Deep in drop : How microenvironments shape reactions and products

The Maillard reaction (MR) is a fundamental non-enzymatic browning process that underpins the generation of flavor, color, and functional compounds in thermally processed foods. While its mechanisms have been extensively studied in homogeneous aqueous solutions, the behavior of the MR in structured, heterogeneous aqueous environments, particularly allaqueous two-phase systems (ATPSs), remains poorly understood. Given the prevalence of such phase-separated aqueous systems in food matrices and biological contexts, this thesis investigates how spatial organization within ATPSs modulates Maillard chemistry, with a specific focus on product profiles, reaction mechanisms, and chemical complexity.To address this, two distinct classes of all-aqueous emulsions were studied: (i) conventional polymer and salt ATPSs based on polyethylene glycol (PEG) and Na₂SO₄, and (ii) natural deep eutectic solvent (NADES) and salt systems based on betaine, choline chloride, glycerol, and propylene glycol formulations combined with K₂HPO₄. These systems differ in their physical properties and molecular interactions, providing complementary platforms for investigating microenvironmental effects on Maillard pathways.In Chapter 2, Na₂SO₄-in-PEG emulsions were used as microreactors to examine how spatial partitioning of reactants affects Maillard product formation. Reactions were either co-encapsulated in the same phase or segregated into separate phases. It was demonstrated for the first time that interfacial reactions can occur in liquid–liquid emulsions due to precursor concentration at the interface, leading to selective product partitioning. For example, Amadori products and beta-carbolines preferentially accumulated in the PEG phase.Building on this, Chapter 3 employed untargeted metabolomics using HILIC-HRMS and molecular networking to systematically profile over 700 Maillard-derived metabolites from PEG and Na₂SO₄ emulsions. Distinct chemical fingerprints emerged depending on the localization of the reactants. Co-encapsulation favored the formation of Amadori rearrangement products and dipeptides, while segregation promoted oxidative degradation pathways and the interface-mediated generation of compounds such as N-formylkynurenine and beta-carbolines.Chapter 4 shifted the focus to NADES-based systems. Four NADESs (Bet:Gly, Bet:PG, ChCl:Gly, ChCl:PG) were synthesized and characterized, and their compatibility with K₂HPO₄ was assessed to form biphasic ATPSs. These systems retained the structural integrity of the NADESs and showed selective solute partitioning based on hydrophobicity. Under alkaline conditions, the Bet:PG-K₂HPO₄ system effectively enhanced the formation and stabilization of Amadori products, likely due to hydrogen bonding and reduced molecular mobility.In Chapter 5, untargeted metabolomics was again applied to explore alanine- and phenylalanine-driven MR in the NADES-based ATPSs. A total of 311 and 355 metabolites were identified in the respective systems. The NADES-rich phase stabilized reactive intermediates and promoted the formation of heterocyclic aromatic compounds, while the salt-rich phase favored glucose fragmentation and polar compound enrichment. Migration of reactive carbonyl species across the phase interface linked early degradation steps with downstream oxidation and pyrazine formation, illustrating how compartmentalized reactions dynamically evolve.Overall, this thesis demonstrates that both synthetic PEG-salt systems and NADES-salt systems function as tunable microreactors for Maillard chemistry. The insights gained provide a foundation for the development of sustainable and precision-engineered food systems. Such systems enable targeted flavor development, reduction of undesirable by-products, and novel strategies to control food quality through microenvironmental design.