Gel Improving Mechanism Study Of α-Amylases On Cross-Linking Tapioca Starch

In current food industry,low degree cross-linking tapioca starch cannot meet the requirements of gel strength,while adding higher degree cross-linking starch will deteriorate the taste of food.The young’s modulus of low degree phosphoric acid cross-linking cassava starch(CS)gel was increased by 100% using Aspergillus Niger α-amylase,and the gel improving mechanism was proposed: the hydrolysis of α-amylase affected the starch structure,leading to the increase of linear chains relative content in the leaching components during starch swelling,and as a result,starch gel network of modified starch was strengthened.However,in present study,only few amylases that had been patented were used to improve CS gel,which cannot meet the demand for wide production of gel-improving CS.In addition,the proposed gel-improving mechanism was not systematic,lacking characterization of starch crystalline structure and detailed molecular structure.According to the gel improving mechanism mentioned above,it was speculated that α-amylases with strong starch molecular degradation ability could hydrolyze the amylopectin in leaching components more efficiently,meeting the mechanism.Based on the screening principles for new amylases,including non-fungal amylase,commercial raw starch digesting amylases and strong starch molecular degradation ability,commercial α-amylase,A-and N-amylase were selected as new tool enzymes to prepare gelimproving CS.Through systematic analysis of modified starch structure,the gel-strengthening mechanism of the amylases was described.Firstly,the strong molecular degradability of A-and N-amylase was verified by multiple hydrolysis mode and molecular weight of starch hydrolysate depolymerized by amylases,followed by modifying phosphoric acid cross-linking tapioca starch using A-and N-amylase.The gelatinization characteristics and swelling behavior of modified starch were characterized:N-amylase decreased CS gelatinization peak viscosity by 20%,final viscosity by 75%,and CS swelling power(SP)and solubility(SOL)(p<0.05),but A-amylase had little effect on the gelatinization viscosity,SP and SOL,indicating that the hydrolysis modes towards CS between A-amylase and N-amylase might be different.The dynamic rheology measurement of CS gel showed that the storage modulus of CS gel could be increased by 10% and 29% by 23 h modification of A-and N-amylase,respectively.Both A-and N-amylase could strengthen the gel of CS,but N-amylase worked more obviously than A-amylase.Considering only molecular structural changes were involved in previous mechanism study,our study deduced the modification mechanism of A-and N-amylases by characterizing the changes of both molecular structure and crystalline structure of modified starch.SEM showed that the hydrolysis pattern of A-amylase on CS was superficial erosion,while the erosion of N-amylase was deeper.XRD patterns of all the samples did not change significantly,and the relative crystallinity of modified starch after enzymatic hydrolysis were higher than CS,indicating relative intact crystalline structure of starch samples,and the hydrolysis of A-and Namylase mainly happened in amorphous region in starch granule.With the measurement of SAXS,N-amylase exhibited higher hydrolysis degree to starch granule,combined with molecular structure determination,which reflected that N-amylase obviously depolymerized the molecular weight of amylose and amylopectin(p<0.05),with no significant change in amylopectin side chain length distribution,confirming that N-amylase hydrolyzed amorphous lamella in crystalline region.With weak depolymerization of amylopectin,A-amylase did not hydrolyze amylopectin significantly(p> 0.05),increasing relative amylose content from 22.59%to 25.51%.The modification mechanisms of A-and N-amylase were deduced: N-amylase extensively hydrolyzed cross-linking starch,producing smaller amylose and amylopectin that promoted molecular rearrangement and improving the gel strength.A-amylase increased the content of linear segments and strengthen the starch gel network,but the gel improvement was limited due to its superficial erosion.The gel improving mechanisms of A-and N-amylase were also applicable to other starch:N-amylase hydrolyzed high degree cross-linking tapioca starch(HCS),promoting swelling behavior and gelatinized viscosity of HCS,and the gel strength was increased from 7 Pa to 4074 Pa,while gel of HCS modified by A-amylase was increased from 7 Pa to 21 Pa,much weaker than N-amylase modification.In addition,A-and N-amylase enhanced the gel of native starch by A,122%,N,2419% for tapioca starch;A,15%,N,23% for wheat starch;A,8%,N,13% for corn starch;A,no gel-improvement,N,7% for potato starch.