Sprouting and fermentation of cowpea: Impact on the physicochemical properties, nutritional composition, and consumer acceptability of composite doughnuts made thereof

Cowpea is a nutrient-rich legume, high in protein and fiber. However, its use in food products is limited due to the presence of anti-nutrients, phytochemicals, a strong beany aroma, and long cooking times. To broaden its application in food systems, various processing techniques—sprouting, fermentation, and a combination of both—were tested for making cowpea-based doughnuts. For the study, cowpea seeds were soaked for 1 hour and blanched at 100 °C for 30 seconds (control). Separate treatments involved sprouting for 3 days (Sprouted, S), fermenting for 3 days using Rhizopus oryzae starter powder (10⁵ viable spores/g seeds) (Fermented, F), and a combined sprouting and fermentation process (S+F). The processed seeds were then blanched, dried, milled into flour, and analyzed for nutrient composition, hexanal content (responsible for beany aroma), anti-nutrients, phytochemicals, and thermal properties. The cowpea flour was mixed with wheat flour (1:1) to prepare composite doughnuts, which were subsequently evaluated for sensory attributes. Among the treatments, the S+F process resulted in the highest protein content (35.65 ± 0.25 g/100 g), followed by S (32.26 ± 0.06 g/100 g), F (31.03 ± 0.03 g/100 g), and the control (24.91 ± 0.02 g/100 g). Processing significantly reduced hexanal levels from 44.41 ± 0.67 µg/100 g (control) to 24.36 ± 2.40 µg/100 g (S), 19.52 ± 0.18 µg/100 g (F), and 14.40 ± 0.66 µg/100 g (S+F). All treatments also led to reductions in oxalates, tannins, flavonoids, polyphenols, and lowered crystallization and decomposition temperatures. Sensory evaluation showed a stronger preference for doughnuts made from fermented cowpea, which had a particulate, rough, and brown appearance, compared to the control, which was beany, sweet, and gritty. These findings suggest that fermentation, alone or combined with sprouting, is a promising strategy to improve the nutritional and sensory qualities of cowpea-based food products.