Elucidating the Maillard Reaction Mechanism in the Hydrothermal Liquefaction of Binary Model Compound Mixtures and Spirulina

Hydrothermal liquefaction (HTL) transforms wet biomass and biowaste feedstock into water-insoluble biocrude and valuable water-soluble organic products. HTL takes advantage of the exceptional properties of water near the critical point to break bio-macromolecule linkages, and as a reaction medium, subcritical water ions allow the reaction and recondensation of intermediates into various structures and product phases. Among the different reaction mechanisms involved in the hydrothermal conversion, the synergetic interaction between protein and carbohydrate intermediates, known as the Maillard reactions, can boost the renewable biocrude yield up to 25% higher than solely free fatty acid hydrolysis from lipids. However, the reaction mechanisms and the effect of biomass composition are not fully understood. In this study, the Maillard reactions have been investigated with a central composite design to validate the impact of mass ratio (carbohydrate/protein) over temperature and residence time, via product response analysis, including product yields, boiling point distribution, aqueous tests, elemental, and chemical compositions. Then, the gas chromatography–mass spectrometry data is used to propose a potential pathway, followed by levulinic acid from carbohydrates as a candidate, to produce proline via Maillard interactions before amino acid condensation into piperazinediones and reduction into pyrazines among other complex degradations. The understanding of Maillard interactions under HTL conditions could lead to more robust compositional models, and the present results showed that a protein-to-carbohydrate mass ratio 2:1 would maximize the volatile fraction, elemental carbon, and higher heating value in the renewable crude, with a 90–95% of the maximum crude yield being obtained. The HTL experiments were run with glucose and soy protein as model compounds and spirulina in a batch reactor at 300 °C temperature, 20 min residence time, and 50% mass ratio as a central point, with an alpha (α) of 20 °C, at 10 min, and a 25% mass ratio.