Technical Abstract: The biochemical determinants of maize kernel hardness and their roles during extrusion processing were explored in an attempt to improve maize hybrid selection criteria for breeders, producers, and end-users. Physical maize kernel properties were evaluated using a tangential abrasion dehulling device, Stenvert grinding test, pycnometer density, test weight, and spectral analysis to characterize grain hardness. These methods were significantly correlated with one another and also with the endosperm protein and zein contents. Maize hybrids were clustered into groups based on hardness-associated properties by using multivariate statistical techniques. A representative hybrid from each cluster was selected for extrusion experiments. Maize hybrids representing each hardness cluster showed unique RP-HPLC chromatograms depending on endosperm texture. Maize kernel hardness significantly affected both the physical and textural properties of extrudates, including expansion ratio (ER), specific mechanical energy (SME), water absorption index (WAI), water solubility index (WSI), and oil absorption capacity (OAC). Extrudates from harder grits had lower protein solubility and greater protein aggregation after extrusion. Proteins in extrudates made from softer grits were more easily dissociated and degraded into smaller molecules as screw speed increased, which resulted in higher protein solubility and greater WAI, WSI, and OAC.