Effect Of Okara On The Quality Of Glucono-?-lactone-induced Tofu And The Optimization Of Okara Adding Process

As the major byproduct of tofu products,okara is conventionally abandoned or simply used as an animal feed.However,abundant dietary fiber and various bioactive compounds also endow the okara with high nutritional value.Therefore,to exploit an effective utilization and value-added processing of okara is vastly concerned.Filter residue is an important processing of traditional tofu,which reduces the utilization rate of soybean and weakens its nutritional value.The high temperature generated by the grinding would negatively influence the gelling property of protein.Therefore,it is a promising alternative to add okara to the processing of tofu by grinding the okara individually.In this paper,the effects of okara on the formation of tofu were studied from four aspects:the drying methods of okara,the size of okara,the additive amount of okara and adding okara at different procedure.The main results were as follows:1.The effects of hot-air dried bean curd,vacuum dried bean curd and vacuum freeze-drying okara on the quality of tofu were studied.The results of content of sulfhydryl group and surface hydrophobicity of cooked soybean milk(CSM)added okara with different drying methods showed that okara inhibited the formation of aggregates during the first heating process.The delayed gelation point of tofu added by vacuum dried okara indicated that the gelation of tofu was inhibited.Further research on soybean dregs replenishment of okara-added tofu(OAT)shows that okara can significantly increase the yield of tofu and the content of protein and cellulose,while decrease the water content of fat.2.The effects of addition amount of okara(5%,10%,15%,20%)on the quality of tofu was studied.The surface hydrophobicity,sulfhydryl group content and viscosity of CSM increased with the increase of okara addtion.The results of rheological properties showed that the G₀'of CSM increased and the phase distribution of soybean milk changed.The results of SDS-PAGE showed that the increase of okara addition led to glycosylation of polysaccharide and soybean protein,then the a*value and b*of OAT increased.The protein content,insoluble fiber content and the soluble fiber content of tofu increased with the increase of soybean dregs.However,the water content of tofu decreased,which induced the water retention and cooking characteristics of tofu increased.When the added amount of soybean dregs was 10%,the gel strength and yield of OAT peaked at 97.64 g and 601.99g/100g soybean,respectively.When the addition amount of okara was 5%,the sensory score of OAT was the highest,even higher than that of traditional tofu.To sum up,addition of okara with an appropriate amount can improve the quality and sensory characteristics of OAT.3.The influence of lyophilized fresh okara with varying particle sizes(40,80,100,and200 meshes)that were added before the heating of okara-free soymilk on the quality of glucono-?-lactone(GDL)-induced tofu was investigated in this study.The oil droplets of the okara-CSM were decreased as the okara particle size reduced,while the sulfhydryl group content and surface hydrophobicity of the okara-CSM first increased and then decreased as the okara particle size decreased.No obvious difference in the protein composition was observed between the OAT and the okara-free tofu.However,the OAT with reduced particle size exhibited a similar microstructural profile to the conventional tofu.Results further demonstrated that the yield,water-holding capacity,cooking loss,and nutritional value of the OAT were better than those of the conventional tofu.The gel strength and sensory score of the OAT were increased as the particle size of okara reduced and these quality attributes of OAT were better than those of the conventional tofu.Theoretically,addition of okara mainly impacted on the interactions between denatured proteins via disulfide bonding and hydrophobic interactions to influence the formation of gel network and finally the textural properties of tofu.In conclusion,addition of okara with appropriate particle size can remarkably improve the processing quality and nutritional value of tofu,which is beneficial to the reasonable exploration of okara for the producers of soybean products.4.It was added to soybean milk before or after to prepare GDL-induced tofu to study the effect of GDL-induced tofu quality by adding okara at different procedure.The water holding capacity,cooking loss and yield of tofu made by adding okara were significantly better than those made without adding okara.Furthermore,the WHC,cooking loss,and the sensory score of tofu made by adding okara before boiling were significantly better than reverse order,and there were significant differences in odor,but no significant differences in yield and color difference.Results of optical microscope,particle size,and viscosity of soybean milk showed that more insoluble particles and micelles were formed and the particle size and viscosity were increased when okara was added before boiling.Results of sulfhydryl group content and surface hydrophobicity of soybean milk showed that adding okara before boiling could lead to protein aggregation.Results of rheological properties showed that the solidification point of soybean milk added with okara before boiling was earlier,and G'was always higher than that of CSM added with okara after boiling.But the protein composition of three types of tofu showed no obvious difference.In conclusion,adding okara before soymilk boiling can improve the quality of tofu to some extent.5.In order to optimize the preparation process of OAT,taking the drying method the addition amount and the particle size of the okara as factors,and the optimal technology parameters were obtained.The sensory score was used as the test indexes.The optimum technology parameters were as follows:addition 10%(w/w soybean)200-mesh okara dried by vacuum freeze-drying.In this condition,the total sensory score of OAT is the highest.The effect of three factors in order:addition amount>particle size>drying methods.Because of the high price of vacuum freeze-drying,the quality of OAT can be improved by reducing the additive amount and particle size of okara treated with other drying methods.