Abstract
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.
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