.At p H 5.0,the PPI-HMP composite emulsion system type was different from that at p H 7.0 and 9.0,and the emulsion droplet breakup potential decreased gradually with p H increasing.The rheological results showed that ultrasound favoured the enhance in apparent viscosity of PPI-HMP composite emulsions.At p H 5.0,PPI-HMP composite emulsions showed typical liquid deformation properties after ultrasound,while at p H 7.0 and 9.0,the composite emulsions exhibited viscoelasticity in the initial frequency range after ultrasound.CLSM presented that at p H 5.0,ultrasound contributed to the formation and adsorption of PPI-HMP soluble complex,while at p H 7.0 and 9.0,ultrasound worked to promote the formation of HMP network structure.(3)The effects of ultrasound on surface tension,interfacial adsorption situation,and the microscopic molecular changes(such as structural changes of interface-adsorbed PPI and non-adsorbed component properties)of PPI-HMP composite emulsions were investigated.The results showed that the surface tension of PPI-HMP composite emulsions significantly increased to 56.31,52.92 and 53.46 m N/m at p H 5.0,7.0 and 9.0 respectively under optimum ultrasound conditions,indicating the effective enhancement of intermolecular forces in the composite emulsions.Ultrasound significantly increased the interfacial adsorption amount of PPI,which was the main component involved in interfacial adsorption,and allowed more PPI to participate in the oil emulsification process.Combined with the analysis of the structural changes of interface-adsorbed PPI,it could be found that at p H 5.0,ultrasound enhanced the emulsion stability through stretching structure of PPI and promoting the formation and adsorption of PPI-HMP soluble complexes;at p H 7.0,ultrasound improved the structure regularity of interface-adsorbed PPI;and at p H 9.0,ultrasound facilitated the synergistic adsorption of hydrophobic PPI and hydrophilic PPI components at the interface.Scanning electron microscopy and fourier infrared spectroscopy of the interface-unadsorbed components revealed that at p H 7.0 and 9.0,ultrasound caused the HMP to form network structure in the aqueous phase through hydrogen bonding,thus providing a certain viscoelasticity for the system,whereas this effect was not presented at p H 5.0. |
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