Cellular architecture and its relationship with mechanical properties in expanded extrudates containing apple pomace

Cereal-based puffed foods can deliver significant amounts of fruit and vegetable fiber, however a major hurdle is the accompanying decrease in expansion and poor textural properties. The objective of this work was to develop a comprehensive understanding of the operative rheology, processing conditions, cellular architecture, macrostructure and mechanical properties of high fiber, starch-based expanded matrices produced using extrusion. The combination of phase transition analysis and non-invasive X-ray microtomography provided a unique analytical approach for this purpose. A lab-scale twin screw extruder was used for processing directly expanded products from corn flour and apple pomace blends containing up to 22.5% total dietary fiber. Different levels of pomace (0-28%) and in-barrel processing moisture (17.5-25%) resulted in extrudates with a wide range of microstructures (average cell size 0.05-3.43 mm, wall thickness 0.12-0.34 mm and void fraction 0.53-0.76). Structural anisotropy biased towards radial expansion was observed in foams without pomace, whereas cellular isotropy, higher cell number density and greater longitudinal expansion were favored in the presence of pomace. A conceptual model for foaming and collapse was developed based on flow temperature of blends (123.7-167.6 degrees C), specific mechanical energy, and anticipated changes in visco-elasticity and extensibility of the high-fiber melt. Cell size was significantly correlated to average crushing force (r = -0.69), crispness work (r = -0.69) and number of spatial ruptures (r = 0.74). Microstructure data in combination with standard theory for brittle foams was employed to understand mechanical properties of extrudates under compression. (C) 2011 Elsevier Ltd. All rights reserved.