Effect Of Concentration-Induced Sodium Alginate Gel On The Retrogradation And Digestion Properties Of Starches Of Different Structure

Firstly,in order to develop a system coexisting starch gelatinization and sodium alginate gelation,the forming conditions of the system were investigated using rheological methods.As shown by the results,sodium alginate-calcium solution formed a solid-like gel structure(storage modulus G'>loss modulus G")after heating for a proper time when the concentrations of sodium alginate and calcium ion were 1%(w/v)and 3.0 mmol/L,respectively.When the concentration of rice starch was 4%-6%,the sodium alginate-Ca2+system containing starch not only converted from a initial liquid state(?>45°)to a final solid-like state(?<45°)after water evaporation for 30 minutes,but also had a good preparation operability.After water evaporation,the sodium alginate-Ca2+system containing starch prepared under the above concentration conditions showed a closely linked network structure under scanning electron microscope,and its G' increased while its flow index n decreased.These results confirmed that the alginate-Ca2+-starch system prepared under the above conditions can make the gelation of sodium alginate and the gelatinization of starch coexist.Secondly,four starch samples of different structure were obtained by acid hydrolyzing for 0,30,60,180 minutes,respectively(recorded as S,S1,S2 and S3 in order).Scanning Electron Microscope(SEM)pictures showed that acid treatment made the surface structure of starch have small change but remain the whole granule.X-ray diffraction(XRD)patterns showed that acid treatment did not change the original A-type crystal structure of the rice starch.Molecular weight distribution of starch was detected using high performance size exclusion chromatography with multi-angle laser light scattering and refractive index detectors(HPSEC-MALLS-RI).As shown by the results,the weight-average molecular weights(Mw)of amylose and amylopectin of the starch samples after acid treatment decreased gradually firstly and then increased sharply with the extensioI of acid hydrolysis time,and S2 had the minimum Mw of 4.027×106g/mol and 5.519×107g/mol for amylose and amylopectin,respectively,while S3 had the maximum Mw of 8.286×106 g/mol and 2.218×108 g/mol for amylose and amylopectin,respectively.The analysis results obtained using rapid viscosity analyzer(RVA)and rheometer showed that the peak viscosity and final viscosity of the acid-treated starch samples decreased significantly,and the maximum reducing rate were 70.84%and 85.64%,respectively;the elasticity of starch paste was weakened after water evaporation for 30 minutes.Thirdly,the pasting properties of rice starch samples of different structure with sodium alginate and Ca2+were studied using RVA(pasting with no moisture evaporation)and rheometer(pasting by moisture evaporated).As shown by the results,for samples without water evaporation,sodium alginate(A)and sodium alginate-Ca2+system(AC)significantly increased the peak viscosity of the starch.The increasing rate of viscosity of the S group was the lowest as the alginate system SA increased by 57.71%and the alginate-Ca2+system SAC increased by 51.90%.The increasing rate of viscosity of the S3 group was the highest as the alginate system S3 A increased by 162.9%and the alginate-Ca2+system S3 AC increased by 147.2%.For samples with water evaporation,before water evaporation,the G' of starch system samples was higher than G,showing a solid state,and the G' of starch-alginate-Ca2+system samples was lower than G" showing a liquid-like state;after 30-min water evaporation,the G' of all starch samples was higher than G,showing a solid-like state.With the extension of water evaporation time,the floxv index n of all the samples decreased and the starch-sodium alginate-Ca2+system samples formed a more compact and stable reticular structure.Fourthly,the effect of concentration-induced sodium alginate gel on the retrogradation of rice starch samples of different structure were investigated by determining free amylose content,moisture distribution,crystal structure and microstructure.As shown by the results,sodium alginate reduced the free amylose content showing the maximum reducing rate of 84.4%for S1 sample,but concentration-induced sodium alginate gel reduced the free amylose content more eftectively showing the laximum reducing rate of 86.9%for S1 sample.In addition,the concentration-induced sodium alginate gel not only significantly delayed the appear time of free water and decreased the content of free water,but also obviously decreased the crystallization peak intensity of starch samples,resulting in the inhibition of the retrogradation of rice starch.The inhibition effect increased with the acid treatment time of the starch samples.Finally,the effects of eoncentration-induced sodium alginate gel on the digestion properties of starch samples were analyzed using in vitro simulated digestion methods.As shown by the results,the digestibility of starch was changed by acid treatment,and S3 showed the maximum digestibility of 89%while S2 showed the minimum digestibility of 68%;concentration-induced sodium alginate gel not only reduced the digestibility of starch effectively,but also reduced the content of rapidly digestible starch(RDS)and increased the content of slowly digestible starch(SDS)and resistance starch(RS)effectively.