The objective of this work was to study the effect of three hydration regimens on the extent of matrix transformation, texture, microstructure and digestibility of a corn-based extruded product blended with apple pomace. Blends containing 0%, 17%, 22%, and 28% pomace were hydrated to 17.2% by adding water either into the preconditioner (P), the extruder (E) or dividing half in each (PE). Increasing the opportunity for hydration (P > PE > E) promoted more complete starch gelatinization at all pomace levels. Apple pomace promoted milder extrusion conditions, resulting in less starch gelatinization and solubilization, poor expansion and reduced starch digestibility. Digestible starch was inversely correlated to cell wall thickness/cell size ratio (r = 0.91), which increased with pomace addition. Thus, pomace decreased digestibility by promoting changes in structure and gelatinization. Delayed water addition (E) promoted broader cell size and cell wall thickness distributions, attributed to poor mixing and hydration. Therefore, water incorporation (not just total content) is a key factor in defining extrudate microstructure, texture and digestibility patterns. (C) 2011 Elsevier Ltd. All rights reserved.