Antioxidant Activity, Chemical Composition, and Antinutrient Content of Selected Underutilized African Crops as Affected by Fermentation

Underutilized crops are indigenous plant species having significant traditional and nutritional values, but are often overlooked and neglected by modern agriculture. Orange-fleshed sweet potato (OFSP), cowpea leaf (CL), and moringa leaf (ML) fall into this category. Despite their year-round availability and low production costs, they are not in the mainstream of the global food market. These three crops are rich in bioactive compounds that may be used in functional foods. The objective of this study was to determine the antioxidant activity, chemical composition and antinutrient content of these bioactive-rich underutilized crops, and examine the effect of fermentation on their compositions. Flours were produced from OFSP, CL, and ML and subjected to analyses. Antioxidant activities, chemical compositions, and antinutrient properties were determined using standard methods. The results showed that fermentation increased the antioxidant capacities of OFSP and ML, while it reduced those of CL. The lycopene content of unfermented OFSP (0.33 mg/g) increased to 0.52 mg/g after fermentation. Phenol (2.30 mg/g) increased to 14.65 mg/g; likewise, the FRAP, DPPH, Fe2+ chelation, and carotene all increased in the fermented samples. The result of the proximate composition showed that there were significant differences among the parameters examined. The moisture content ranged from 8.53 to 14.07% with the unfermented OFSP (UFOSP) having the highest, while fermented ML (FML) had the least. In each case, the moisture content of the fermented samples was far lower than their unfermented counterparts. The protein content of all the samples, which ranged from 5.61 to 23.69% was significantly higher in the fermented samples. Conversely, the ash and crude fibre contents, which ranged between 2.71 and 16.18%; 2.79 and 14.99% respectively, were observed to be lower in the fermented samples; this may be attributed to leaching during the fermentation process. While alkaloid was not detected in CL, its content in OFSP and ML was drastically reduced in the fermented samples. Saponin decreased from 18.36 mg/g to 10.18 mg/g in OFSP, and from 7.27 mg/g to 3.27 mg/g in ML, respectively. In conclusion, fermentation enhanced the protein content of OFSP, CL, and ML, reduced their antinutrient composition, and improved their antioxidant bioavailability.