Biofuels can be made via processing in hydrothermal media, which have the potential of providing an efficient environment for reforming biomass to fuels. Hydrothermal media refers to processing in high-temperature (200-600C), high-pressure (3-30 MPa) sub- and supercritical water. These technologies have been demonstrated to produce a range of fuels, from gaseous fuels such as hydrogen and methane, to liquid fuels such as diesel and "biocrude". While the chemical reactions of individual species, such as glucose or cellulose, are fairly well known under these conditions, interactions between species are generally uncharacterized and can lead to unexpected reactions. We have studied the chemistry of glucose and glycine as model compounds of two of the most common categories of biomass, carbohydrates and proteins, and have confirmed a strong interaction under hydrothermal conditions similar to the well-known Maillard reaction that occurs at lower temperatures and pressures. However, we report behavior in which the reaction kinetics have an unexpected dependence on the initial concentrations of the reactants.