Compartmentalization vs. segregation of reactants: Accomplishment of the Maillard reaction at the water-water interface

All-aqueous (water-in-water) emulsions are increasingly used as droplets reactors. The present communication reports that precursors of a reaction segregated by partitioning between emulsion phases can undergo reaction at the interface, i.e., on droplet surface, while the interface remains liquid. Na₂SO₄-in-polyethylene glycol (PEG) emulsions were prepared, and precursors (glucose, asparagine, and tryptophan) of the Maillard reaction were partitioned either inside the droplets (co-encapsulation) or segregated between the emulsion interior and exterior phases. It was found that following the interfacial (i.e., on-droplet) reaction of the segregated precursors, $\sim$99 % of the Amadori product N-(1-deoxy-D-fructos-1-yl)-L-tryptophan (Fru-Trp) partitioned into the PEG phase. Also, hydrophobic advanced reaction products including β-carboline derivatives and Strecker aldehyde, alongside melanoidins, showed a clear affinity towards the PEG phase. Once the precursors were co-encapsulated within Na₂SO₄ droplets, following their generation succinimide and pyridine derivatives remained partitioned within the droplets, whereas N-hydroxysuccinimide, pyrrole derivatives, and melanoidins predominantly partitioned into the PEG phase.