Legumes are a highly valued food source, recognised for their rich protein content and beneficial bioactive compounds. However, their full nutritional potential can be limited by the presence of anti-nutritional factors (ANFs). Fermentation is a crucial processing method that effectively reduces ANFs while improving the bioavailability of beneficial nutrients, thereby maximising legumes' health benefits. Optimising these fermentation processes, tailored to specific legume species, is essential to exploit their inherent advantages fully.

This study investigated the impact of natural fermentation, involving the spontaneous microbial activity present in raw legume flours, carried out at 21 °C for 14 days, on fava beans (Vicia faba), chickpeas (Cicer arietinum), and grass peas (Lathyrus sativus), focusing on their microbiological and physicochemical transformations. Egocultum provided raw legume flours. Microbiological assessments revealed a consistent proliferation of lactic acid bacteria (LABs) across all the fermenting samples, with fava beans and grass peas demonstrating robust LAB growth. Chickpeas, while exhibiting fewer LABs, showed a notable presence of moulds. Yeasts were predominantly observed in fava bean samples. Physicochemical analyses revealed a decrease in the pH and a significant increase (p < 0.05, Tukey test) in the titratable acidity, with increases of 113% in fava beans, 60% in grass peas, and 53% in chickpeas. The soluble solid (°Brix) content was reduced across all the legume species. Notably, the total phenolic content significantly increased (p < 0.05) by 224% in chickpeas and 155% in grass peas, while fava beans showed a slight increase of 13%. The antioxidant activity, measured using DPPH, ABTS, and FRAP assays, showed varied responses: fava beans' activity decreased, grass peas' ABTS activity increased (with its other activities decreasing), and chickpeas' activity consistently increased across all three assays.

These findings highlight the diverse species-specific responses to this natural fermentation process. While further optimisation is required, natural fermentation presents a promising method for enhancing the functional potential of legumes, particularly by increasing the acidity and phenolic content in certain species.