Encapsulation Using Plant Proteins: Thermodynamics and Kinetics of Wetting for Simple Zein Coacervates

Traditionally, complex coacervates of oppositely charged biopolymers have been used to form coatings around oil droplets for encapsulation of oil-soluble payloads. However, many proteins can form coacervates by themselves under certain conditions. Here, we revisit the well-known simple coacervates of prolamins such as zein in mixed solvents to explore whether they can be used for plant-based encapsulation systems. We show that, for zein in mixed water/propylene glycol (PG) solvents, we can encapsulate limonene droplets but only under specific conditions. We illustrate that this limitation is due to the very different physical properties of the simple zein coacervates as compared to those of the more extensively studied complex coacervates. Droplets of simple coacervates of zein can carry a significant net charge, whereas complex coacervates are usually close to being charge-balanced. In particular, we demonstrate that the spreading of zein coacervates at the interface of the droplets is thermodynamically favorable due to their extremely low interfacial tensions in both the dispersed (∼0.24 mN/m) and oil phases (∼0.68 mN/m), but the kinetics of coacervate droplet deposition and the interactions among coacervate droplets that oppose coacervate droplet coalescence are highly pH-dependent, leading to a sharp pH optimum (around pH 8) for capsule formation.