Influence of extrusion on expansion, functional and digestibility properties of whole sweetpotato flour

Beerwah Gold, Northern Star, Snow White, and L49 cultivars of sweetpotato from Australia and Papua New Guinea, were studied for their extrusion behaviours in a co-rotating twin-screw extruder at three moisture (30, 35, 40 g/100 g) and screw speed (150, 220, 300 rpm) levels with a slit die. Low moisture increased the die pressure (2-6 bar) and specific mechanical energy (280-600 kJ/kg) of the extruder. Expansion, functional and digestibility properties of the extrudates were extrusion-dependent and cultivar-specific. Extrusion moisture increased the longitudinal expansion (15-30 m/kg) of the extrudates, which were almost completely gelatinised (100 g/100 g degree of gelatinisation). In-vitro starch digestion revealed that salivary-gastric digestion in the extrudates ranged from 8 to 18 g/100 g dry starch, while the rate of starch digestion was 3.0-3.7 min(-1). Salivary-gastric digestion in the non-extrudates was from 2 to 11 g/100 g dry starch, with the rate of starch digestion being 0.1-0.8 min(-1). Estimated glyceamic index of the extrudates ranged from 87 to 124 g/100 g, higher than in the non-extrudates and dependent on extrusion moisture and screw speed. This is the first study on extrusion-property relationships of the cultivars to guide global utilisation of sweetpotato. (C) 2014 Elsevier Ltd. All rights reserved.