A molecular mobility approach to describing the role of moisture in the Maillard browning reaction rate

In foods, water activity (a_w) can correlate to reaction rate, yet no theory conclusively links them. Mobility via the glass transition temperature (T_g) or solvation of reactants are two candidate mechanisms relating moisture content to reaction rate. A previous study suggested adding humectants with both plasticizer and solvent properties allowed nonenzymatic browning at very low aw.; Hypothesis. Both moisture and non-moisture solvents in foods affect Maillard browning reaction rate by allowing greater molecular mobility. Objectives. In a model system, determine the effect of humectants, a_w, and moisture content on physical state; Maillard browning reaction rate; and translational and rotational diffusion of a solute.; A freeze-dried model system mimicked a high protein/carbohydrate bar containing humectants. The reactants were sodium caseinate and glucose. Humectants were glycerol, exhibiting solvent character, sorbitol, a non-solvent type, or a control with no humectant. Physical state was characterized over multiple a_w via the T_g (DSC and DMTA), microstructure (enviro-SEM), and density (pycnomentry). Browning was measured by L, a, and b values and by A₄₂₀ of a filtered aqueous extract. Translational diffusion of 14C-mannitol was measured through a tube packed with the model system (omitting glucose) by taking scintillation counts of tube sections over time. Rotational mobility of glucose was measured by cross-polarization/magic angle spinning NMR spectroscopy. T_1ρ and T_CH were used as indicators of relative mobility.; Results show plasticization of the caseinate matrix with adsorbed moisture and added humectants. In comparison to the control, there was an increase in browning in the glycerol formulation over all a_w and moisture contents with a maximum at a_w = 0.33, and a slight or negligible increase for the sorbitol formulation at equal moisture contents. For the sorbitol, different T_g yet equal reaction rate suggested an alternate mechanism. Translational diffusion was insufficiently rapid to measure. Rotational mobility, theorized to indicate dissolution of glucose, increased with a _w, and was higher in the glycerol and sorbitol formulations than the control at equal a_w. Neither a_w, moisture monolayer value, nor T_g was a consistent predictor of reaction rates. Rotational mobility could be indirectly measured with no correction for humectancy, but results suggest a solvation mechanism.