Effects Of Soymilk Protein And Lipid Composition On The Preparation And Quality Of Yuba

Yuba is a protein-lipid film formed on the surface of soymilk during heating.The composition and properties of soymilk directly affect the preparation and quality of yuba.There are some problems in the production of yuba,such as low yield,unstable quality and limited utilization rate of soymilk.Aiming at improving the yield and quality of yuba in the production,the ratios of protein to lipid in soymilk and the composition and structure of soy protein are studied.The purpose of this paper is to elucidate the effects of soymilk composition and protein structure on the preparation and quality of yuba.The main research results were as follows:Firstly,soymilk with different ratios of protein to lipid(2.04,2.21,2.83,4.69)could be prepared by controlling centrifugal conditions.The protein composition of soymilk obtained by centrifugation was not significantly different from that of original soymilk.On this basis,the effects of ratios of protein to lipid in soymilk on the preparation and quality of yuba were investigated.The results showed that the yield of yuba and the protein utilization of soymilk could be improved by controlling the ratios of protein to lipid in soymilk.Soymilk with higher ratio of protein to lipid could prepare yuba with higher ratio of protein to lipid.With the increasing ratio of protein to lipid in soymilk,the tensile strength,oil seepage and boiling resistance of yuba were improved(Taking the second yuba as an example,the tensile strength of yuba increased from 10.81 g/mm~2 to 16.58 g/mm~2,the oil seepage rate of yuba decreased from 72.26%to 0%,the cooking loss rate of yuba decreased from 44.30%to 35.53%),which could also be witnessed by the denser and continuous microstructure of yuba.The protein composition in the boiled liquid from yuba showed thatβsubunit of 7S and B polypeptide of11S contributed most to the texture of yuba.Secondly,the change of physiochemical properties of soymilk during preparing yuba was studied.During yuba preparation,the protein content and viscosity of soymilk decreased,while the average particle size of soymilk increased.The content of insoluble aggregates in soymilk protein increased significantly during preparing yuba.Compared with other protein subunits,βsubunit and B polypeptide in the soymilk were rapidly consumed during preparing yuba,indicating that they were more likely to participate in the formation of yuba.The content of free sulfhydryl group of soymilk protein decreased during preparing yuba.The instability of yuba quality was caused by the decrease of soymilk protein proportion,the accumulation of insoluble protein aggregates,the change of protein composition and the consumption of free sulfhydryl group.With the increasing ratio of protein to lipid in soymilk,soymilk protein and itsβsubunit and B polypeptide were consumed rapidly,and the reduction rate of free sulfhydryl group content increased.The increase of thermal aggregation of soymilk protein further promoted the formation of insoluble aggregates,which led to the restriction of protein utilization in soymilk with high ratio of protein to lipid.Finally,defatted soy flour was used as raw material to simulate soymilk C8000(the ratio of protein to lipid:2.83)and prepare yuba.The effects of soy protein structure on the preparation and quality of yuba was further studied.The results showed that the protein structure of defatted soy flour was easily changed during storage.The expansion of protein hydrophobic region promoted the further hydrophobic aggregation of protein,leading to the decrease of protein solubility.At the same time,the content of free sulfhydryl group decreased due to protein oxidation,which further reduced the film forming ability of protein in defatted soy flour.Compared yuba prepared from defatted soy flour with yuba prepared from soymilk(C8000),there was no significant difference in yuba yield and protein utilization rate of soymilk,but the former was better than the latter in tensile strength and boiling resistance.Compared with yuba prepared from fresh defatted soy flour,yuba prepared from aged defatted soy flour were lower in the yield and protein utilization rate of soymilk.The protein content,tensile strength and boiling resistance of yuba prepared from aged soy flour were worse than those of fresh soy flour.