Solid-state fermentation with Rhizopus oligosporus and Yarrowia lipolytica improved nutritional and flavour properties of okara

Okara (soybean pulp) is a processing by-product generated in considerable amounts by the soy food industry. Due to its fibrous mouthfeel, low digestibility, presence of anti-nutrients and grassy off-odour, it is often under-utilised. To improve its composition, okara was fermented using a fungal monoculture of Rhizopus oligosporus (RO), and a coculture of R. oligosporus and yeast Yarrowia lipolytica (ROYL). After fermentation, the digestibility, nutritional value and flavour of okara were enhanced by RO and ROYL. Coculture of fungi and yeast seemed to exert synergistic effects. Compared to the unfermented okara, ROYL coculture contained 33% less insoluble dietary fibre, 16% less phytic acid, 176% more soluble dietary fibre, 254% more free amino acids, 179% more isoflavone aglycones and 197% more free phenolic acids. After fermentation, RO monoculture contained more alcohols and esters, while ROYL coculture contained more acids and volatile phenols. These improvements in the nutritional and flavour properties of the fermented okara could expand its valorisation by the food industry.