Elsevier

Journal of Cereal Science

Kinetics of starch digestion and functional properties of twin-screw extruded sorghum☆

Abstract

The time-course of starch digestion in twin-screw extruded milled sorghum grain was investigated using an in-vitro procedure based on glucometry. The sorghum grains were hammer-milled, and extruded at three levels each of moisture and screw speed. Irrespective of the extrusion conditions, extruded and non-extruded milled sorghum grain exhibited monophasic digestograms, and the modified first-order kinetic and Peleg models adequately described the digestograms. Extrusion increased the rate of digestion by about ten times compared with non-extrudates. Starch gelatinisation varied in the extrudates, and microscopy revealed a mixture of raw, gelatinised and destructured starch and protein components in the extrudates. Starch digestion parameters significantly ( p  < 0.05) correlated with extruder response and various functional properties of the extrudates. Extrusion conditions for maximum starch gelatinisation in milled sorghum grain for fastest digestion as an efficient animal feed were interpolated, as well as the conditions for directly-expanded extrudates with potential for human food, where minimum starch digestion is desired.

Introduction

Sorghum (Sorghum bicolor (L.) Moench) is widely used for animal feed and human food, and its nutritionally bioactive components might increase its food uses (Awika et al., 2005). However, as a food or feed, digestibility of sorghum continues to be a concern because of pronounced starch–protein interactions, and formation of intermolecular disulphide bonds in the proteins upon high-moisture heat treatments, both of which render its starch and protein less digestible compared to other cereals (Ezeogu et al., 2005). Improvements in sorghum digestibility are important for its utilisation and, consequently, maximisation of its nutritional benefits. Apart from agronomic and genetic approaches, low-moisture processing techniques that include pressure-cooking, popping, extrusion, baking, nixtamalisation, and flaking with and without reducing agents, have been suggested as ways of increasing sorghum digestibility (Glennie, 1987, Gomez et al., 1989, Elkhalifa et al., 2006, Ezeogu et al., 2005).

Extrusion of sorghum with and without other grains and additives has been studied (Anderson et al., 1969). Fapojuwo et al. (1987) observed that extrusion temperature improved in-vitro protein digestibility of two low tannin sorghum varieties. Falcone and Phillips (1988) reported changes to the physical (expansion and textural) and sensory properties of extruded sorghum–cowpea blends, but neither protein nor starch digestibility of the extrudates was studied. However, low moisture extrusion enhanced starch digestibility of low amylose sorghum (Gomez et al., 1988). Although Phillips and Falcone (1988) did not investigate digestibility, they observed that low to medium extrusion temperatures and moistures enhanced expansion and texture of sorghum or sorghum–peanut mixtures. However, extrusion studies need to be cautiously compared because of differences in screw configuration and overall shear effects. This notwithstanding, temperature and moisture were reported (Dahlin and Lorenz, 1993) to have the greatest influence on improving in-vitro starch digestibility in low- and high-tannin sorghum and other cereals (wheat, rye, corn or maize, and millet). Extrusion of sorghum improves bioavailability and digestibility of nutrients in various animals (e.g. Gu et al., 2008). Both in-vivo and in-vitro digestion techniques feature in digestibility studies, but the latter is more common, where both single-point and time-course measurements are used (Sopade and Gidley, 2009). Time-course measurements produce digestion profiles (digestograms), from which parameters of digestion kinetics can be estimated for better understanding of the digestion process. Although time-course starch digestion of processed and non-processed sorghum grain, and sorghum starch has been reported with limited modelling (e.g. Ezeogu et al., 2005), further understanding of sorghum digestion would benefit from mathematical description of digestograms, and an examination of the dependence of the resulting parameters on process conditions to guide optimisation strategies. It is generally recognised (Hernot et al., 2008) that knowledge of digestion characteristics of whole grains and their components are vital to enhancing their full utilisation in food and feed. The objectives of this study, therefore, were to (1) understand how extrusion conditions affected the kinetics of starch digestion in milled sorghum grain, (2) investigate the suitability of digestion models in describing sorghum digestograms and (3) examine the relationships between digestion and functional properties for an insight into possible end-uses.

Section snippets

Pre-extrusion preparation

Sorghum (var. Buster) was obtained from the Queensland Department of Primary Industries and Fisheries, Brisbane, Australia, and its physicochemical characteristics have been described before (Mahasukhonthachat et al., 2010). The grain was hammer-milled using a 2-mm retention sieve, and stored in polythene-lined plastic bags at room temperature before extrusion. This is referred to as a non-extruded (feed) sample, and its moisture content averaged 10.2 ± 0.25%, while its particle size

Digestion kinetics

Irrespective of the extrusion conditions, the extrudates exhibited monophasic digestion characteristics (Fig. 1a) as did the non-extrudates (feed and RGE). As expected, the extrudates were more rapidly digested than the non-extrudates. The parameters (D o, K and 1/K 1) of the modified first-order kinetic and Peleg models for the extrudates were substantially greater (5–35 times) than the corresponding ones from the non-extrudates (D o = 0.26–1.97 g/100 g dry starch; K = 0.0016–0.0017 min−1; 1/K 1

Conclusions

Extrusion can yield products from sorghum with variable digestion properties, which can be described by digestion models with parameters that can significantly correlate with functional parameters and extruder response. The relationship between digestion kinetics and SME is of practical significance as extrudates with predictable digestibility can be processed at set SME. However, with increasing interests in digestibility of food/feed, the developments of user-friendly, rapid and

Acknowledgements

The authors are grateful to the Cooperative Research Centre for an Internationally Competitive Pork Industry for financial support, the Queensland Department of Primary Industries and Fisheries for material support, Dr Olena Kravchuk for statistical assistance and Dr John Black for useful comments.

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