Abstract
Thermal degradation of sugars and amino acids, and depolymerization of macromolecules such as starch, proteins and fibre occasioned by high-temperature short-time extrusion cooking modify the physicochemical and functional properties of raw materials. High-temperature short-time extrusion cooking holds promise for the expanded use of non-conventional ingredients as food/feed due to its practicality, increased productivity and efficiency, and ability to retain thermally degradable nutrients during cooking. However, little is known about the effect of the high-temperature short-time extrusion cooking process on the physicochemical properties and starch digestibility of lesser-known grain legumes such as African yam beans (Sphenostylis stenocarpa), Pigeon pea (Cajanus cajan), and Bambara peanut (Vigna subterranean). In this study, we investigate the effect of high-temperature short-time extrusion cooking and extrusion cooking temperature; low (100°C) vs high (140°C) temperatures in a single screw extruder, on hydration characteristics, viscoamylolytic properties, in vitro starch digestibility and digestion kinetics of these grain legumes. We show that water holding capacity and swelling power increased (p < 0.05) with increasing extrusion temperature for Sphenostylis stenocarpa and Vigna subterranean but not Cajanus cajan extrudates. Significant effects of extrusion cooking (i.e unextruded vs 100°C and unextruded vs 140°C) and extrusion temperatures (i.e. 100°C vs 140°C) were observed in peak, trough, final and setback viscosities of all extrudates. Starch digestibility and digestion characteristics were modified with increase in extrusion temperature, however, no effect of extrusion temperatures (i.e. 100°C vs 140°C) on starch digestion kinetics was observed for Sphenostylis stenocarpa and Vigna subterranean except for hydrolysis index (34.77 vs 40.77%). Nutritional and physiological implications of extruded grain legumes in monogastric animal feeding were also highlighted. The Information presented herein will influence expanded use of extruded grain legumes as feed ingredients for intensive monogastric animal feeding.
Highlights
Legume grains are protein-energy rich seeds, members of the dicotyledonous family Fabaceae or Leguminosae commonly found in the tropical rain forests of the Americas and Africa [1]
This study describes the influence of high-temperature short-time extrusion cooking temperatures on the hydration and viscoamylolytic properties of Sphenostylis stenocarpa, Cajanus cajan, and Vigna subterranean, and their susceptibility to enzymatic hydrolysis in a monogastric digestive system model
Proximate composition of the raw and extruded Sphenostylis stenocarpa (African yam beans), Vigna subterranean (Bambara groundnut) and Cajanus cajan (Pigeon pea) products are shown in Table 1, and have been discussed in detail in a previous publication [15]
Summary
Legume grains are protein-energy rich seeds, members of the dicotyledonous family Fabaceae or Leguminosae commonly found in the tropical rain forests of the Americas and Africa [1]. Starch is the most abundant complex carbohydrate fraction (22–45% of legume grains), followed by non-starch polysaccharides ((NSPs) loosely translated as dietary fibre) representing 10–20% of complex carbohydrates and the balance being sugars [4,5]. Oligosaccharides, polysaccharides (e.g. cellulose, hemicellulose, pectic polysaccharides, and resistant starch) and associated plant substances[6], constitute the NSPs of grain legumes. Carbohydrates of grain legume origin are often termed “low glycaemic” as their consumption produce a smaller rise in blood glucose relative to a reference, often white bread or pure glucose, owing to their high soluble and insoluble fibre, and resistant starch content which range from 5–35% by weight [8,9].
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