3.1.3. Encapsulation of Quercetin

Quercetin was used as a model bioactive flavonoid to be encapsulated in hollow zein particles. The loading efficiencies of quercetin were 72.71–79.86% in HZ particles, while the loading efficiencies increased to 90.58–93.86% at the flavonoid concentrations of 100–200 μg/mL and 85.84% at 250 μg/mL in HZ-chi particles (Table 2). The higher loading efficiency of quercetin in the presence of chitosan is attributed to its interaction with chitosan through electrostatic and hydrogen bonding, in addition to hydrophobic interaction with zein [33,34]. The loading capacity of quercetin in the absence and presence of chitosan remained similar and increased from about 2% to 6%. This is different from a decreasing trend that was commonly observed in composite protein particles with increasing the content of the wall materials [35,36].


**Table 2.** Loading efficiency and loading capacity of quercetin in hollow zein and zein-chitosan particles.

Values with different letters (upper case A, B and C for the concentration of quercetin in column; lower case a and b in the row for the type of particles) are significantly different (*p* < 0.05).

<sup>250</sup> 72.71 <sup>±</sup> 3.82 Aa 85.84 <sup>±</sup> 4.16 Bb 6.29 <sup>±</sup> 0.98 Aa 5.89 <sup>±</sup> 1.08 Aa

SEM images showed that quercetin-loaded HZ, HZ-chi and HZ-chi/pec were spherical with a smooth surface (Figure 2). The loading of quercetin increased the size of HZ particles without and with chitosan and/or pectin coating (Figure S1A). A similar trend was previously reported for the encapsulation of resveratrol and curcumin in composite hollow zein particles [11,37]. The loading of quercetin did not affect the ζ-Potential of HZ particles but increased ζ-potential absolute values of HZ-chi and HZ-chi/pec particles (Figure S1C), possibly due to rearrangement and exposure of more charged groups upon flavonoid inclusion [33].

**Figure 2.** SEM images of quercetin-loaded hollow zein (qHZ, (**A**)), zein-chitosan (qHZ-chi, (**B**)) and zein-chitosan/pectin (qHZ-chi/pec, (**C**)) particles. The concentrations of chitosan and pectin were 1 and 0.1 mg/mL, respectively.
