| Soy Protein Isolate(SPI),a natural emulsifier,can adsorb on the oil-water interface in order to keep droplet aggregation away,which is a common use in vegetable protein beverages.However,there is no denying that SPI is susceptible to the influence of external environment such as pH and ionic strength of the solution,the stability of the emulsion is affected,especially when the pH is acidic,the protein adsorption layer on the liquid surface will decrease or even cause flocculation.Besides,it is believed that the majority of protein beverages are acidic and limited to the application of SPI,therefore,more and more studies have been made based on the combination of anionic sugars and proteins for its high water solubility,low viscosity,and sound stability as stabilizers which are able to form a protective film around the oil droplets so as to stop drops gathering.Soy oligosaccharides,consist of stachyose and raffinose,can promote the proliferation of beneficial bacteria such as bifidobacteria,it is capable of developing health-care dairy products with special functions.In consequence,this article focused on the effect of ratio of oligosaccharide to SPI on the its emulsification under acidic conditions in a systematical way,providing a theoretical basis for the development of natural emulsifiers with health efficacy.Firstly,different ratios of stachyose and raffinose were selected to study the effect of oligosaccharides on the emulsification and emulsifying stability of SPI.Secondly,studies on the effect of the pH,ratio of oligosaccharide to SPI,heating temperature as well as ionic strength on the emulsification and emulsifying stability of SPI were carried out.Moreover,electrostatic compounding process and its corresponding behaviors between SPI and oligosaccharide were examined and discussed by Zeta electric potential and turbidity measurement and laser confocal microscopy,aiming to explain the mechanism and formation conditions of the formation of soluble electrostatic complexes between SPI and oligosaccharides and to analyze the interaction degree between SPI and oligosaccharides by the way of endogenous fluorescence chromatography,at the same time,the particle size distribution of the composite and the dispersion were analyzed by measuring the particle size distribution,providing further theoretical supplements for the compounding of SPI and oligosaccharides;Secondly,for the purpose of acquiring the formation conditions of SPI and oligosaccharide composite solution with the best emulsification through the response surface optimization experiment on the basis of controlling the effect of the pH,ratio of oligosaccharide to SPI,heating temperature and ionic strength on the emulsifiability of the SPI-oligosaccharide composite solution as mono-factor.Lastly,the changes of emulsifying properties,solubility,foamability and apparent viscosity of the SPI and oligosaccharide composite solution after one month of storage at room temperature compared with 4 °C were investigated.The main research results are as follows:(1)When the pH was 6.5,ratio of stachyose to raffinose was 7:3,the emulsification and emulsification stability of the SPI-oligosaccharide composite solution reached the maximum values of 0.699 ± 0.009 and 5.53 ± 0.15% respectively,which were increased by 39.96% and79.89% higher than SPI under the same conditions.(2)The results illustrated that the SPI was inclined to form an electrostatic complex with oligosaccharides under acidic conditions,compared with SPI,the isoelectric point shifted to acidity.When the pH was 6.0-7.0,the SPI-oligosaccharides mixture,to a large extent,is preferred to be a soluble electrostatic compound.When the pH was 6.0,in contrast to SPI,the emulsification and emulsification stability of SPI-oligosaccharides raised by 34.33% and 97.32%.When the pH was 7.0,the emulsification and emulsification stability of SPI-oligosaccharides raised by 13.44%and 29.94%.When the composite ratio was 0.15,the emulsification and emulsification stability would increase by 61.69% and 95.66% respectively.While during the heating process,when the temperature reached 75°C,the emulsification and emulsifying stability of the SPI-oligosaccharide composite solution reached the maximum value,which was 32.33% and 79.98% higher than that of the SPI.When the ionic strength of the solution was changed and controlled it to be 0.10,it could be found that the SPI-oligosaccharide composite solution was provide with sound emulsification and emulsification stability,which is 52.97% and 49.78% higher than SPI.(3)The response surface optimization analysis method was used to conduct the experiment,varieties of factors affecting the emulsification were obtained,in addition,the primary and secondary order of these factors were pH>ionic strength>composite ratio>temperature;as a result,the final optimal preparation conditions for the SPI-oligosaccharide composite solution were that pH is 6.4,ratio of oligosaccharide to SPI is 0.14,treatment temperature is 74°C and ionic intensity is 0.09,in this way,the emulsification of SPI-oligosaccharide composite solution reached its maximum value of 0.933±0.002.(4)The results showed that in contrast to SPI,the SPI-oligosaccharide composite solution possesses higher emulsification,emulsification stability,solubility,foamability,foaming stability and apparent viscosity.Furthermore,these properties is more evident at the temperature of 4°C.Thus,the conclusion that electrostatic compounds formed from oligosaccharides and SPI can improve the functionality of SPI in an effective way and also prolong its storage period can be induced. |
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