**1. Introduction**

Hollow zein particles have been fabricated by wrapping sodium carbonate (Na2CO3) nanoprecipitate as sacrificial templet with zein in ethanol–water binary mixture followed by antisolvent precipitation [1]. Hollow particles in the loading and controlled release of bioactive components were preferred over their solid counterpart for more surface area and low density [2], but proteinaceous nature limits their utility as an efficient delivery system due to destabilization around pI (5–6.5), presence of counterion and high temperatureinduced denaturation [3,4]. Numerous strategies have been adopted to overcome instability issues of zein particles, for instance, coating with proteins [5,6], polysaccharides [7] and lipids [8]. Pectin is an anionic biodegradable polymer found in the plant cell wall and mainly made up of methyl esterified 1-4 linked α-D-galacturonic acid and 1-2-linked α-Lrhamnopyranose. Composite hollow zein particles with casein and pectin were developed by heating at 80 ◦C and 6.2 pH for 1 h to attain outstanding stability under simulated gastrointestinal conditions. Still, these composite particles were limited only to encapsulate and deliver heat-sensitive bioactives [9].

Chitosan-coated solid zein particles were fabricated through hydrophobic, hydrogen and van der Waals interactions at pH 4, improving the entrapment, photo/thermal

**Citation:** Khan, M.A.; Zhou, C.; Zheng, P.; Zhao, M.; Liang, L. Improving Physicochemical Stability of Quercetin-Loaded Hollow Zein Particles with Chitosan/Pectin Complex Coating. *Antioxidants* **2021**, *10*, 1476. https://doi.org/10.3390/ antiox10091476

Academic Editor: Elisabetta Esposito

Received: 26 August 2021 Accepted: 13 September 2021 Published: 16 September 2021

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protection and controlled release of bioactive components [10–12]. Chitosan, a N-acetyl-Dglucosamine and D-glucosamine β1-4 linked cationic polymer, is obtained by deacetylation of chitin and considerably used for the stabilization of delivery systems [13]. However, the deprotonation of the amine groups of chitosan above pka (~pH 6.5) reduces charge density, and chitosan competes with counterions with the increase in ionic strength, heading towards destabilization [12,14]. It has been reported that pectin imparted good pH and heating stability to zein core–shell nanoparticles, but the endurance for increasing ionic strength was extremely weak [15,16]. Chitosan and pectin could form polyelectrolyte complexes via electrostatic interaction [17], which was used to improve the physiochemical stability of nanoliposome [18]. Therefore, a combination of chitosan and pectin may synergistically and resourcefully bear variation in pH, temperature and counterions.

Quercetin is a flavonoid with antioxidant, anticarcinogenic, antiviral and anti-inflamm atory properties. Its low solubility in water and chemical instability have been addressed through biopolymer-based nano/micro-delivery systems for the application in functional foods [19,20]. In the current work, composite hollow zein particles were fabricated with chitosan-pectin complex coating for the encapsulation and protection of quercetin. The particles were characterized for size, ζ-potential and loading efficiency of quercetin. The lyophilized samples of quercetin-loaded composite particles were analyzed with infrared and X-ray diffraction techniques. Moreover, the particle dispersions were subject to varying pH, ionic strength and temperature conditions to assess physical stability. Finally, the antioxidant activity, photochemical and storage stability of quercetin encapsulated in composite hollow zein particles were examined to judge the protective effects of the particles. This study focused on the fabrication of composite hollow zein particles with improved physical stability and better protective effect on flavonoids through tailoring a complex polysaccharide interfacial layer.
