Effects of extrusion processing on the physiochemical and functional properties of lupin kernel fibre
Lupin kernel fibre is an underutilised by-product of lupin protein isolation rich in insoluble dietary fibre. By means of extrusion technology, insoluble fibres can be converted in soluble fibres, which are considered the most effective dietary fibre fraction for human health. Lupin kernel fibre was processed at various barrel temperatures, feed moistures and screw speeds. The physiochemical (dietary fibre composition, colour, water and oil binding capacities) and functional (viscosity and bile acid interactions) properties of the lupin fibres after extrusion were evaluated compared to the non-extruded fibre. Due to extrusion processing, the soluble fraction of dietary fibre was increased from 1.9 g/100 g DM to up to 37.7 g/100 g DM, water binding capacity was increased by up to 95%, while oil binding capacity significantly decreased. Moisture content, followed by barrel temperature were identified as the most relevant extrusion parameters to influence physiochemical properties. To estimate effects of extrusion on fibre functionality, extrudates were digested under simulated gastro-intestinal conditions. Viscosity of in vitro digesta was increased for most extruded fibres. Accordingly, diffusion of bile acids was decreased, which may improve cholesterol lowering properties. Molecular interactions of lupin compounds with bile acids were not affected by the extrusion treatment. The results indicate that extrusion caused solubilisation of hydrophilic pectin-like polymers, which exhibit high hydration properties and thus increase the viscosity at physiological conditions. These findings suggest that extrusion could be a practical technology to enhance health benefits of lupin kernel fibre.