Synergistic co-culture of Rhizopus oryzae and Rhodotorula babjevae transforms faba bean and oat into nutrient-rich...

This study explored a novel fermentation combining Rhizopus oryzae and Rhodotorula babjevae to produce tempeh-like products from faba bean and wholegrain oat. While R. oryzae is established in tempeh production, this work focused on functional interactions and synergistic effects of co-culturing, as well as the impact of pre-soaking with or without Lactiplantibacillus plantarum (LAB). Unfermented and single-culture (R. oryzae) samples served as controls. The results showed that co-fermentation with R. babjevae significantly enhanced the nutritional profile. The faba-oat tempeh produced by the co-culture achieved the highest total free amino acid concentration (24 mg/g DM) compared with the single culture (14 mg/g DM). Gamma-aminobutyric acid (GABA) increased to 503 mg/kg (wet) in co-fermented faba bean tempeh, compared with 290 mg/kg (wet) in the single culture. In contrast, LAB pre-soaking reduced protein content, total free amino acids, GABA levels, and the extent of phytic acid degradation. Phenolic compounds were also modulated by co-culture; sinapic acid increased approximately sixfold, while protocatechuic acid and 4-hydroxybenzaldehyde increased in all fermented samples compared with the non-fermented faba bean and faba-oat samples. Co-fermentation with R. babjevae was effective in degrading antinutritional factors in faba bean tempeh, showing complete phytic acid reduction, whereas single-culture fermentation with R. oryzae resulted in a 60% reduction. Vicine and convicine were reduced in all fermented samples. Structural analyses confirmed that co-culture fermentation produced a cohesive, compact, and chewy matrix with extensive cell wall disruption, reflecting enzymatic activity. The novel co-culture demonstrated a clear synergistic effect, improving nutritional composition while maintaining a tempeh-like structure and appearance.