Study On Enzymatic Improvement Of Surface Activity Of Soy Protein And Its Effects On Properties Of Whipping Cream

In recent years,with the increasing demand for plant-based diets and food for people with lactose intolerance,improving the functional properties of plant proteins,especially the cheap and fine soy protein,and replacing animal protein with soy protein have attracted more attention.However,due to the compact structure and high hydrophobicity,surface activity of natural soy protein isolate(SPI)is relatively low.Heating and enzymatic hydrolysis are common modification methods.However,it’s difficult to control the formation of aggregates during heating,leading to an increased viscosity of SPI solution.During the process of enzymatic hydrolysis,the increased small molecule peptides could cause the formation of aggregates by hydrophobic interaction and even lead to protein precipitation,resulting in a low hydrolysis yield.Furthermore,the formation of small molecule peptides could also make the interface film thinner.Due to limited surface activity,it’s difficult to apply the SPI hydrolysate to whipping cream without small molecule emulsifiers.According to the principle of whipping cream,the protein in the emulsion system requires different surface activities for competitive adsorption at the oil/water interface.To address these problems,this thesis aims to improve the surface activity and hydrolysis yield of SPI and expand its application in whipping cream.The surface hydrophilic-lipophilic balance of heated proteins with different denatured degrees was regulated by protein-glutaminase(PG)The hydrophilic-lipophilic balance and molecular weight distribution of SPI during the papain hydrolysis process were also regulated by PG.Moreover,the effects of the combination of macromolecule protein and small molecules on the whipping performance of whipping cream without the addition of small molecular emulsifiers were discussed.Results may provide a basis for expanding the application of SPI in whipping cream.The main research contents and results are as follows:Firstly,the effects of PG-deamidation on the aggregate formation and surface activity of SPI with different thermal denaturation levels(70°C,100°C,and 121°C).The results of deamidation degree showed that moderate denaturation and structural unfolding of SPI(preheating at 70°C)accelerated the PG-deamidation reaction,and excessive protein aggregate(preheating at 121°C)inhibited the PG-deamidation reaction.The results of particle size,molecular weight distribution,and electrophoresis showed that heat-treatment led to an increased size of aggregates and PG-deamidation induced the unfolding of protein structure,resulting in the formation of soluble aggregates through hydrophobic interactions and disulfide bonds.The surface activity and interfacial rheology at oil/water interface results demonstrated that the emulsifying activities of NSPI and preheated SPI were significantly improved by PG-deamidation.Especially for the SPI preheated at 70°C,the adsorption rate at the oil/water interface and the elastic modulus of the interface film both reached the highest level after PG-deamidation,specifically,it obtained the best emulsifying activity(90.19 m~2/g)and emulsifying stability(113.79%),which were 100.0%and 22.4%higher than NSPI,respectively.Mild aggregation did not hinder the adsorption of proteins at the oil/water interface,while severe aggregation(deamidation treatment after heating at 121°C)would weaken the strength of the oil/water interface film and thus make the emulsifying properties worse.The interfacial rheology at air/water interface results demonstrated that the foaming properties of heated SPI were not improved by PG-deamidation,which could be explained by the fact that the adsorption behavior of heated SPI at air/water interfaces was not improved by PG.The above results indicated that while the PG deamidation reaction increased the protein charge,the protein structure was partially unfolded,which increased the surface hydrophobicity of the protein and induced the formation of aggregates.The moderate formation of aggregates was beneficial to the increase of surface activity.Then,using PG to regulate the hydrophobicity of SPI during the papain hydrolysis process,the effects on the aggregate formation and surface activity were investigated.Compared with papain-modified protein,the addition of PG led to the decrease of surface hydrophobicity and promoted the formation of protein aggregates.Fortunately,the interfacial rheology at the air/water interface exhibited that PG with addition of 20 U/g(moderate formation of protein aggregate)is beneficial to improve the speed of protein adsorption to the interface and the elastic modulus of the interface film.However,when the amount of PG added was increased to50 U/g or 100 U/g,the further increased formation of aggregates decreased its adsorption rate at the air/water interface,and the emulsifying activity also decreased significantly.In addition,during the process of compound enzymatic hydrolysis,as the amount of papain added increased,the proportion small-molecule peptide boosted,resulting in the improvement of foaming capacity,but destruction of emulsifying properties and foaming stability.As for the enzymatic hydrolysis time,30 min was the best.If the incubation time was too long(such as 120 min),both small molecule proteins and aggregates would be further generated,leading to a significant decrease in the adsorption speed at the air/water interface and the elastic modulus of the interface film.Based on the results of emulsifying and foaming properties,the optimal conditions of enzymatic hydrolysis were determined as follows:papain and PG were added at100 U/g and 20 U/g,respectively,and enzymatic hydrolysis was carried out at 50°C for 30 min.Under this reaction conditions,the foaming capacity of the modified SPI was 162.50%,the foaming stability was 94.33%,the emulsifying activity index was 65.45 m~2/g,and the emulsifying stability index was 95.92%.Furthermore,the hydrolysis yield almost reached100%.Finally,the modified protein with better surface activity was used to prepare whipping cream without the addition of small molecule emulsifiers,and based on the modified protein,macromolecule aggregates or small molecule polypeptides were added to change the protein composition of emulsions,the effects of the combination of macromolecule protein and small molecules on the whipping performance of emulsions were investigated.Results showed that the modified protein obtained from the previous chapter could be applied to whipping cream without the addition of small molecule emulsifier due to its good surface activity and the presence of large and small molecule proteins in the same emulsion system.The interfacial film of fat globules was strengthened and whipping overrun was increased by the moderate addition of aggregates(the ratio of modified protein to aggregates was 4:1).Furthermore,the cream stability was improved and the whipping time was shortened.On the contrary,the addition of small-molecule polypeptide made the interface film weaker,leading to a serious coalescence of the fat globules and even leading to defoaming,causing longer whipping time and poor stability of the cream.The above results demonstrated the benefits of the appropriate existence of aggregates once again,and could provide more ideas for improving the application of soy protein to clean-label whipping cream.In conclusion,the SPI preheated at 70°C after PG-deamidation exhibited the best emulsifying properties.In addition,the modified protein obtained by the combination of PG deamidation(20 U/g)and papain hydrolysis(100 U/g)for 30 min,which showed good surface activity and was suitable for the preparation of whipping cream without emulsifier addition.Furthermore,the properties of whipping cream got further improved when the ratio of modified protein to aggregates was 4:1.