Study On The Storage Stability And Rheological Properties Of The Powder Mixed With Soybean Powder And Wheat Flour

In our country, wheat flour has a wide range of consumer group. In fact, the content of nutrient lysine is very low in wheat flour. Therefore, a kind of efficient strategy of adding soybean protein powder has been developed to compensate the shortage or lack of nutrients in wheat flour. In this dissertation, using six kinds of soybean powders including whole soybean powder, inactive whole soybean powder, defatted soybean powder, inactive defatted soybean powder, soybean protein concentrate powder or soybean protein isolate powder as one of components to form the soybean-wheat mixed powder, the tests were firstly performed to analyze the storage stability of mixed powder, and dough rheology properties of the six kinds of mixed powders was also investigated, and then the impact mechanism was revealed. Based on the results of the study on dough rheology properties and the storage stability, noodles were used as an example to investigate the application of the mixed powder. The main findings were as follows:(1) Through the experiment of storage stability, it could establish the safe storage period of soybean-wheat mixed powder. When the percentage of whole soybean powder increased from 5% to 35%, the safe storage period of soybean-wheat mixed powder was shortened from 50 d to 30 d; but for adding inactive whole soybean powder, the storage period decreased from 55 d to 45 d. As for adding inactive defatted soybean powder, when its percentage in the mixed soybean-wheat powder was 35%, the safe storage period was not more than 55 d; but if reducing its percentage, the storage period could be more than 60 d. If adding defatted soybean powder, soybean protein concentrate powder or soybean protein isolate powder into wheat flour, when their percentage did not exceed 35%, they could all be safely stored over 60 d.(2) The brightness value L, red-green value a, yellow-blue value b and Hunter whiteness value of the mixed new soybean-wheat powders was related to the colors and mass of each individual powder in the mixed powder. When the color values of L, a, b and H of each individual powder were constant, the color value of the mixed powder was only related to the mass percent of each individual powder, shown a good linear relationship.(3) It was found that G' and G" of the mixed powder decreased along with the increase of the percentage of soybean powder added into the mixed soybean-wheat powder, but Tanδincreased. Adding soybean powder could not only reduce the elasticity of the mixed soybean-wheat mixed dough, but also lead to the decrease of its liquidity (or viscosity). The descent speed of the elasticity was faster than its viscosity, and with the increase of the proportion of viscous liquid, it trended to form "class liquid".(4) It was found that hardness, flexibility,ε_H,σ_max, G' and G", as for defatted soybean powder, inactivated defatted soybean powder, whole soybean powder, or inactivated whole soybean powder, had different variations with the increase of the adding percentage. When defatted soybean powder, soybean protein isolate or soybean protein concentrate powder was added less than 4%, or whole soybean powder less than 3%, the values of these parameters were higher than the control (only wheat dough). However, these parameters fell rapidly when the adding percentage was more than 9%, and then slowly fell down. The hardness, flexibility,ε_H,σ_max, G' and G" properties of the soybean-wheat mixed powder added inactive defatted soybean powder or inactive whole soybean powder were less than the control when the adding percentage was in the range of 0²0%.(5) Adding defatted soybean powder, soybean protein isolate powder, soybean protein concentrate powder or whole soybean powder into wheat flour (the percentage of defatted soybean powder, soybean protein isolate powder, soybean protein concentrate powder was not more than 4%, while whole soybean powder was not exceeded 3%), the reasons for the improved rheological properties were mainly that hydrogen peroxide oxidation which was produced from fat under the action of lipoxygenase oxidized–SH bond of gluten protein to form -S-S- bond, which leaded to the strengthened three-dimensional structure of gluten protein.(6) When the percentage increasing from 4% to 9% for defatted soybean powder, soybean protein concentrate powder or soybean protein isolate powder, and increasing from 5% to 9% for whole soybean powder, the reason for the weakened dough rheological properties of the mixed soybean-wheat powder was mainly attributed to the dilution of soybean protein for gluten protein; when the adding percentage of soybean powder was more than 9%, however, the protein aggregation which was from the interactions between soybean protein and wheat gluten protein resulting in the breakdown of gluten network mainly contributed to the weakened dough rheological properties.(7) Through the experiments of the sensory evaluation of noodles, it was found that defatted soybean powder was the best soybean powder as one of the components added into wheat flour, and its optimal percentage accounted for the mixed soybean-wheat powder was established 4%. Compared with the control, its chewing and hardness properties increased by 16.1% and 15.5%, respectively, its tensile breaking strength increased by 8.5%, and its sensory evaluation increased by 5.8%, which reached 91.(8) It was found that if adding defatted soybean powder into wheat flour, when the initial moisture of the mixed soybean-wheat powder was relatively low, soybean protein with high water absorption capacity competed the moisture with gluten protein macromolecular complex in the small regional competition, and then the bound water failed to arrival at the maximal content, leading to the uncompleted and discontinuous gluten structure and the appearance of massive objects at various sizes in cooked noodles, which finally resulted in the loss of noodles without any improvement.