Background and purpose: Globally, macadamia nut production generates substantial amounts of by-products, particularly macadamia husk, which is rich in phenolic compounds known for their antioxidant and health promoting properties. This study aimed to encapsulate a phenolic-rich extract from macadamia green husk to protect their stability for food applications. Phenolic compounds are often unstable and susceptible to environmental conditions such as heat, light, enzymes and pH.

Experimental approach: New Zealand macadamia husk phenolic extract (MHPE) was encapsulated into liposomes using high shear mixing in aqueous media (pH 6.5) and subsequently, high-pressure homogenization (2000 MPa), aiming to improve the liposomes colloidal stability. The surface of anionic liposomes was coated with cationic chitosan (0.4%, w/v) to increase the physical stability of liposomal structure. The liposomes were incorporated into yogurt as a food carrier to enhance the bioavailability of macadamia husk phenolics. To evaluate the characteristics of liposomes, particle size, zeta potential (ζ-potential), polydispersity index (PDI), encapsulation efficiency (EE), and morphology were determined. The in vitro cytotoxicity study of non-encapsulated extract and the liposomes containing MHPE were also investigated using Caco-2 cells. In vitro bioaccessibility was evaluated by measuring total phenolic content release during simulated gastrointestinal digestion of liposomes alone or when incorporated into a low-fat yogurt.

Key results: Liposomes, whether empty or loaded with MHPE, exhibited nanoscale particle sizes, low polydispersity, and negative surface charge (ζ-potential). The mean diameters of empty and MHPE-loaded liposomes were approximately 112 nm and 77 nm, respectively. Chitosan coating significantly increased vesicle size (empty: 522 nm; loaded: 501 nm) and broadened the size distribution (PDI, 0.41). Uncoated liposomes were strongly anionic ( −62.55 mV for empty; −43.11 mV for loaded), whereas chitosan coating reversed the surface charge to strongly positive values (+48.22 mV for empty; +49.99 mV for loaded), confirming successful adsorption of the cationic polymer. Transmission electron microscopy (TEM) showed that the vesicles were predominantly spherical in morphology, and their observed sizes were consistent with the size measured by dynamic light scattering (DLS). Coated liposomes achieved higher EE (91.15%) compared to uncoated liposomes (81.50%). Furthermore, both free extract and MHPE-loaded liposomes exhibited no cytotoxicity in Caco-2 cells. Simulated digestion of fortified yogurt demonstrated that liposomal encapsulation provided substantial protection under gastric conditions (low pH) but lost membrane integrity under intestinal conditions, releasing phenolic compounds. Importantly, coated liposomes exhibited a slower release of phenolics compared to uncoated liposomes.

Conclusion: These findings support the potential utilisation of macadamia husk as a readily available and rich source of phenolics. Liposomal encapsulation of these phenolic offers an effective strategy to protect them from negative environment (gastrointestinal) conditions and to enhance their delivery and bioaccessibility through a yogurt product.

Keywords: Macadamia husk phenolics; Chitosan-coated liposomes; Encapsulation efficiency; In vitro digestibility; Bioaccessibility.