Extrusion cooking of pearl millet: effects on sensory attributes of porridges made from pre-cooked flours

Abstract

In the face of climate change and rapid urbanization in Africa, there is a growing need to produce convenient, nutritious, and safe novel foods from climate-resilient, under-utilized cereals like pearl millet. However, the perceived poor sensory quality, the high lipid content and resulting susceptibility to rancidity development, and content of goitrogenic flavonoids of pearl millet have constrained its commercial utilization. Extrusion cooking is a thermal process that in addition to yielding a convenient ready-to-eat food product, could also prevent the development of rancidity, resulting in a product of improved and stable sensory quality. Two varieties of pearl millet were extrusion-cooked at Purdue University (TIF Leaf 3 variety in wholegrain and decorticated forms) and separately at the University of Pretoria (OKASHANA 1 variety in wholegrain form), using different extruders and extrusion conditions (a friction heated single-screw extruder at Purdue University, and an electrically heated co-rotating twin-screw extruder at Pretoria). The resulting products were then stored at 4 °C, ambient (25-30 °C) and elevated (50 °C) temperatures for 24 weeks following a reversed storage design. Porridges prepared from stored flours were evaluated for changes in their sensory profiles using descriptive sensory evaluation. Sensory attributes were developed and rated on an 11-point intensity scale by a trained quantitative descriptive analysis panel. The results were analysed for significant (p≤0.05) changes in the intensities of the attributes as functions of storage time and temperature. Stored flours were also evaluated for total phenolic content, antioxidant activity and C-glycosyl flavone content. Significant increases in sensory attributes indicative of rancidity development (chemical, painty, and soapy flavours) and a concomitant decrease in the intensities of attributes indicative of freshness (maize meal, sweet, cooked grain, canned sweetcorn, and wheaty aromas) were only detected in porridges prepared from Purdue-prepared ambient- and elevated-stored raw flours, and Pretoria-prepared elevated-temperature-stored extrusion-cooked flour. Chemical tests for fat acidity and fatty acid composition also confirmed the development of rancidity due to lipid oxidation. Unexpectedly, no indications of rancidity development were detected in the Pretoria-prepared raw flours. This was explained by the high phenolic content and antioxidant activity of this flour. Contrastingly, while extrusion cooking prevented rancidity development in the Purdue-prepared flours, it resulted in the oxidation of lipids in the Pretoria-prepared flours. The latter effect was primarily explained by residual lipase activity and reduced phenolic content and antioxidant activity in the flour following extrusion cooking. The absence of noticeable residual lipase activity in the Purdue-prepared flours was likely due to difference in extruder type and extrusion-cooking conditions used. The findings from the Purdue extrusion process demonstrate that extrusion cooking can increase the sensory quality and stability of wholegrain and decorticated pearl millet flours. Thermally induced reactions are responsible for flavour development, inactivation of lipolytic enzymes, and degradation of potential goitrogenic factors. However, the findings from the Pretoria extrusion process demonstrate that these effects are dependent on extrusion cooking conditions, especially feed moisture content and extrusion temperature. Therefore, extrusion cooking conditions must be optimized to produce convenient, ready-to-eat, and safe foods from wholegrain and decorticated pearl millet flours, with improved sensory quality and stability.