WAX-STABILIZED WATER-IN-OIL HIGH INTERNAL PHASE EMULSIONS: UNRAVELING STRUCTURE–DIGESTIBILITY RELATIONSHIPS THROUGH STATIC AND MICROFLUIDIC IN VITRO DIGESTION

Water-in-oil high internal phase emulsions (W/O HIPEs) have emerged as promising fat replacers for functional foods due to their ability to incorporate large amounts of water while maintaining desirable structural and textural properties. However, the impact of their microstructure on gastrointestinal digestion remains poorly understood. Therefore, this study investigated the stability and digestibility of W/O HIPEs structured with polyglycerol polyricinoleate (PGPR), sunflower wax (SW), and hydrolyzed sunflower wax (HSW) under static and semi-dynamic in vitro digestion conditions. W/O HIPEs containing 80% (w/w) aqueous phase and 20% (w/w) oil phase were produced by gradual water incorporation into the oil phase under rotor-stator homogenization at 80 °C. Static digestion (INFOGEST protocol) included oral, gastric, and intestinal phases, while semi-dynamic digestion combined a static gastric phase with a microfluidic intestinal chip operating under continuous flow. The microchip was designed to reproduce physiological intestinal phase, allowing controlled mixing of gastric chyme and intestinal fluids under laminar flow conditions. Microstructure, droplet size distribution, and lipolysis were evaluated and monitored throughout digestion. Fresh emulsions exhibited distinct structural characteristics depending on the stabilizing system. SW promoted the formation of a crystalline network in the oil phase, generating smaller and more homogeneous droplets, whereas HSW produced larger and more heterogeneous structures. During digestion, all HIPEs underwent droplet coalescence during the gastric phase and phase inversion in the intestinal phase. Nevertheless, SW-containing formulations retained greater structural integrity, while HSW systems formed larger aggregates and showed enhanced susceptibility to enzymatic hydrolysis. Under static digestion, HSW-HIPEs exhibited the highest lipolysis (33.5%), whereas SW-HIPEs and PGPR-HIPEs displayed lower values (17.8% and 12.6%, respectively), demonstrating an inverse relationship between structural stability and lipid digestion. Fluid-dynamic analysis revealed marked differences between digestion models, with the microfluidic chip operating under physiologically relevant laminar flow (Re ≈ 0.4) and the static model under highly turbulent conditions (Re ≈ 13,000). Despite these contrasting hydrodynamic conditions, both models exhibited similar overall digestion trends across the formulations. Wax type, concentration, and digestion conditions influenced W/O HIPEs digestibility, while the microfluidic model provided enhanced discrimination of formulation effects, supporting its application in lipid-based functional food evaluation.