Effects Of Ultrasound-assisted Transglutaminase Crosslinking And Maillard Reaction On Structure And Functional Properties Of Soy Protein Isolate

Soybean protein is one of the most important plant proteins with a protein content of more than 90%(dry basis).The commercial soy protein isolate products are treated by acid precipitation,resulting in varying degrees of denaturation.Therefore,some properties are difficult to meet the market demand.In recent years,a large number of modifications have been made to obtain protein products with good functional properties,such as emulsifiers and biofilms.As is known to all,the functional properties are determined by the composition,structure and conformation of the protein.However,there is currently a lack of systematic studies on the effect of modifications on the structure of soy protein isolate.Therefore,based on providing a basis for further understanding and better explaining the relationship between functional properties(e.g.solubility,surface hydrophobicity(H0),emulsification,etc.)and protein structure,the paper did a research on structure of SPI through the safe and effective modification methods,such as ultrasonic treatment,transglutaminase,Maillard reaction and even a combination of the above methods.The main contents and conclusions in the thesis are summarized as follows:1.Ultrasonic treatment conditions were 40 k Hz,at with different ultrasonic power(0,80,120,160,200 W)for 20,40,60,80 min.The solubility,intrinsic fluorescence,UV–vis absorbance,SH,secondary structure,H0 and emulsification properties of SPI with or without sonication were investigated.The experimental results suggested that with the increasing ultrasonic power,the solubility of SPI was continuously enhanced,but the ultrasonic time(0~80 min)had little impact on the solubility.The H0 of the SPI increased firstly and then decreased.The ultrasonic treatment exposed the hydrophobic groups.When the ultrasonic power exceeded 120 W or was 200 W for 20~80 min,because of non–covalent interactions between proteins such as electrostatic and oscillatory movement,hydrophobic groups were embedded into the protein molecules again.Ultrasonic power less than 160 W had little influence on the UV absorption of SPI.At 200 W,the UV absorption increased significantly,and the UV absorption peak intensity increased with the increasing ultrasound time.Ultrasonic treatment could expose more sulfhydryl group than NSP sample,but the power had little effect on the ultrasound SPI sample.Ultrasonic treatment made the intrinsic fluorescence of SPI quenched weakly,but the ?em was almost not shifted.Synchronous fluorescence results also confirmed that ultrasonic treatment made no significant difference on the hydrophobicity and polarity of the microenvironment surrounding tryptophan and tyrosine residues.The lower ultrasonic power(200 W)had little effect on the secondary structure,which means that the degree of structural damage was light,but it significantly improved the solubility and emulsification.Therefore,the 200 W ultrasonic was used for 20 min as the pretreatment method for subsequent experiments.2.On the basis of 200 W ultrasonic pretreatment,TG catalyzed the reaction of SPI.The SDS–PAGE and free amino groups confirmed that TG used a covalent cross–linking mechanism in the range of 0~8 U/g SPI.The optimized TG reaction conditions were at p H 7.4,50 °C for 30 min.The changes of SPI solubility,intrinsic fluorescence,secondary structure,rheology,thermal stability,H0 and emulsification properties with or without TG were measured.At p H 4.5,the solubility was the lowest,and when the p H was increased to 7.5,the solubility became stable.At different p H,TSPI solubilities were reduced to varying degrees.TG induced SPI cross–linked,SPI structure changed,not only cross–linked amino,but also exposed tryptophan,tyrosine residues,resulting in increased fluorescence intensity.With the increase of TG,the contents of ?–sheet and ?–helix decreased continuously,and the random coil content increased.This indicated that TG made the SPI unfold,so secondary structure of SPI became irregular.Heating and ultrasound treatment reduced the apparent viscosity of the SPI.The addition of TG caused a dramatic increase in the viscosity.The results about tan? indicated that the TG promoted the SPI to form a gel.The Td of 7S and 11 S in the NSP protein were 70.06 °C and 82.67 °C,respectively.The Td of the TG treated SPI(10 U/g SPI)were 79.5 °C and 83.0 °C.It indicated that TG treatment increased the thermal stability of the SPI.After TG modification,the emulsification increased.However,the emulsification stability tended to rise firstly,and then fell.3.The Optimization conditions of wet Maillard reaction: Study of ultrasonic power(0 W,200 W),reaction times(0~90 min,1.5 h,2 h,24 h),reaction temperatures(60 °C,70 °C,80 °C,90 °C,95 °C)and mass ratios of sugar to SPI(2:1,1:1,1:2)on the degree of grafting,browning,Maillard reaction products.The results showed that ultrasonic–assisted modified SPI reacted at 95 °C for 1.5 h exhibited the highest degree of grafting,which the ultrasonic enhanced the graft action.Comprehensive analysis of above parameters,the experimental conditions were: 1:1 mass ratio of sugar to SPI,95 °C,reaction 60 min.We verified that under the optimized conditions,the Maillard reaction taken only 1 hour to reach the middle and final stages.4.The Maillard reactions of three monosaccharides(galactose,glucose,fructose)were performed under optimized conditions.The structure and properties of the modified SPI which combined with TG treatment were analyzed.As a result,it was found that TG could accelerate glycosylation,and the degree of glycosylation is as follows: galactose> glucose> fructose.Both Maillard reaction and TG action could increase the intensity of SPI intrinsic fluorescence and reduce the content of SPI free amino groups.Maillard reactions increased SPI solubility and reduced the H0,while TG treatment decreased solubility and increased the H0.The combination of these two methods had little effect on SPI solubility,but decreased the H0.Both Maillard reactions and TG treatment increased the emulsifiability and emulsion stability.Simultaneously with both methods,the SPI's emulsification and emulsion stability increased.