*3.5. Stability of Quercetin*

### 3.5.1. Photochemical Stability

Quercetin is prone to degradation upon exposure to ultraviolet light due to oxidative decarboxylation of the C ring [22]. The retention of unencapsulated quercetin sharply decreased to 25% after 120 min of irradiation (Figure 10A). The quercetin loaded in HZ and HZ-chi/pec particles was more stable, with retention of about 80% after 120 min of irradiation. These results suggesting the hollow particle provide the excellent stability of quercetin against ultraviolet light. The protection results from the physical barrier and light-scattering effect of the particles [61]. The similar retention of quercetin in HZ and HZchi/pec particles (Figure 10A) indicates that the protection was fundamentally attributed to zein. A comparable protective effect has previously been reported by encapsulating quercetin in WPI/lotus root amylopectin, pea protein-isolated and zein/soluble soybean polysaccharide composite nanoparticles due to hydrogen and hydrophobic interaction between quercetin and proteins [39,43,64].

**Figure 10.** Retention of quercetin free and encapsulated in hollow zein particles coated without (qHZ) and with (qHZ-chi/pec) 1 mg/mL chitosan and 0.1 mg/mL pectin under irradiation (**A**) and during storage at 25 ◦C and 45 ◦C (**B**).

#### 3.5.2. Storage Stability

Liu and coworkers reported a drastic decrease in the content of quercetin to around 38% in 3 days of storage at room temperature [39]. The degradation of quercetin was faster at 45 ◦C than 25 ◦C, with the retention of quercetin being 42% and 16% after 2 days (Figure 10B), respectively. Its complete loss was observed at 25 ◦C and 45 ◦C after 4 days. The degradation of quercetin is attributed to its auto-oxidation in the aqueous medium, which is much pronounced at elevated temperature [19,65]. The retention of quercetin was significantly greater in HZ-chi/pec particles during storage, being 84% and 67% at 25 and

45 ◦C after 27 days (Figure 10B), respectively. The ABTS<sup>+</sup> scavenging capacity of quercetinloaded HZ-chi/pec particles was lower than quercetin alone (Figure 9), indicating that the entrapment of quercetin reduces its accessibility to ABTS+ [30]. It is thus speculated that the entrapment in HZ-chi/pec particles (Table 2) provides a physical barrier between quercetin and environment-sensitive factors, resulting in the improved stability of quercetin (Figure 10B). Moreover, the hydrophobic interaction and hydrogen bond of quercetin with excipient biopolymer (Figure 4) may inhibit the flavonoid autoxidation and prolong its storage life.
