## 2 Starch Modification Processes
### Chemical Modifications
#### 2.4.1 Crosslinking  
Crosslinking of starch is the reaction between the hydroxyl groups of the starch molecule and a compound with two or more functional groups [61]. A crosslink is an ionic or covalent bond that connects one starch chain to another. Crosslinking agents bridge the adjacent starch chains [9]. For example, a bifunctional or multifunctional crosslinking agent forms an intermolecular linkage with the primary hydroxyl groups at C2 and C3 carbons and the secondary hydroxyl group at C6 carbon [60], as shown in Figure 6.  
Widely used crosslinking agents are citric acid, glutaraldehyde, phosphorus oxychloride, sodium trimetaphosphate (STMP), sodium tripolynphosphate (STPP), and epichlorohydrin (EPI) [9; 60]. Crosslinked starch has remarkable properties compared to native starch, such as high mechanical, thermal properties, and water stability, as shown in Table 4. Citric acid (see Figure 7a) is one of the commonly tested crosslinking agents, which forms a strong hydrogen bond with starch, giving improved thermal and water stabilities and inhibiting retrogradation [62]. For example, [PERSON] and [PERSON] used citric acid as a crosslinker for corn starch and reported increased water and acid stabilities (see Table 4) [61]. [PERSON] et al. [63] conducted a study to explore the effect of citric acid on the biodegradable and non-retrograde starch-glycerol-based films. This work indicated that the addition of citric acid as a crosslinking agent decreased the water vapor permeability (WVP) by 35.71% while avoiding starch retrogradation and maintaining degradability during composting. Such observations were ascribed to the substitution of hydrophilic -OH groups by the hydrophobic ester groups. In addition, citric acid has also been used as an antibacterial agent.  
Epichlorohydrin (EPI), also known as 2-((chloromethyl)oxirane), is an organochlorine compound, see Figure 7b. The reaction of EPI with starch is considered a multifunctional reaction since one or two EPI molecules react to produce only one crosslinked bond (glycerol bond). However, starch crosslinked with EPI is not homogeneous [64]. [PERSON] et al. showed the improved tensile strength of high amylose corn starch thin films [65]. [PERSON] and coworkers exhibited a decrease in the moisture absorbance for banana starch upon crosslinked with EPI [64], as shown in Table 4.  
The blending of starch films with other synthetic or natural polymers is another common technique to improve the mechanical properties of starch. During mixing, EPI is used as a crosslinking agent. For example, potato starch-polyethene blended films, which were crosslinked with EPI, showed increased tensile strength and elongation at break of 54.48% and 8.32%, respectively [66]. [PERSON] et al. [67] displayed enhanced tensile strength of the starch thin films due to crosslinking with EPI, STMP, and STMP/STPP, as depicted in Table 4.  
Figure 5: The reaction mechanisms of different chemical modification processes.  
[PERSON] [9] compared the mechanical properties of oxidized and crosslinked sucrose films with non-crosslinked films. Oxidized sucrose acted as a crosslinking agent due to the presence of aldehyde groups [9]. The corn starch films crosslinked with oxidized sucrose exhibited tensile strength of 22.9 MPa and 59.5% elongation at break [9]. EPI crosslinked rice starch films also showed superior water barrier properties compared to untreated rice starch films, as shown in Table 4[67].