The effect of extrusion on the nutritional, functional and structural properties of gluten-free extruded snacks fortified with whey protein concentrate and cowpea flour : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Rising health concerns along with changes in lifestyles and diets have increased the demand for ready-to-eat snacks with increased the protein, fibre, and bioactive compounds through extrusion processing across the globe. Therefore, this study focused on determining the technical feasibility of adding varying levels of cowpea and whey protein concentrate (WPC) to rice flour to produce extruded snacks and assess the impact of extrusion processing and fortifications of cowpea and WPC on the nutritional and physicochemical properties. Further, the effect of fortifications of cowpea and WPC on textural and sensory properties of rice-based snacks was evaluated. This research also extended to investigate more nutritional aspects of cowpea flour during germination under different LED lights. Rice flour was blended with cowpea flour and WPC at weight ratios of 90:10:0, 80:15:05, 70:20:10, 60:25:15, and 50:30:20 to prepare composite flours. Rice flour was used as a control. The effects of different levels of cowpea flour and WPC additions on the nutritional components, pasting properties, in vitro starch and protein digestibility of rice-based blends were investigated. Results indicated that the peak, breakdown, setback, and final viscosities were significantly lower, and the pasting temperature was significantly higher with cowpea and WPC fortifications compared to the control. The observed total reduction in reducing sugars being released during the in vitro starch digestion for the extruded samples was in the range of 14-41% at the inclusion levels of 10-30% CP and 5-20% of WPC, respectively. An increase in the ratio of cowpea flour and WPC increased the protein and fibre contents and decreased the starch content of raw blends. The increased protein content had an impact on protein digestibility. Raw blends were extruded using a Clextral twin-screw extruder with co-rotating and intermeshing screws. The processing conditions used a die temperature of 111.5 °C, a screw speed of 252 rpm, a feed rate of 7.98 kg h-1, a water rate of 90.67 kg h-1, and a die diameter of 3 mm. Nutritional, physicochemical, and textural properties of the extrudates were evaluated. The protein and fibre contents of the extrudates significantly increased (p<0.05) with cowpea (10-30%) and WPC (5-20%) fortifications compared to those of the control. Increased cowpea and WPC contents decreased lightness and increased the bulk density and hardness of the extrudates. The crispiness associated with the number of peaks during the compression test increased with higher levels of cowpea and WPC. Correlation analysis revealed that the protein and fibre contents were significantly correlated to hardness, crispiness, bulk density, water absorption index and colour properties of the extrudates. The essential and non-essential amino acid contents increased proportionally to the increases of the cowpea and WPC fortifications in the extrudates. Extrusion increased the oligosaccharides (2-3-fold) and resistant starch (1-3-fold) contents, whereas the insoluble fibre content was not significantly affected. Extrusion increased the protein digestibility (p˂0.05) and amino acid composition in the snacks. Extruded and raw samples enriched with cowpea and WPC had an increase in the total phenolic content (TPC) and antioxidant activity. Extrusion significantly reduced the TPC, and antioxidant properties of the extruded snacks compared to their raw counterparts. The observed total reduction of released reducing sugars during the in vitro digestion was in the range of 14-41% at the inclusion levels of 10-30% cowpea flour and 5-20% of WPC, respectively. After the in vitro digestion process, the antioxidant activity of the cowpea and WPC fortified snacks was more than 2-fold higher compared to their extruded counterparts. The effects on sensory properties of cowpea and WPC fortification in rice-based ready-to-eat extruded were investigated. Six samples of extruded snacks were evaluated by the 70 consumers for acceptability using a 9-point hedonic scale. The sensory properties were assessed using the check-all-that-apply (CATA) method and the just-about-right (JAR) scale. The cowpea and WPC fortified samples had higher scores for overall liking than the control sample made with 100% rice flour. The 15-25% cowpea and 5-15% WPC fortified samples had the highest JAR frequencies in the penalty analysis for colour and texture attributes. According to Cochran’s Q test, panellists were able to discriminate different textural attributes, being the terms soft and crunchy terms significant for the extrudates. Overall, the analysis of all sensory attributes demonstrated that the formulation of 15% cowpea and 5% WPC had a higher acceptance by the consumers involved in this study. A principal component analysis (PCA) revealed that there were positive associations between crispiness and L* value, and hardness b* and a* values, respectively. The final experiments were extended to investigate more extensively about nutritional properties of cowpea flour during germination under different LED light conditions. Light quality and intensity are vital for plant development and pigment biosynthesis. The effects of a dark condition and various light-emitting diodes (LEDs), such as red, blue, and white fluorescent lamps were evaluated on the nutritional components, mineral content, levels of phenolic compounds and antioxidant activities of cowpea sprouts. The protein and total dietary fibre concentrations of the cowpea sprouts increased significantly with the application of blue and red LED lights compared to the dark condition. The phytic acid, trypsin inhibitory activity, raffinose series oligosaccharides and starch fractions of cowpea sprouts germinated under the light conditions were not significantly different from those germinated under the dark conditions. The total phenolic and antioxidant properties of cowpea sprouts under the white and red LED lights were significantly higher when compared to that of the blue and dark environments. The red LED grown cowpea sprouts had the highest mineral content followed by the white, blue, and dark conditions, respectively. These results show the potential beneficial use of cowpea and WPC, as functional ingredients to improve the nutritional profile and reduce the glycaemic index of rice-based extruded snacks. In the cowpea sprouts study, the application of red LED light and white fluorescent light sources improved the nutritional, total phenolic content, and antioxidant properties of cowpea sprouts. This experiment confirmed the recent findings on the beneficial effects of LED light on plant growth and quality of the crops, including the accumulation of phytonutrients in sprouts. Incorporation of natural phytonutrients rich foods in the human diet in different ways exert beneficial effects for human health.