Thermodynamic Modelling of Water-Starch-Protein Interactions in Extruded Nigerian Cereal-Legume Blends

The extrusion cooking of cereal-legume blends involves complex thermodynamic and structural transformations driven by temperature, moisture, and shear, and the interactions among water, starch, and protein macromolecules determine the texture, digestibility, and functional properties of the extrudate. Existing thermodynamic models for extrusion have been developed primarily for single-component or simple binary systems, leaving the multi-component interactions in cereal-legume blends inadequately theorised. This study develops a thermodynamic framework for water-starch-protein interactions in extruded blends of maize, sorghum, and cowpea, which are the principal cereal-legume pairs used in northern Nigerian food product development. The theoretical contribution is a state diagram-based thermodynamic model that predicts glass transition temperature, gelatinisation enthalpy, melt viscosity, and structural organisation as functions of blend composition, moisture content, and processing temperature profile. The model is parameterised using thermodynamic measurements from differential scanning calorimetry, dynamic mechanical analysis, and time-domain nuclear magnetic resonance spectroscopy on extrudates produced across a temperature range of 120 to 180 degrees Celsius and moisture content of 14 to 22 percent. Molecular dynamics simulations complement the experimental data by modelling starch-protein hydrogen bonding interactions at the molecular scale. The thermodynamic model is validated against expansion ratio, bulk density, and in vitro digestibility measurements from a purpose-designed extrusion experiment. The study contributes an original thermodynamic framework and comprehensive experimental dataset for the extrusion of Nigerian cereal-legume blends, advancing food structure design methodology. Keywords: thermodynamics, extrusion, starch-protein interactions, cereal-legume blend, food structure