## 2 Starch Modification Processes
### Chemical Modifications
#### 2.4.3 Alkali Treatment  
Bases such as potassium hydroxide (KOH) and sodium hydroxide (NaOH) are widely used to modify starch. However, the alkali medium works more effectively on dry starch. After the addition of alkali, the mixture should be neutralized using hydrochloric acid or acetic acid. [PERSON] et al. [74] investigated the effect of alkali treatment on the properties of thermoplastic starch films prepared by melt extrusion. The data showed that increasing NaOH concentration changed the tensile strength of high amylose corn starch (HAS) and gradually increased the elongation at break. Moreover, the tensile strength of the HAS film with 2% of NaOH slightly increased up to 10.03 MPa (see Table 4), while its elongation at break also increased up to 40%. The increased tensile strength and elongation at break were attributed to the low alkali concentration promoting the gelatinization of HAS. The gelatinization facilitated the leaching of amylose from the crystalline structure while forming more hydrogen bonds with amorphous regions. Besides, NaOH also rearranged the molecular chain of HAS and changed the crystal structure of starch, increasing the space within molecular chains and improving the elongation at break. The HAS film treated with 10% NaOH exhibited the lowest tensile strength of 7.96 MPa and the highest elongation at break of 60%. During the melt extrusion process, NaOH oxidized the anhydroglucose of starch, causing irregular crosslinking of the molecular chains while increasing the degree of branching and molecular gap of the starch chain, resulting in increased flexibility and reduced tensile strength [74]. According to the research carried out by [PERSON] et al. [71], the increase in pH reduced the tensile strength and the modulus while exhibiting an increase in the elongation at break. The highest elongation at break and the strength of the films were obtained at pH 11 and 5, respectively. Moreover, compared to the elongation at pH 7, the pH 11 films showed a significant increase in elongation at break. Furthermore, the elongation of the films varied from 1.9 to 2.5%, which was considerably lower than synthetic polymer-based films. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Property of Starch** & \multicolumn{2}{c}{**Chemical Modification**} & \multicolumn{2}{c}{**Values**} & **References** \\ \cline{2-5}  & **Reaction Type** & **Reagent** & **Before** & **After** & \\ \hline  & Oxidation & Sodium hypochlorite & 173.8 mPa-s & 151.8 mPa-s & [73] \\ \hline Viscosity & Grafting & Cassava starch grafted with Polyacrylamide (CS-p-PAM) & 1717 mPa-s & 4178 mPa-s & [76] \\ \hline  & Crosslinking & Epichlorofri (EPT) & 10.04 MPa & 15.51 MPa & [66] \\ \hline  & Crosslinking & Epichlorofri (EPT) & 5.01 MPa & 7.99 MPa & [67] \\ \hline  & Crosslinking & Sodium Trimetphosphate (STMP) & 5.01 MPa & 8.23 MPa & [67] \\ \hline  & Crosslinking & Sodium Trimetphosphate (STMP)/ Sodium Drop Dropbeat (STTP) & 5.01 MPa & 7.57 MPa & [67] \\ \hline  & Alkali treatment & Sodium Hydroduce & 9.51 MPa & 10.03 MPa & [74] \\ \hline  & Etherification & Carboxymethyl starch & 1.1 MPa & 0.2 MPa & [77] \\ \hline  & Oxidation & Sodium hypochlorite & 3.8 MPa & 1.8 MPa & [73] \\ \cline{2-5} Tensile strength & Oxidation & Sodium hypochlorite & 3.53 MPa & 6.07 MPa & [78] \\ \cline{2-5}  & Grafting & Thermoplastic Starch & 2.40 MPa & 0.06 MPa & [79] \\ \cline{2-5}  & Oxidation & Sodium hypochlorite & 3.88 MPa & 5.05 MPa & [80] \\ \hline  & Oxidation & Sodium hypochlorite & 4.66 MPa & 8.39 MPa & [81] \\ \hline  & Oxidation & Hydrogen peroxide & 18.8 MPa & 24.7 MPa & [82] \\ \hline  & Etherification & Hydroxyproylated starch & 3.1 MPa & 4.9 MPa & [83] \\ \hline  & Grafting & Starch grafted with polystyrene & 43 MPa & 25 MPa & [84] \\ \hline  & Etherification & Carboxymethyl starch & 31 MPa & 14 MPa & [83] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Changes in the properties starch upon chemical modification.