Effect Of Cross-linking With Transglutaminase And Thermal Processing On Quality And Potential Allergenicity Of Tofu

Food allergy is a common food safety problem in the world,and the growth of morbidity has been a widely concerned.Soybean can be used to produce all kinds of soy products because of rich and high quality protein.However,it is also one of the eight food allergens certified by the World Food and Agriculture organization,and it can cause severe allergic reactions,anaphylactic shock and even endangers life,which is unfavourable for soybean allergy sufferers.Because of the variety of allergens in soybean,it is of practical significance to investigate the effects of soybean protein and the processing method on the allergencity of soybean.Thermal process is an indispensable step in the preparation of tofu,and it is also one of the physical methods to reduce the allergencity of food.Microbial transglutaminase(MTG)is widely used in food processing,because it can improve the gel properties,rheological properties of protein,and the quality,structure,and water retention of food.Moreover,some researchers found that MTG could reduce the allergenicity of food.However,the effect of MTG on the allergenicity of soybean allergens is still poorly understood.In this paper,the effects of thermal processing on the structure and potential allergenicity of soybean milk protein were discussed,and the effects of MTG crosslinking on the structure and potential sensitization of soybean allergen were further evaluated.A new type of tofu was made by thermal process and MTG crosslinking,and the physical characteristics,texture and microscopic characteristics of tofu were characterized.The digestibility of tofu in the human gastrointestinal tract was evaluated In vitro simulation,and KU812 cells were used to evaluate the potential allergenic changes of tofu digestive products.The main methods,results and conclusions are as follows:1.The heat processing of soybean milk was carried out in different ways,including one-step heating and two-step heating.After thermal treatment,the change in viscosity of soymilk was dicussed.The polymerization,particle size and structure of heated soybean milk have been analyzed by SDS-PAGE,dynamic light scattering,circular binary chromatography,UV absorption spectra and surface hydrophobicity.The results show that the viscosity of soybean milk after two-step heating is significantly greater than that of one-step heating.After the processing of soybean milk,the protein would accumulate and form some polymers with high molecular weight.The average particle size of soybean milk treated by two-step heating was larger,so the polymerization degree was greater.Thermal processing can change the secondary and tertiary structure and surface hydrophobicity of the protein.Under heating condition,the spatial structure of the protein molecules was unfolded,and the hydrophobic residues embedded in soybean milk protein were exposed to the outside of the molecules;Compared with the unprocessed soybean milk,heat processing can significantly reduce the allergenicity of soybean milk,and the potential allergenicity of soybean milk treated by two-step heating is slightly larger.2.After soybean milk treated by two-step heating at 75℃-5 min/95℃-10 min,the process conditions of MTG crosslinked soybean protein was screened by the IgE binding ability of soybean milk protein and the sera of the patients with soybean allergy.The optimum cross-linking conditions were as follows: the p H value of soybean milk was adjusted to 6.0,MTG was added with the ratio of 5 U/g enzyme activity and crosslinked for 3 h in water bath at 50℃.The structure of enzymatic cross-linked products was characterized by Far UV CD spectrum,UV absorption spectrum and surface hydrophobicity.The results showed that the contents of a-helix and β-corner were increased,and random coil was reduce in the secondary structure,the UV absorption value was increased,and the hydrophobic of crosslinked product was enhanced.3.Single coagulant(MTG,bittern,gluconic acid lactone)tofu and compound coagulant(bittern+MTG,MTG+GDL)tofu were prepared on the basis of the optimization conditions on thermal process and the enzymatic cross-linking.The texture,physical properties,sensory and surface microstructure of different tofu were analyzed.The results showed that the single MTG crosslinked tofu had better color,texture and texture,but the texture was the worst.Compared to a single bittern,gluconic acid lactone tofu and its composite with MTG tofu,it found that the tofu added MTG could improve the flexibility,chewiness,tissue morphology,the yield and physical properties of tofu.Furthmore,the digestibility of soybean,soybean milk and bean curd was simulated in vitro.The results showed that the digestibility of the protein in the soybean milk in the digestive stage of the stomach was the highest,the salt brine,gluconate lactone bean curd was the second,the digestibility of the compound bean curd was weaker,and the protein in the soybean is most resistant to the stomach digestion.In the gastrointestinal digestive stage,there is no significant difference in the digestion of different tofu,and there are some intact protein bands was still observed in lane of the electrophoresis even after 2 h in the digestion of the intestines.4.The IgG and IgE binding ability of digestive products were detected by ELISA method.The results showed that there was no significant difference in IgG binding ability of the gastric digestive products of tofu,but there were significant changes in IgG binding ability of gastrointestinal digestive products,among them the lowest IgG binding ability was observed in tofu(salt brine and MTG).The IgE binding ability of gastric digestive products of tofu had significant difference.The digestive products of the GDL and MTG tofu showed a low of IgE binding ability.There was no significant difference in IgE binding ability of gastrointestinal digestive products.Moreover,The degranulation of KU812 cells was carried out.The results showed that content of bioactive medium(β-HEX、Histamine、IL-6 and IL-4)in digestive products of the untreated soybean was the highest,and its degranulation effect was the most significant.Compared with MTG,salt and GDL tofu,the gastrointestinal digestion of compound coagulant tofu showed a lower allergenicity,especially the digestive product of GDL+MTG tofu,which had less change in the degranulation of KU812 cells.