Studies On Chemical Modification Of Cassava Starch And Structure, Properties And In Vitro Digestibility Of Its Derivatives

In this study, three chemical methods including citric acid esterification, carboxymethylation and OSA esterification were used to modify and prepare cassava starch with different degree of substitution (DS). The influences of various factors and levels on DS of modified cassava starch in the process of reaction were investigated, and the structure, properties and in vitro digestibility of native cassava starch (NCS) and modified starch samples with different DS were compared. Moreover, the influence mechanism of substituent group and DS on the structural characteristics of starch derivatives were explored.Firstly, citric acid esterification was used to prepare citrate cassava starch (CCS) and the effects of esterification time, esterification temperature, esterification pH and citric acid content (ratio of starch dry basis) on the DS of CCS samples were investigated. The results showed that under the reaction conditions of esterification time 5 h, temperature 130 ℃, pH 3.0 and citric acid content 30%, the DS of CCS sample reached up to 0.178. In addition, compared with NCS, the infrared spectrum of all CCS samples appeared obvious characteristic peak in 1724 cm-1, which demonstrated that the esterification reaction happened. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) results showed that citric acid esterification occurred both in the amorphous and crystalline regions, and the relative crystallinity was reduced from 37.64% to 1.76%. DSC results showed that when the DS was lower, citric acid esterification could reduce gelatinization parameters of cassava starch, and when the DS was too high, starch samples could not be gelatinized. Moreover, after citric acid esterification, the freeze-thaw stability of starch paste had been improved, but the paste clarity was greatly reduced. Citric acid esterification modification could also increase the content of SDS and RS, when the DS was 0.178, SDS and RS content increased from 8.01% and 5.92% to 14.24% and 69.53%, respectively.Secondly, carboxymethylation was used to prepare carboxymethyl cassava starch (CMCS) and the effects of reaction time, reaction temperature, isopropyl alcohol concentration, nNaOH/nAGU and nMCA/nAGU on the DS of CMCS samples were investigated. The results showed that under the reaction conditions of reaction time 3 h, temperature 45 ℃, isopropyl alcohol concentration 80%, nNaOH/nAGU=2.5 and nMCA/nAGU=1-2, the DS of CMCS sample reached up to 0.674. In addition, compared with NCS, all CMCS samples appeared two obvious characteristic peaks in 1604 and 1426 cm-1, which demonstrated that the starch carboxymethylation happened. XRD and SEM results showed that the carboxymethylation reaction also occurred both in the amorphous and and crystalline regions, and the relative crystallinity reduced from 36.14% to 2.32%. DSC result showed that CMCS samples without gelatization peak. Carboxmethylation also raised the freeze-thaw stability and clarity of starch paste. Moreover, carboxymethylation significantly increased the content of RS and SDS in starch samples, when the DS was 0.674, the RS content reached up to 53.13%, and the DS was 0.536, SDS content was increased to 19.84%.Finally, OSA esterification was used to prepare octenylsuccinylated cassava starch (OSCS) and the effects of starch concentration, esterification time, esterification temperature, esterification pH and OSA content on the DS of OSCS samples were investigated. The results showed that under the reaction conditions of starch concentration 30%, esterification time 3 h, temperature 35 ℃, pH 8.5 and OSA content 9%, the DS of OSCS sample reached up to 0.035. In addition, compared with NCS, the infrared spectrum of all OSCS samples appeared two obvious characteristic peaks in 1724 and 1572 cm-1, which demonstrated that esterification reaction happened between OSA and hydroxyl groups on the starch. XRD and SEM results showed that OSA esterification only occurred in the amorphous region, it did not destroy the crystalline structure of starch granules. DSC results showed that gelatinization parameters of OSCS samples were lower than NCS, and with the increase of DS, gelatinization parameters decreased gradually. OSA esterification also raised the freeze-thaw stability and clarity of starch paste. In addition, the OSA esterification significantly increased the content of starch in RS and SDS, when DS was 0.035, the RS content reached up to 39.51%, and DS was 0.018, SDS content increased to 23.59%.In addition, three chemical modifications, namely citric acid esterification, carboxymethylation and OSA esterification, were compared in this study. The results showed that the modified mechanisms of three methods were different, and influence of these three methods on the structure, properties and in vitro digestibility were also different. Furthermore, citric acid esterification was a suitable method for preparation of RS, and OSA esterification was a suitable method for preparation of SDS.