| Dioscorea esculenta(Lour.)brukill grows mainly in Hainan and the south of Guangdong and Guangxi,with a few planted in Fujian and Jiangsu in China.In recent years,with the support of the national poverty alleviation policy,the amount and production of D.esculenta has increased year by year.however,their post-production processing rate is extremely low,which seriously restricts the development of their production.The D.esculenta is rich in protein,fibre,starch,polysaccharides and various essential trace elements.It is particularly rich in starch at 19%.Therefore,it is necessary to develop D.esculenta starch to improve its storability and broaden its processing varieties.Preparation of D.esculenta starch by the lye method using Na OH.Preparation of D.esculenta resistant starch by autoclave and autoclaved-enzymatic method.The microstructure,iodine absorption properties,thermodynamic properties,pasting properties,rheological properties and other physical properties of starch and resistant starch were then studied in comparison.The main experimental results are as follows.(1)The best colour protection process for D.esculenta starch: The best process conditions for colour protection of D.esculenta starch were determined by conducting L9(34)orthogonal test with four colour protection agents,namely Vc,citric acid,Na Cl and phytic acid as factors: i.e.Vc 0.2%,phytic acid 0.2%,Na Cl 0.2% and citric acid 0.3%.The whiteness of D.esculenta starch extracted under these conditions was 92.3±0.125%.The main order of influence of each factor on the whiteness of D.esculenta starch was: citric acid > phytic acid > Na Cl > Vc.(2)The optimal process conditions for the preparation of D.esculenta starch: A response surface optimisation test was conducted using p H,liquid to solid ratio,extraction temperature and extraction time as response factors.The optimum preparation conditions were: p H 10.5,liquid to solid ratio 6:1,extraction temperature 25℃ and extraction time 84 min.The main order of influence of the factors on the extraction rate of hairy potato starch was: p H>liquid to solid ratio>extraction time>extraction temperature.(3)The optimum process conditions for the preparation of D.esculenta resistant starch by autoclave method: The process of preparing resistant starch from D.esculenta by autoclaving was optimised by L9(34)orthogonal test with starch emulsion concentrations,p H value,autoclaving temperature and autoclaving time as factors.The main order of influence of the factors on the yield of D.esculenta resistant starch was: starch emulsion concentrations > autoclaving time > autoclaving temperature > p H.The optimum process conditions for the preparation of D.esculenta resistant starch by autoclavinging were: starch emulsionconcentrations of 20%,p H 6,autoclaving temperature of 115°C and autoclaving time of 60 min,and the yield of resistant starch was 25.92%±0.09%.(4)The optimum process conditions for the preparation of D.esculenta resistant starch by autoclaved-enzymatic method: The process of preparation of resistant starch from D.esculenta by autoclaved-enzymatic method was optimized by L9(34)orthogonal test with starch emulsion concentrations,enzyme addition,enzymatic hydrolysis time and autoclaving time as factors.The main order of influence of the factors on the yield of D.esculenta resistant starch was: starch emulsion concentrations > enzyme addition > enzymatic hydrolysis time > autoclaving time.The optimum process conditions for the preparation of D.esculenta resistant starch by the autoclaved-enzymatic method were: starch emulsion concentrations of 20%,enzyme addition of 30 U/g,enzymatic hydrolysis time of 12 h,autoclaving time of 50 min,and resistant starch yield of 35.32%±0.18%.Compared with the autoclave method,the yield of resistant starch was increased by 9.4% in the autoclaved-enzymatic method.It can be seen that the combined effect of the autoclave and enzyme methods can significantly improve the yield of D.esculenta resistant starch.(5)The microstructure of D.esculenta starch and resistant starch was analysed by electron microscopy,laser particle size,X-ray diffraction and infrared spectroscopy.The results were as follows: D.esculenta starch granules were polygonal and belonged to C-type starch with an average particle size of 4.636μm,which was a super small granule starch;resistant starch had a multi-layer and multi-fold structure and belonged to C-type starch with an average particle size of 60.25μm.The differences between the two in the infrared spectra were not significant,and no new groups were produced or disappeared,except for the C-H at 2927 cm-1,C=O at 1647 cm-1 and D-pyran structures of resistant starch,which were damaged.Indicating that the autoclaving treatment only changed the physical properties of the D.esculenta starch.(6)Analysis of iodine absorption characteristics: the strong absorption peak of the original starch appeared at 580~620nm,while the strong absorption peak of the resistant starch appeared at 560~600nm.Compared with the original starch of D.esculenta,the absorption curve of resistant starch is narrower and less intense,indicating that the molecular mass of resistant starch is more concentrated and the enzyme and autoclave treatment formed straight chain starch molecules with less polymerisation.(7)DSC thermodynamic characteristics analysis: the thermal absorption peak of the original starch was between 70~81℃;the thermal absorption peak of the resistant starch was between 106~151℃,and Tp,To and Tc were higher than the original starch.And the △H of resistant starch is 2901J/g,which is much higher than the original starch and the resistant starch of maize,wheat,potato and wai yam.It can be seen that D.esculenta resistant starch has highthermal stability.(8)Analysis of RVA pasting characteristics:RVA eigenvalues Except for the valley viscosity,the paste-forming temperature,peak viscosity,disintegration value,final viscosity and regeneration value of the D.esculenta original starch are all greater than those of the resistant starch.This indicates that the thermal and cold stability of the resistant starch is stronger than that of the original starch,and the short straight chain starch formed reduces the viscosity of the system.(9)Rheological characteristics analysis: The Herschel-Bulkley fitting parameters of the rheological curves of both D.esculenta starch and resistant starch satisfy the yield-pseudoplastic fluid law.The resistant starch has a smaller hysteresis area and yield stress than the original starch,and the fluid index n is larger,indicating that the shear stability of the starch is improved and the pseudoplasticity is reduced after autoclaving.(10)Analysis of other physical properties: a.The solubility and swelling power of both D.esculenta starch and resistant starch increase with the temperature.Before reaching the pasting temperature of D.esculenta starch(80℃),the solubility and swelling power of the original starch are smaller than that of resistant starch;after reaching the pasting temperature,the solubility and swelling power of the original starch rise suddenly and exceed that of resistant starch.b.When D.esculenta starch is not resting,its transmittance is lower than that of footpad potato,mung bean,wheat and potato starch.During the 0-12 h resting process,the transmittance of both the original starch and resistant starch decreased with time,and the transmittance of the original starch was always greater than that of the resistant starch.c.The retrogradation of the resistant starch of D.esculenta was much greater than that of the original starch,and the retrogradation was basically completed within one hour;the original starch of D.esculenta stabilised only after 10 h.d.The freeze-thawing property of D.esculenta starch was stronger than that of the resistant starch,and the water precipitation rate reached 6.43% after three times of freeze-thawing.6.43%;the precipitation rate of resistant starch was 11.24%.The precipitation rate of the D.esculenta original starch of is also smaller than that of the starch of lentils,mung beans and Chinese yam,which makes it more suitable for use in frozen foods. |
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