## 3 Characterization of Food-Grade Systems
### Technological Characterization
#### 3.1.2 Surface Charge and Mucoadhesion
The mucoadhesive formulation obtained from the combination of the two polysaccharides proved to be an effective strategy for the delivery of curcumin targeted to the colon [92]. The surface properties expressed in the absolute value of zeta potential also determine other properties of nanosystems such as the stability of the colloidal system in the gastrointestinal tract and the redispersibility of lyophilized samples [93]. Zeta potential values indicating stability of NPs and limiting aggregation phenomena should ideally be higher than \(\pm\)30 mV [94; 95]. Zein has been widely applied for the development of new biomaterials such as NP. However, zein NP are prone to instability and premature release of active molecules in gastric fluids. For this reason, zein is often used combination with other hydrophilic anionic polysaccharides such as pectin and dextran sulfate [55]. In addition to its mucoadhesive properties, pectin is often used as a surface coating of nanosystems due to its stabilizing properties and low cost [56]. In study by Wang et al., core-shell NPs with a zein core and a pectin coating were prepared for delivery of the flavonoid hyperoside to the colon. The pectin coating resulted in a change in surface charge from positive (+28.9 mV) to negative values (approximately \(-\)30 mV). It was observed that the positive zeta potential of the zein NPs was neutralized as the anionic pectin was adsorbed onto the surface of the NPs. Different concentrations of pectin were studied (0.2 to 2 g/L) and it was found that when the concentration of pectin was so high as to cover the entire surface, the zeta potential maintained constant negative values. Although even the highest concentration completely coated the surface of NPs it could induce depletion flocculation and was discarded from the experiments. The pectin concentration of 1 g/L was selected because it stabilized the structure through hydrophobic effects, hydrogen bonds and electrostatic interactions and provided the highest encapsulation efficiency. In addition, the strong electrostatic repulsion between the particles induced steric stabilization that ensured excellent redispersibility of lyophilized samples and stability under simulated gastrointestinal conditions [96]. A similar study was conducted by Yuan et al., where dextran sulfate-stabilized zein NPs (ZNPs) were fabricated for curcumin delivery. By increasing the concentrations of the anionic stabilizer, the surface charge of the nanosystems gradually shifted toward negative values (Figure 12), indicating as in the previous case that the deposition of dextran sulfate on the surface of the NPs was driven by electrostatic and hydrophobic interactions. The addition of dextran sulfate not only improved the encapsulation efficiency and retention of curcumin in NPs, but also ensured increased bioaccessibility of the molecule in an in vitro model of simulated digestion. The increased bioaccessibility of curcumin could potentially promote absorption into the systemic circulation and thus improve in vivo bioavailability [60]. Therefore, appropriate surface properties, such as resistance to gastrointestinal digestion and mucoadhesive and mucopentrating features, need to be imparted to the nanosystems to ensure that the carriers can be effective colon tract-specific delivery systems.