Unveiling Microbial Dynamics in the Spontaneous Fermentation of Oat and Rice Okara Sourdoughs

Sourdough fermentation is increasingly explored as a sustainable strategy for the valorisation of cereal-based by-products, although okara from oat- and rice-based beverage production remains largely underexplored. This study investigates the microbial evolution and nutritional characteristics of oat and rice okara sourdoughs obtained by spontaneous fermentation using the back-slopping technique. High-throughput sequencing revealed dynamic but matrix-dependent microbial composition. At the beginning of fermentation, oat okara was dominated by the Bacillus genus, while the Streptococcus genus was the most abundant in rice okara. After 30 days of back-slopping, the bacterial communities of both matrices were dominated by Lactobacillus, accounting for 80.1% and 73.3% of the relative abundance in oat and rice okara sourdoughs, respectively. Secondary bacterial taxa differed between matrices, with Weissella prevailing in oat okara (7.0%) and Acetobacter in rice okara (11.2%). Yeast communities showed a substrate-dependent temporal succession, being initially dominated by Pichia in both oat and rice okara sourdoughs (96.6% and 97.1%, respectively), whereas Saccharomyces became predominant at later fermentation stages, reaching 54.8% in oat okara and 83.5% in rice okara. From a nutritional perspective, okara sourdoughs exhibited promising characteristics, being rich in proteins and free amino acids, particularly glutamic acid, aspartic acid and leucine. The fatty acid profile was marked by oleic, linoleic and stearic acids, while nutritionally important minerals associated with musculoskeletal and immune function, such as calcium, zinc and selenium, were present in relevant quantities in the sourdoughs. These findings provide new insights into oat and rice okara sourdoughs and support the use of fermented okara as a sustainable ingredient with potential functional relevance.