Efect Of Heat Treatment Intensity On The Gel Properties Of Soybean Protein And Study On Its Mechanism

As one of the important functional properties,soy protein gel properties have been studied at home and abroad for many years,and the temperature of most studies are below 100°C.However,the sterilization temperature of long shelf life foods is usually higher than 100°C.This high-intensity heat treatment can make a certain effect on protein gel.Therefore,it is guiding significance in practical production to systematically study the effect of different different heat treatment intensity on the properties of soybean protein gel.In this study,changes of the surface properties and endogenous fluorescence properties of proteins from soy protein isolate solutions and emulsions with different heat treatment intensities were studied.The gel properties,gel force and microstructure of the acid gels and salt gels were evaluated.The reasons why the heat treatment intensity affects the gel properties were analyzed,and equations were established to describe the relationships about the heat treatment intensities which the gel hardness and water holding capacity were optimal.Firstly,the complete denaturation times of the 7S and 11S components in the pure protein solution and emulsion with different heat treatment intensities were studied,and two fitting curves were obtained to describe the complete denaturation time of 7S and 11S at different temperatures.Then,the changes in the surface properties of the protein were further studied within the two curves.The results showed that the protein surface properties in protein solution and emulsion increased with the heat treatment intensity,showing a trend of increasing first and then decreasing.The surface properties of proteins in protein solutions and emulsions increased at first and then decreased with the increase of heat treatment intensity.The maximum surface hydrophobicity were 364.4±23.76 and 407.0±18.17,respectively.The maximum surface thiol content were 6.936±0.0507?mol·g^-1 and 7.124±0.0042?mol·g^-1.The surface hydrophobicity of proteins was increased due to the presence of oil.And the changes of endogenous fluorescence intensity showed that the degree of protein unfolding increased with the enhancement of the heat treatment intensity.At the same time,the interaction between protein molecules were increased and the formation of aggregates were accelerated.Secondly,the hardness and water-holding capacity of the protein gels and emulsion gels,induced by acid and salt with different heat treatment intensities,were studied.The results showed that the overall change trend of the two indicators were consistent with the changes of the protein surface properties.After rising to the highest value,the two indicators decreased as the heat treatment intensity continues increasing.It indicates that with the heat treatment intensity from weak to strong,the gradual exposure of the hydrophobic region of the protein will improve the hardness and water holding capacity of gel.However,if the heat treatment intensity is too high,a large amount of aggregates will be generated in a relatively short period of time,which is disadvantageous to the formation of a gel network with dense structure,uniform pores,and strong skeleton.In addition,the gel hardness of the emulsion gel was lower than the protein gel,but the water holding capacity was slightly higher than it.In order to describe the relationships about the heat treatment intensities with gel hardness and water holding capacity,the optimal heat treatment temperature and time for the emulsion gel hardness and water holding capacity were fitted to obtain the equations as follows:1)Acid-induced gel optimal hardness:t=9.977×10⁷T^-3.272?R=0.9537?;2)salt-induced gel optimal hardness:t=1.071×10¹²T^-5.364?R=0.9556?;3)the optimal water holding capacity of acid-induced gel:t=6.643×10¹⁰T^-4.783?R=0.9715?;4)the optimal water holding capacity of salt-induced gel:t=9.077×10¹⁶T^-7.914?R=0.9690?,where T is the heat treatment temperature and t is the time.Thirdly,further studies on the gel force and microstructure have been carried out and found that the contribution of the forces in gel can be ranked as disulfide bonds>hydrophobic bonds>hydrogen bonds>ionic bonds.Disulfide bonds and hydrophobic bonds in acid gels revealed the greatest effect on the formation of gel structures while the effects of hydrogen bonds and ionic bonds in salt-induced gels increased significantly.The microstructure of the gel also showed that when gel hardness and water holding capacity were in good conditions,the microstructure of the gel was denser,vice versa.Finally,when the equations,obtained from the emulsion gel,about the gel hardness and water holding capacity were applied,it was found that the optimum fitting equation of the water holding capacity of the emulsion acid-induced gel was in good agreement with the real system.Dietary fiber has a greater effect on the hardness of the gel,resulting in a certain deviation in the fitting equations about the hardness of the acid gel and salt gel,which can be improved through further study.