Study On The Mechanism Of Preparing High Protein Content Rice Flour By Commercial Raw Starch-active Enzymes

Rice flour,as the main raw material of rice-based foods,has the native defects of high starch content but low protein content,which not only cannot meet the needs of balanced nutrition,but also has poor processability in gluten-free food.However,there are many limitations in the current methods to improve the protein content in rice flour.Raw starch-active enzyme can directly act on specific areas of raw starch to avoid starch gelatinization,and has the advantages of maintaining pasting viscosity and other physicochemical properties of starch,which are more conducive to subsequent processing.Therefore,in this study,the commercial raw starch-active enzymes(Novozyme 5009,named N5009)containing fungal α-amylase and amyloglucosidase were selected.On the basis of hydrolyzing rice starch granules to analyze the hydrolysis regulation,the enzymes were applied to the preparation of high-protein rice flour,and its preparation mechanism was analyzed.Finally,the effect of this method on the processability of high protein content rice flour was evaluated based on the changes of physicochemical properties.The main research contents and results are as follows:(1)The analysis of the hydrolysis regulation to rice starch by N5009.N5009 showed the effective degradation of rice starch granules,and the hydrolysis degree reached 44.54% at 24 h.The releasing products of oligosaccharides were mainly glucose and maltose.The changes in their relative contents indicated that the relative activity of endo-acting α-amylase decreased faster than that of exo-acting amyloglucosidase,while the total hydrolytic activity decreased.The results from scanning electron microscopy(SEM)showed that N5009 hydrolyzed the starch granular surface through holes,pits and surface erosion.At the early stage of hydrolysis(1-6 h),the very short chains(DP < 6)increased from 3.65% to a maximum of 19.87%,and then decreased slowly to 13.91% in the late stage(9-24 h),in which the amorphous region was preferentially hydrolyzed.The weight-average molecular weight of starch decreased from 2.48× 107 Da to 1.41 × 107 Da during the hydrolysis process,indicating that the raw starch was significantly degraded at the molecular level.(2)Preparation and mechanism analysis of high protein content rice flour.The protein content of rice flour was increased from 8.52% of the native rice flour to 13.17% by N5009 hydrolysis at 55 ℃ for 24 h,which proved that this method could be used as an effective way to prepare high protein content rice flour.By analyzing the changes in the micromorphology of rice flour in the hydrolysis process and combining with the hydrolysis regulation of enzymes,the possible mechanism of preparation was deduced: The enzymes eroded the surface of starch particles through hydrolysis pattern similar to that of rice starch,resulting in the surface of starch agglomerates in rice flour loose and some starch granules falling off.As a result,the particle size distribution changed from unimodal to trimodal distribution,and the median particle size decreased significantly from 63.24 μm to 33.14 μm.X-ray diffraction(XRD)and Fourier transform infrared spectroscopy(FTIR)showed that the enzymatic hydrolysis increased the hydrogen bonding and short-range order in rice flour.In addition,the secondary structure of rice protein did not change significantly with the change of only about 2% of the random coil to β-sheet,indicating that the method could effectively maintain the structure of rice protein.(3)Study on the processability of high protein content rice flour.Differential scanning calorimetry(DSC)showed that the gelatinization temperature of high protein rice flour increased,and the gelatinization temperature range decreased by more than 6 ℃.The swelling capacity of high protein rice flour decreased significantly at 65 ℃,while the water solubility increased firstly and then decreased to a similar to that of native rice flour with hydrolysis time.Rapid viscosity analyzer(RVA)showed that the peak viscosity decreased to 37.54% of native rice flour after 1 h hydrolysis,and then recovered slowly to 51.89% of native rice flour at 24 h.The results in rheological experiment showed that the storage modulus and loss modulus of high protein content rice paste decreased,but the shear stress did not change significantly at low shear rate.These results reflected that the processability of rice flour was partially maintained.After hydrolysis by N5009,the change trend of rice starch was similar to that of rice flour,but the change degree was higher,indicating that starch components still played a leading role in the processability of rice flour.Therefore,the high protein content rice flour can be used in the production and processing of rice-based food.