The development of safe and sustainable coatings derived from food by-products represents an important strategy within circular bioeconomy frameworks. In this study, high internal phase Pickering emulsions (HIPEs) were developed using ultrasound-modified high-methoxyl pectin extracted from citrus processing waste combined with commercial zein from maize. The resulting polysaccharide–protein complexes were investigated as stabilizing particles for oil-in-water emulsions intended for edible coating applications on fresh-cut pineapple. Pectin (2.0% w/v) was extracted from citrus fresh-produce waste and structurally modified using high-power ultrasound. The modified pectin was subsequently combined with increasing concentrations of zein (0.0–2.0% w/v) to form pectin–zein complexes that stabilized emulsions with a fixed oil fraction (φ = 0.6). The influence of environmental pH on emulsion stability was evaluated by measuring droplet size distribution and ζ-potential at pH 4, 7, and 10 using electrophoretic light scattering (Zetasizer Nano, Malvern Instruments Ltd., UK). Results demonstrated a strong pH-dependent stability of the Pickering emulsions. The smallest droplet sizes were observed at pH 4, where positively charged zein interacts electrostatically with negatively charged pectin, promoting the formation of strong interfacial complexes and enhanced emulsion stability. Near neutral pH (≈7), close to the isoelectric point of zein (pI ≈ 6.2–6.8), electrostatic interactions weakened due to partial charge neutralization, resulting in increased droplet growth, aggregation, and reduced stability. Under alkaline conditions (pH 10), both pectin and zein carried negative charges, leading to electrostatic repulsion and a renewed decrease in droplet size. At this pH, stability was primarily governed by repulsive forces, as confirmed by the more negative ζ-potential values, although interfacial cohesion between biopolymers was reduced compared with acidic conditions. Overall, the findings highlight the critical role of environmental conditions, as the pH, in modulating polysaccharide–protein interactions and the stability of pectin–zein Pickering emulsions. These results support the potential of ultrasound-modified citrus pectin and zein as sustainable stabilizing particles for the design of circular edible coatings and thin films based on high internal phase emulsions.