Optimization Of Processing Conditions Of Okara-Reserved Soymilk And Effects Of Ultrasonic And High-Pressure Homogenization Treatments On Its Quality

As a new type of vegetable protein drink,okara-reserved soymilk retains the maximum extent of dietary fiber,protein,and other biological active substances during processing,thus improving the nutritional value.In order to overcome the adverse effects on processing and sensorial quality led by the insoluble okara components,the factors of flour particle size,the ratio between the material and water,cooking temperature,and cooking time were optimized by the combination of single factor experiment and response surface methodology(RSM).Moreover,the quality attributes of okara-reserved soymilk were further improved using ultrasonic treatment or high-pressure homogenization.The main findings are as follows:1)The optimal processing conditions were determined as follows:soybean powder passed through a mesh sieve of 120 mesh,the ratio of material to water was 1:10,the cooking temperature was 90℃and the cooking time was 6 min.Under these conditions,the protein content and precipitation rate of the obtained okara-reserved soymilk were5.82%and 24.8%,respectively.Moreover,the nutritional value of okara-reserved soymilk was higher than that of traditional soymilk.The contents of fat content,protein,total dietary fiber,soluble dietary fiber and the insoluble dietary fiber increased by 64.29%,76.90%,202.72%,2.33%and 200.96%,respectively.In addition,the average particle size,centrifugal precipitation rate,and viscosity of the okara-reserved soymilk were higher than those of traditional soymilk.The microbial propagation rate of the okara-reserved soymilk was higher than that of the traditional soymilk during storage;however,the sensory quality was not significantly different.2)Ultrasonic treatment could improve the quality attributes of okara-reserved soymilk.When treated at an ultrasonic power of 500 W for 40 min,the free sulfhydryl content and particle size of the raw or cooked okara-reserved soymilk decreased by 41.18%and13.56%or 19.53%and 26.52%compared with those of the control,respectively.Moreover,the corresponding centrifugal precipitation rate,soluble protein content,and protein surface hydrophobicity increased 26.71%,250%,and 32.83%or 18.26%,49.35%,and22.07%,respectively.After ultrasonic treatment,the L*and a*values of okara-reserved soymilk increased,while b*values decreased.The viscosity decreased after ultrasonic treatment.Native-PAGE results showed that the protein subunit composition of okara-reserved soymilk was not changed under different ultrasonic treatment conditions.3)High-pressure homogenization could also improve the quality attributes of okara-reserved soymilk.The soluble protein content in raw or cooked okara-reserved soymilk treated at 60 MPa for 20 times increased by 157.14%and 44.68%compared to that of the control.With the increasing intensity of high-pressure homogenization,the average particle size of the okara-reserved soymilk decreased significantly.However,the content of the free sulfhydryl group decreased,and the L*and?_ivalues of the okara-reserved soymilk showed a significantly increasing trend(P<0.05),while the a*and b*values decreased.The viscosity of the okara-reserved soymilk increased after high-pressure homogenization.Native-PAGE results showed that the basic and acidic subunit of protein of okara-reserved soymilk was decomposed after high-pressure homogenization.In conclusion,under the optimal preparation conditions,the nutritional value of okara-reserved soymilk was higher than the okara-filtered soymilk.Furthermore,ultrasonic and high-pressure homogenization treatments can further improve the physicochemical quality of okara-reserved soymilk by increasing the brightness and soluble protein content and decreasing the particle size.Results provide useful information for developing innovative healthy soymilk products.