A Research On Aggregation Characteristics Of Wheat Globulin And Its Effects On Noodle Processing Quality

Gluten protein has long been regarded as the key factor in the viscoelasticity of dough and the structure and quality of flour products.However,the wheat dough is a complex system,more and more researches indicate that gluten protein is not the only factor affecting the quality of wheat flour.The role of non-gluten proteins（albumin and globulin）in flour processing can not be ignored.Research on the structure and functional properties of non-gluten proteins in flour processing has a significant meaning in comprehensive apprehension about the relationship between wheat proteins and flour processing and the improvement in the quality of wheat flour.Wheat globulin is one of the important non-gluten proteins in wheat,which plays a potentially significant role in the formation of dough protein network and the quality of wheat flour.However,compared with gluten protein,the interactions of various components in the complex dough system may mask the effects of globulin.Researches on the structure and functional properties of globulin in dough and flour products are limited,without figuring out whether globulin participates in the formation of gluten protein network and how it interacts with gluten protein.In this paper,the aggregation characteristics of wheat globulin and the structure and functional properties of wheat globulin under different noodle processing conditions will be systematically explained,and the interaction mechanism between wheat globulin and gluten protein will be revealed.The research results are helpful for people to have a new understanding of the structure and functional properties of wheat globulin,to improve the basic theory of flour processing,and to provide a theoretical basis for noodle processing and wheat breeding.The main research results of this paper are as follows:（1）Both alkali and salt induction could promote the aggregation of wheat globulin.Wheat globulin was prepared from wheat variety QL 2122.The basic structure of globulin was analyzed,and the aggregation behavior of globulin under different concentrations of Na Cl（salt）or Na₂CO₃（alkali）was systematically studied.The results showed that Na Cl and Na₂CO₃with different concentrations could promote the transformation of globulin conformation fromβ-turn andα-helix toβ-sheet and random coil.In particular,theβ-sheet of globulin increased significantly under the impact of Na₂CO₃with different concentrations,indicating an enhanced hydrogen bond interaction between globulins,which promoted the aggregation of globulin molecules.At the same time,Na Cl with higher concentrations（≥500mmol/L）could induce globulin to covalently aggregate through disulfide bonds.Treated with Na₂CO₃,disulfide bond covalently aggregates through disulfide bond at lower concentration（≤500 mmol/L）.With the increase of concentration（≥500 mmol/L）,disulfide bond was destroyed and non-disulfide bonds covalently aggregated.In addition,the surface hydrophobicity of globulin treated with Na₂CO₃was lower than that of untreated samples.This proved that the hydrophobicity between globulin molecules was enhanced.Besides,it was observed that both alkali and salt promoted the agglomeration of globulin by AFM.（2）Heat treatments combined with alkali induction could promote more extensive aggregation behavior of globulin.Based on the research in the previous chapter,the effects of heat treatment（35℃,50℃,65℃,80℃,100℃）in combination with Na Cl or Na₂CO₃induction on the aggregation of wheat globulin were further explored.The results showed that high-temperature heating（100°C）could induce covalent aggregation of macro-globulin via disulfide bonds,as the treatment temperature increased.At the same time,the surface hydrophobicity increased,and the protein conformation changed fromβ-turn toβ-sheet.Under heating in combination with Na Cl treatment,the protein conformation changed fromα-helix and random coil toβ-turn,and other changes were similar to those in the condition of heating alone.However,when heating in combination with Na₂CO₃treatment,higher temperature（≥65°C）could induce disulfide bonds reduction and non-reducible covalent aggregation.In addition,bothβ-sheet andβ-turn increased in protein conformation,and the protein surface hydrophobicity was lower than that of heat treatment alone and heat treatment in combination with Na Cl,which further confirmed that heat treatments in combination with Na₂CO₃could promote more extensive aggregation of globulin.（3）Wheat globulin could affect dough rheology and noodle processing quality.Wheat globulin was added to flour to further explore the effects of globulin on dough rheology and noodle processing quality.The internal mechanism of quality changes was explored from the perspective of the interaction between globulin and gluten protein.The results showed that the stiffness,hardness and maximum tensile resistance of dough and cooked noodles were enhanced,when the amount of globulin was 1.5%or higher.The ductility of cooked noodles also improved when the addition reached 3.0%.Further analysis showed that in the system of dough,adding globulin tended to weaken the SS bond in the gluten network and mainly cross-linked with SDS soluble gluten protein through non-covalent interactions,especially through hydrophobic interactions.At the same time,CLSM observed a more rigid microstructure of protein network.In addition,in cooked noodles,the extraction rates of noodle protein with globulin were reduced,indicating that cooking accelerated the co-aggregation of globulin and noodle protein network.The protein conformation changed fromβ-sheet and random coil toβ-turn when the amount of addition reaches 3%.These results might account for the fact that the extension of noodles are more ductile than dough.（4）The improvement effects of globulin on dough processing quality was enhanced by alkali or salt induction,especially by alkali induction.Based on the research in the previous chapter,the functional properties of globulin induced by alkali or salt in dough processing and the interaction mechanism between proteins were further explored.The results showed that compared with adding globulin alone,G’and G"of dough increased significantly in the presence of Na₂CO₃,while tanδdecreased.This suggested that the functional properties of globulin in noodle processing were enhanced in the presence of alkali.In the presence of Na Cl,the enhancement effect was not so obvious as that in the presence of alkali.Further analysis found that the surface hydrophobicity of dough protein decreased by 38.71%in the system of dough,when globulin was added.This proved that the hydrophobicity between proteins was enhanced after adding globulin.Under the condition of alkali and salt,the hydrophobicity might be further weakened,butβ-sheet of the protein conformation increased by 3.17%and 1.68%respectively,indicating that the hydrogen bonds interaction between proteins was further enhanced by adding globulin under the condition of alkali and salt.In addition,disulfide covalent interaction between globulin and gluten protein occurred in the presence of alkali.These results were closely related to the aggregation of protein network observed by CLSM.It was further explained that a more obvious polymeric reaction occured between globulin and gluten protein induced by alkali,which might be the main reason why alkali significantly improved the functional properties of globulin.（5）The improvement effects of globulin on noodle quality was further enhanced by heating combined with alkali induction.Founded on the research in the previous chapter,the influence of adding globulin on the quality of noodles was further investigated in the presence of alkali and salt.The results suggested that globulin significantly enhanced the hardness,chewiness,springiness and resilience of noodles induced by alkali.In addition,compared with the tensile properties of dough,the ductility of cooked noodles improved significantly,which indicated that heating in combination with alkali further improved the functional properties of globulin,while the effect of salt is not so obvious as that of alkali.Further analysis of the type and degree of cross-linking between dough and noodle protein showed that the synergistic effect of heating and alkali can further the covalent cross-linking of disulfide bonds between noodle proteins.At the same time,the occurrence of non-disulfide covalent cross-linking（dehydroalanine cross-linking）might be produced,and it was analyzed that such cross-linking might contribute more to the cross-linking of noodle protein network than disulfide cross-linking.The further research on the contribution of non-covalent interaction between proteins proved that under the condition of heating in combination with alkali,adding globulin promoted the significant increase ofβ-sheet structure in noodle protein,which was conducive to the enhancement of inter-molecular hydrogen bonds,further strengthened the noodle protein network structure,and improved the quality of noodle after cooking.While the contribution of electrostatic and hydrophobic effects were weak.