Physicochemical Modification Of Starch And Selective Hydrolysis Mechanism Of Amylase

The applications of natural starch in some special foods are greatly limited because of its some physiochemical properties,such as viscoelasticity,retrogradation,and digestibility.In this paper,natural starch was modified by biological and physical methods to meet the application requirements of different industrial fields.The experimental results of this paper are as follows:Natural starch modified by fermentation.The effects of yogurt containing active lactic acid bacteria on starch viscosity were investigated.Corn starch was used as raw material,and 1%(V/V)yogurt was added respectively,and kept at 37?for 1 h;Study on rheological properties of fermented corn starch by rheometer.The shear rheological curve obeys Herschel-Bulkley model(R~2>0.90).Yield stress?0,viscosity coefficient k increased,and flow index n decreased(n<1),which indicated that fermented corn starch was still a non-Newtonian,yield pseudoplastic fluid.Oscillatory rheological curves showed that the storage modulus G'and loss modulus G''of the sample group added yogurt were larger than those of the blank group,and both increased with the increase of shear frequency.Corn starch fermented by lactic acid bacteria has higher viscoelastic properties.Physical modification of natural starch.The effects of peach gum with different concentrations(0%,4%,8%,12%,16%)on the dynamic rheology,thermal properties and digestibility of wheat starch in vitro were studied.Compared with wheat starch alone,the mixture of wheat starch and peach gum showed higher pseudoplasticity.With the increase of peach gum concentration,the consistency coefficient(k)of the mixture gradually increased and the flow index(n)gradually decreased,which indicated that peach gum increased the consistency and pseudoplasticity of the mixture.When peach gum exists,the rheological parameters of wheat starch,such as storage modulus(G')and loss modulus(G''),increase.Peach gum reduced the gelatinization enthalpy of the mixture from 9.56 J/g to 7.85 J/g(p<0.05).However,with the increase of peach gum concentration,the content of fast digestible starch decreased from 87.2%to 50.5%,while the content of slow digestible starch and resistant starch increased(p<0.05).The results of IR showed that no new covalent bond was formed between peach gum and wheat starch,but peach gum could help the mixture to form a more orderly structure.Peach gum has the characteristics of improving the viscoelasticity of wheat starch and reducing the digestibility,and has potential application value in developing soft quality control sugar wheat food.Enzymatical modification of natural starch.The hydrolysis effects of the mutant amylase on pullulan and?-cyclodextrin were determined respectively.The viscoelastic properties of potato modified by amylase were evaluated.The results showed that compared with wild-type amylase,the hydrolysis ability of mutated amylase to pullulan and?-cyclodextrin decreased.With pullulan as substrate,the specific activity of mutated amylase was less than 5.447 U/mg.With?-cyclodextrin as substrate,the specific activity of the mutated amylase is less than 0.72 U/mg,and even the mutated enzyme has no hydrolysis ability to?–cyclodextrin.This provides a theoretical basis for the enzymatic preparation of cyclodextrin.The amylase reduced the rheopectic property,storage modulus and loss modulus of potato starch.