Spatial distribution of glucose and amino acids within all-aqueous emulsions directs the Maillard reaction and oxidation pathways

All-aqueous emulsions represent a versatile platform for studying and controlling chemical reactions in foods and biological systems. Through the compartmentalization and segregation of reactants, distinct molecularly crowded microenvironments enable the generation of unique reaction products. We explored how spatial organization within all-aqueous emulsions composed of polyethylene glycol (PEG) and sodium sulfate (Na2SO4) modulates the Maillard reaction and oxidation reactions between glucose and amino acids. Using untargeted metabolomics and molecular networking, we characterized the chemical diversity of reaction products formed when the reactants were either co-encapsulated within the droplet phase or distributed (segregated) between the two phases of the emulsions. Over 500 compounds were annotated across both systems, revealing distinct molecular profiles driven by reactant localization and phase partitioning. When the precursors were segregated (tryptophan and glucose), oxidation products as aminobenzoyl-, hydroxy- and hydroperoxy-derivatives such as β-carbolines and N-formylkynurenine accumulated preferentially in the PEG phase. Conversely, when the reactants were co-encapsulated (asparagine and glucose) within Na2SO4 droplets, enhanced formation of the Amadori products and dipeptides was observed, guided by phase-specific microenvironment. Our results demonstrate that the reactant location, in addition to time and temperature, plays a critical role in modulating food-relevant reactions, with a new framework for controlling the formation of glycation compounds via emulsion-based microreactors.