Effect Of Alkali/Salt Treatment On The Structure Of Cassava Starch

As the second largest natural polysaccharide polymer compound after cellulose,starch was widely used in food,chemicals,pharmaceuticals,etc because of its abundant resources,low price,renewable and biodegradable.However,since the starch structure was a semi-crystal mainly composed of hydrogen bonds,the cold water solubility,biocompatibility,etc were poor,which limited the development of starch.Therefore,in order to expand the application range of starch,the cassava starch was used as raw material and 90%ethanol was used as solvent,the structure of starch was modified with alkali/salt.The structure of starch samples was investigated by using polarized microscopy,scanning electron microscopy?SEM?,laser confocal microscopy?CLSM?,X-ray diffraction?XRD?,simultaneous thermal analyzer,Fourier transform infrared spectroscopy?FTIR?,ultraviolet-visible spectroscopy?UV-Vis?,laser confocal micro-Raman spectroscopy?LCM-Raman?and ¹³C uclear magnetic resonance spectroscopy(¹³C NMR).The amylose content?AM?and reducing sugar content of the samples were detected.The main research contents and results were as follows:The native cassava starch was treated with different concentrations of sodium hydroxide ethanol solution at 120°C for 4 h.The results showed that the relative crystallinity?RC?of the processed cassava starches could be reduced from 25.56%to 17.90%by adjusting the concentration of sodium hydroxide while keeping the integrity of the starch granule.During the decrystallization process,as the sodium hydroxide concentration was increased from 0.25 to 1.25 mol/L,the polarized light of the starch granules disappeared from the inside to the outside,and the XRD peaks of native cassava starch disappeared sequentially from 17.16,23.04,15.08,to 17.83°.On the other hand,the FTIR peaks of the native cassava starch centered at 1645 cm^-11 was shifted to about 1600 cm^-1,and the intensity of the peaks at 3424,2929,1645,and 1017 cm^-11 become weaker with decrystallization process.It was indicated that different concentrations of sodium hydroxide had a significant effect on the starch structure.In order to further understand the differences effect of different alkalis on the structure of cassava starch,the cassava starch was treated with different alkalis?lithium hydroxide,sodium hydroxide,potassium hydroxide,rubidium hydroxide,and caesium hydroxide?with a concentration of 0.2 mol/L under the same reaction conditions.The results showed that the integrity of cassava starch particle morphology was not impacted as treated with different alkalis treatment.The result of FTIR showed that no new functional group formed,but the peak at 3412cm^-11 of treated cassava starches shifted to the high wavenumber and their intensities were lower than the native cassava starch.Moreover,as the period of the corresponding element of the alkali metal increased,the destruction of the internal structure of starch was obviously enhanced.The crystalline form of cassava starches was unchanged and its RC decreased from 22.03%to 15.07%.The AM of the cassava starches was gradually decreased from 22.30%to 16.51%.The total reducing sugar content of all treated samples was higher than the native cassava starch.In summary,the effect of different alkalis treatment on the structure of cassava starch showed significant differences.In order to understand whether the salt had a similar alkali effect on the structure of cassava starch,the cassava starch was treated with four kinds of salt alcohol solutions of lithium chloride,lithium nitrate,calcium chloride and calcium nitrate under the same reaction conditions.The results showed that although the integrity of cassava starch particles was maintained after the treatment,the internal structure of its was heavily destroyed.The crystalline form of the treated starches was not damaged,but the RC were sequentially reduced in the order of lithium nitrate,lithium chloride,calcium nitrate to calcium chloride.The FTIR peaks at3424 cm^-11 and 1645 cm^-11 were shifted to the low wavenumber after the salt treatment.In addition,compared to the native cassava starch,except for the AM of the sample treated with lithium nitrate was increased,the rest were decreased.All the decomposition temperature after water loss of treated starches were lower than the native cassava starch,which the order was consistent with their relative crystallinity.In summary,the effect of different salts on the structure of cassava starch showed significant differences in the ethanol solution.Moreover,the order of strength of different salts was:calcium chloride>calcium nitrate>lithium chloride>lithium nitrate.When studying the effect of salt on the structure of cassava starch,it was found that the effect of ferric chloride treatment on the crystalline form of cassava starch showed significant differences compared with other salt treatments.Therefore,the structure of cassava starch treated with different concentrations of ferric chloride was systematically studied and its mechanism was explored.The results showed that with increasing the ferric chloride concentration,the morphology of the cassava starches was damaged,the crystalline form changed from A type to A+V type,a new absorption peak of UV at 281 nm appeared,the peaks of FTIR at3424 cm^-11 and 1642 cm^-11 shifted to lower wavenumbers,the full width at half maximum?FWHM?of LCM-Raman band at 476 cm^-11 increased from 19.86 cm^-11 to 23.40 cm^-1.The starch treated with ferric chloride showed new chemical shifts at 19.0,56.5,79.8,77.2 and 70.4 ppm.In summary,ferric chloride treatment significantly impacted the structure of cassava starch,and its mechanism was to hydrolyze it.