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

Abstract

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 0.05–3.43mm, wall thickness 0.12–0.34mm 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°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.