Studies On Gel-forming Mechanism Of Tofu And The Relation Of The State Of Water And Quality

Our country is rich in soybean resources.Soybean products have a history of more than 2000 years.Tofu is one of the most popular soybean products.It contains much protein,but without cholesterol.And it has many excellent processing qualities.What’s more,tofu is rich of isoflavone which plays an important role in restraining cancers. The gel characteristic of soybean protein is one of the important characteristics in the process of soybean produce.In the protein gel,the interaction of protein and water decides and maintains the three-dimensional structure of protein.It not only influences the function of protein,but also influences the food sensory evaluate.The reports about tofu focus on the compose and structure of soybean proteins, function characteristic of soybean proteins,the gel-forming mechanism and processing technology,but little on the relationship between the state of water in tofu and the gel’s texture properties.Thus,studies on affecting factors of tofu gel forming and the relationship between the state of water and the gel’s texture properties are very important to effectively control the gel forming and develop tofu products with high quality. Zhongxiang Tofu is one of the special local products in Hubei.Setting conditions and relative ground works of tofu gel,the interaction between metal ion,cooking oil and soybean protein and its influence on tofu gel forming,the relationship between the state of water in tofu and the gel’s structure and texture properties were investigated.The results were as follows:1.Basis studies on Tofu gel forming Gel forming and effects of molecular forces on gelation process and texture properties of soy protein gels induced by multifold factors were studied with the soy protein isolate simulating the gelation process of traditional tofu.It was shown that during course,the electrostatic forces,hydrophobic interactions and hydrogen bonds and sulphydryl conversion function have given important contribution for the gelation process of soybean protein gels coagulating by plaster.2.Protein’s change in the gelation process of soybean protein After soybean’s milling,there were not anyα-helix,β-sheet andβ-turn and the secondary structure of soybean protein was mainly coil.When the boiled soybean milk was cooled to 80℃, plaster was put into it and the content ofα-helix was larger.Twenty minutes later,the content ofα-helix andβ-turn was a little larger,the content ofβ- sheet was clearly smaller and the content of coil was larger.Twenty minutes later,steady gel net came into being on the whole.After press,tofu came into being and content ofα-helix was smaller and content ofβ- sheet was larger.At the same time,the content of non- freezing water increased and the content of frozen water decreased in the gel net.3.Effects of metal ion and cooking oil on the state of water of soy protein gels Different metal ions had different effects on the swelling ratio and equilibrium water content of soy protein gels.With the concentrations of Na⁺,Mg²⁺ and cooking oil increasing,the swelling ratio and equilibrium water content of soy protein gels descended.On the other hand,the swelling ratio and equilibrium water content of soy protein gels increased with the increasing of the concentrations of Ca²⁺.With the adding of metal ion,the content of frozen water soy protein gels descended. With the concentrations of Ca²⁺ increasing,the W_nf value was more and more large. However,the effect of Na⁺ and Mg²⁺ on the content of non- frozen water in the gel was opposite.On the other hand,the time for gel formation was shorten with the addition of metal ions,which mean that metal ions could promote the formation of gel.4.The relationship between the state of water and the microstructure in the SPI gel by plaster Through the analysis of electron microscope,it was observed that with the increasing of the concentration of metal ions,the gel’s hole diameter decreased and the W_fro value of gel gradually decreased.We believe that with the macromolecular chain becoming closer together to form a gel network,its hole diameter became smaller and the space would be smaller,space resistance would be larger,and the combined water that could be tied into the holes would be less and as a result the content of frozen water decreased.5.Effects of metal ion and cooking oil on the secondary structure of SPI gels by plaster In the SPI gels by plaster,the proportions ofα-helix,β-sheet,β-turn and random coil were 4.9%、33.0%、14.6%and 47.5%.After the addition of Na⁺,Mg²⁺ and Ca²⁺,the proportions ofα-helix slightly increased,while theβ-sheet structure declined in the proportion.On the other hand,with the increasing of the concentration of metal ions, protein molecular chain connected more closely,resulting in great changes in the space. In the molecular chain,the distance between the levels reduced,damaging layered structure ofβ-sheet,therefore the proportion ofβ-sheet structure decreased.The addition of cooking oil made the proportion ofα-helix,β- sheet,β-turn and random coil in the entire system were 4.0%,26.0%,22.6%and 47.4%.It shown that the addition of cooking oil made the proportion of theα-helix andβ-sheet structure decreased slightly in the system.6.Effects of metal ion and cooking oil on gelation process and texture properties of SPI gels by plaster Effects of metal ion and cooking oil on gelation process and texture properties of soy protein gels were studied with the soy protein isolate simulating the gelation process of traditional bean curd.It was shown that when the concentration of Na⁺ was between 0 and 0.05mol/L,the clotting time was shorten with the addition of natrium ion and when the concentration of Na⁺ was between 0.05 and 0.075mol/L,the clotting time was prolonged with the addition of natrium ion.When the concentration of Ca²⁺ was between 0 and 6.0mmol/L,the clotting time was obviously shorten with the addition of calcium ion.When the concentration of Mg²⁺ was between 0 and 0.03mol/L, the clotting time was shorten with the addition of magnesium ion.To sum up,the change of gel characteristic was complex with the addition of the concentration of metal ion because of the effects on protein structure from negative ion.After the addition of metal ions,the hardness of SPI gel reduced and with the concentration of metal ions increasing,the springiness and adhesiveness decreased. When the concentration of Ca²⁺ gradually increased,the adhesiveness of SPI gel gradually reduced.Na⁺,Mg²⁺ and Ca²⁺ had not obvious effect on the cohesiveness of SPI gel.With the increasing of the concentration of the cooking oil,the hardness of tofu gel gradually decreased,springiness and cohesiveness increased slightly and adhesiveness decreased significantly. 7.The relationship between the texture properties and structure of SPI gel by plaster The addition of sodium ion,magnesium ion and calcium ion made the proportion ofα-helix structure slightly improve,while the proportion ofβ-sheet structure relatively decline.With the increasing of the concentration of cooking oil,the hardness of tofu gel gradually decreased,the springiness increased,adhesiveness decreased clearly and the cohesiveness increased slightly.The addition of cooking oil made the proportion ofα-helix structure andβ-sheet structure declined.In our opinion,a layer of lipid molecules membrane came into being on the surface of the gel network after the addition of oil.The membrane played a role in locking water to strengthen the reversion ability of gel system and increased the springiness of the SPI gel.8.The relationship between the texture properties and structure of SPI gel by plaster Generally,the addition of sodium ion,magnesium ion and calcium ion made the W_nf and W_fro value declined and the hardness and springiness of the gel declined.The change of adhesiveness was ruleless and the cohesiveness had not clear change.It was presumed that the reduction of W_nf value and the content of water in the hole of the gel reduced the role of support and stability,so the hardness and springiness of the gel declined.On the aspect of adhesiveness,with the increasing of the concentration of Ca²⁺,the W_nf value of the gel gradually decreased,the adhesiveness of the gel decreased,possibly because of the formation of calcium bridges.The changes of water content were ruleless with the change of the other two metal ions and their impact on the adhesiveness should be studied further.9.Effects of different dehydration conditions on the state of water in tofu In the course of tofu dehydration,with the increasing of the stress and press time,the water in gel network have been extruded continuously.The water content,equilibrium water content,frozen water content and swelling ratio in tofu gel declined clearly and the non-frozen water content in tofu gel increased slightly.From the appearance of the tofu, when the press time was 20 minutes and stress was 700g and 900g,tofu had good shape. When press time changed from 10 minutes to 40 minutes,press time had not obvious effect on tofu.10.Effects of the state of water on the texture properties of tofu gel With the increasing of pressure,in the gel system the frozen water content reduced,the hardness of the gel increased continuously,the springiness and cohesiveness tended to increase and the adhesiveness declined after increasing.With the pressure time prolonged,in the gel system the adhesiveness declined all the while.The water in the gel separated out.At the same time,the network shrank continuously and the water in the network became unsaturated.Therefore during this time period,the water content decreased,the hardness and springiness tended to declined. We believe that the quality of the gel that has just been produced was not the best quality status.The water content and the water state of tofu gel had an important effect on the edibility and stability of quality.This experiment showed that during the facture of Tofu in small sample,the diameter of the hole in the network structure of tofu 11.4μm when the quality was the best.With this water content,the appearance,hardness and springiness of Tofu was better.11.The factors of the stability of tofu The factors affecting the stability of tofu had temperature,water content,salt concentration and so on.This research showed that the temperature had obvious effect on the stability of tofu.The shelf-life of samples at 4℃were prolonged for 9 days relative to the samples at room temperature and at the same time the water loss rate was reduced and the quality was maintained 4℃.Different water contents had some effects on the stability of tofu.The results of stability experiment on non-fermented soybean products from the market showed that with the decreasing of water content of samples,the shelf-life reduced from six to three days at room temperature.When the concentration of salt was 0.1%,the shelf-life of tofu was prolonged for one day and the water loss rate was reduced for about 4%.It showed that the addition of salt could suppress the growth of mildew,salt had a certain role of locking water and holding shape and the shelf life of foods was extended. When the concentration of salt increased to 0.4%,the gel could not come into being.It was presumed that the network rebuilding from the protein chain after protein denaturalization would be held back if the concentration of salt was great excessively.