The quest for wheat flour alternatives : sensory characterisation and volatile compound profiling of composite flours and flatbread from sorghum, cassava and cowpea flours

Abstract

Sorghum, cowpea, and cassava are underutilised gluten-free sources of flour that have the potential to be used in bread products in sub-Saharan Africa. Heavy reliance of sub-Saharan Africa (SSA) on wheat imports and the Russian-Ukraine crises affect the economies of countries in sub-Saharan Africa, driving the search to explore new flour ingredients from climate-resilient crops locally produced in sub-Saharan African countries for wheat-based bread products. To extend the use of sorghum, cowpea, and cassava flours toward bread production, it is vital that the sensory properties of bread produced from these flours are better understood in order to develop non gluten-containing bread that can be comparable to wheat bread. The main focus of this study was to determine how sorghum, cassava, and fractions (whole and dehulled) of two cowpea varieties cowpea flours and designated flour composites affect the sensory properties of bread based on a flatbread-type food model using descriptive sensory evaluation and gas chromatography analysis. Refined wheat flour and wheat flatbread were used as the reference. The study's first objective was to evaluate the sensory properties of flatbreads prepared from red non-tannin sorghum, cassava starch and fractions (whole and dehulled) of two cowpea varieties and designated flour composites using a trained sensory panel (n=12). The composites were prepared using cassava starch and sorghum flour at 0 %, 35 %, and 70 %, respectively, with 30 % cowpea flour. The effect of cowpea variety and dehulling on sensory properties of cowpea flatbread were investigated. The sensory properties of flatbread prepared from 7 single flour samples and 12 composite flour samples were evaluated by the trained panel by using 10 appearance, 6 aroma, 5 in-mouth texture, 6 flavour and 5 aftertaste descriptors. Flours and flatbreads were subjected to instrumental colour analysis and all flour samples to proximate analysis to aid in explaining sensory attributes of flatbreads. The second objective was to determine the contributions of the sorghum, cassava starch and cowpea flours to the aroma profile of flatbreads prepared from the sorghum, cassava and cowpea composite flour flatbreads by correlating the sensory properties with volatile compounds extracted using solid-phase microextraction (SPME) technique and determined by gas chromatography high-resolution time of flight mass spectrometry (GC-HRTOF-MS). In the sensory study, results showed that the addition of sorghum intensified sorghum aroma in flatbread, while cowpea flours contributed a beany flavour. There was a significant high beany aroma in white cowpea-only flatbreads compared to the red cowpea flatbreads. Sorghum (70 %) and whole cowpea (30 %) flatbreads had intense sorghum aroma, dry appearance, grainy texture and left residual particles after swallowing. Cassava (70%) and (30 %) cowpea flatbreads had a chewier and rubberier mouthfeel, an intense fermented aroma and flavour, a sour aftertaste but were most similar to the wheat flatbread, with a residual beany flavour. The cassava-dehulled cowpea flatbreads had a hue similar to that of the wheat flatbread, and the cassava-dehulled cowpea flours had protein content of 8 – 9 % similar to wheat flour. A total of 283 volatile compounds were detected in flour and flatbread samples, 91 of them having flavour or aroma characteristics reported in literature. Volatile compounds chemical classes included hydrocarbons, aldehydes, alcohols, acids, esters, ketones, benzene derivatives, sulphur and nitrogen-containing compounds, terpenes and terpenoids. Variety influenced the aroma profile of cowpea flour and the resultant flatbread. Whole white cowpea flour had significantly more hexanal and nonanal (p <0.05) relative to whole red cowpea flour. The red cowpea flatbreads had significantly more 1-octen-3-ol, phenylethyl alcohol and decane which differentiated them from the white cowpea flatbreads (p < 0.05). Soaking and dehulling the cowpeas reduced flour hexanal levels while significantly increasing 1-hexanol and 1-octen-3-ol levels in flours and derived flatbreads (p<0.05). Cowpea flatbreads were also characterized by pyrazines with higher levels in flatbread from the dehulled cowpea flour. The main compounds identified in the flatbreads with beany flavour characteristics were dimethyl trisulfide, hexanal, nonanal, 1-octen-3-ol, heptanal, 1-(2-furanyl)-2-butanone and 2-pentylfuran. Dehulling cowpeas increased alcohols and esters in cowpea flours and promoted the formation of pyrazines in cowpea flatbreads. This is the first study that characterised volatile compounds and the effect of dehulling cowpea on volatile compound generation in cowpea bread. Based on partial least square regression, volatile compounds were predicted to contribute to the aroma from sorghum, cassava and cowpea composite flour flatbread. The addition of sorghum flour to a composite flour had a diluting effect on the generation of aldehydes and contributed 2-methoxyphenol, a sweet medicinal aroma and 2-methoxy-4-vinylphenol, a clove-like aroma to the flatbreads. The cassava starch presented a high level of acetic acid and aldehydes, which contributed to the fermented and green aroma in the cassava and cowpea composite flour flatbreads. Fermented attributes were contributed by acetic acid, hexanoic acid and 2-hexenal. The cassava-dehulled cowpea flours have the potential to be used as a wheat flour replacement in flatbread. The sensory properties and the underlying volatile compounds responsible for the aroma perceived from sorghum, cassava and cowpea flatbreads will guide food product developers toward developing new bread products from sorghum, cassava, and cowpea composite flours.