Effects Of β-amylase On The Structural Properties,Gelatinization Properties And Retrogradation Properties Of Wheat Starch

As one of the most common grain crops in China,wheat is an important raw material for the production of a variety of traditional pasta products.Retrogradation is one of the most important factors to affect the the quality of pasta products during the storage process,which could lead to the decline of product sensory characteristics and texture characteristics.Therefore,it has been an important area of research to improve the quality of starch products by inhibiting starch retrogradation.β-amylase has a good potential to resist retrogradation as it could affect the orderly arrangement and recrystallization of starch by trimming the branched chains of amylose and amylopectin and changing the proportion of amylose and amylopectin.However,the current researches onβ-amylase mostly focused on the direct application,and there was no detailed and systematic study on the effect and the law of action in various systems.In this paper,the effects of differentβ-amylase concentrations and enzymatic hydrolysis time on the structural properties,gelatinization properties and retrogradation characteristics of wheat starch were investigated.Meanwhile,the relationship between the enzymatic hydrolysis conditions and the changes of starch properties was analyzed.On the basis,the effects ofβ-amylase in starch systems and protein-starch systems with different moisture contents were explored.The main research contents and results are as follows:1.The effects of different enzymatic hydrolysis times(10 min～300 min)and different enzyme concentrations(10 U/g～50 U/g)on the structural properties of wheat starch were studied.With the increase of enzymatic hydrolysis time,the degree of hydrolysis of the enzymatic hydrolyzed starch sample(BA-WS)increased,the ratio of amylose/amylopectin decreased,the relative content of low DP branched chains increased and the hydrolysis rates were higher in the early stage of enzymatic hydrolysis.After enzymatic hydrolysis for 300min at 20 U/g,the starch content decreased from 98.56%to 73.60%,the amylose/amylopectin ratio decreased from 26.91%to 17.92%;the heavy average molecular weight decreased from2.390×10~6to 1.355×10~6,and the dispersibility coefficient showed a trend of increasing first and then decreasing.There was a similar effect of increasing enzyme concentration on the structural properties of BA-WS.Higher enzyme concentration made starch have higher hydrolysis degree and lower heavy average molecular weight,with starch content decreasing to 85.85%and heavy average molecular weight decreasing to 2.112×10~6after 30 min of enzymatic hydrolysis at 50 U/g.There was a stronger correlation between enzymatic hydrolysis time and changes in the structural properties of starch than enzyme concentration.2.The gelatinization characteristics of BA-WS were studied by differential scanning calorimetry(DSC),rapid viscosity analysis(RVA)and dynamic rheological testing,etc.After enzymatic hydrolysis,the viscosity of the starch system decreased,the swelling power decreased and the solubility index increased,the storage modulus(G’)and loss modulus(G’’)decreased and the gelatinization enthalpy decreased.The gelatinization properties of BA-WS changed gradually with increasing enzymatic hydrolysis time,and the difference in gelatinization properties between enzyme concentration groups was small.3.The effects of different enzymatic hydrolysis conditions on the retrogradation properties of BA-WS were investigated combining methods such as X-ray diffraction(XRD),DSC,confocal laser scanning microscope(CLSM),low-field nuclear magnetic resonance(LF-NMR)and total texture analysis(TPA),etc.The crystallinity and retrogradation enthalpy of BA-WS decreased significantly during storage.After enzymatic hydrolysis at 50 U/g for 30min,the crystallinity of the BA-WS decreased by 48.49%stored for 14 days,and the retrogradation enthalpy decreased by 35.64%after 21 days of storage compared with the blank group.The enzymatic hydrolysis of BA-WS at 20 U/g for 300 min resulted in a 68.30%decrease in crystallinity after storage for 14 days and a 60.06%decrease in retrogradation enthalpy compared with control sample stored for 21 days.Moreover,the molecular aggregation was reduced and water migration was inhibited..The Avrami model proved that the rates of nucleation and crystallization of wheat starch were decreased,and the dynamic retrogradation processes were slowed down.On the macroscopic side,the starch gel had lower hardness and higher water holding capacity after enzymatic hydrolysis.β-amylase treatment could effectively inhibit the short-term and long-term retrogradation of wheat starch.The inhibition effect increased with increasing enzymatic time,and higherβ-amylase concentration had stronger inhibition effect at the same enzymatic time.4.The anti-retrogradation effects ofβ-amylase in pure starch systems and protein-starch systems at different concentrations(6%,20%,40%)were studied and contrasted.β-amylase retarded the retrogradation process in pure starch systems at all concentrations significantly,and the increasing of relative crystallinity,enthalpy of retrogradation and hardness during storage was significantly suppressed.For the protein-starch system,β-amylase could produce a better synergistic effect with protein molecules and jointly retard the occurrence of starch retrogradation.The samples stored for 14 days after enzymatic hydrolysis produced a reduction in relative crystallinity ranging from 7.22%to 27.68%compared with the non-enzymatic group.The gels had softer texture during storage compared with control protein-starch system meanwhile,indicating that the value ofβ-amylase for anti-retrogradation applications in complex product systems.