Biotransformation of flaxseed oilcake using Rhizopus oryzae: A sustainable way to enhance its nutritional and functional properties

• Solid-state fermentation improved the nutritional profile of flaxseed oil cake. • Incubation for 120 h increased protein digestibility and reduced antinutrients. • SDS-PAGE confirmed fermentation-induced structural modifications in proteins. • Fermented flaxseed oil cake can act as a functional food ingredient. Waste valorization represents a critical strategy to address the sustainability challenge while simultaneously harnessing the nutritional profile of agro-industrial by-products. Solid-state fermentation (SSF) offers a promising solution to upcycle various agro-industrial waste into edible biomass, with enhanced nutritional bioavailability and functional properties. In the present study, flaxseed oil cake (FOC) was subjected to monocultural fermentation using Rhizopus oryzae for different incubation times. The highest protein (42.67%) and fat content (10.05%) were observed at 72 and 120 h of fermentation, respectively. An increase in free phenolic content (23% increase) after 72 h, whereas a decrease in bound phenolics (20% decrease) was observed after 120 h of fermentation. The fermented FOC exhibited significantly higher antioxidant activity with increased DPPH (18.09%) and FRAP (56.63%) than unfermented FOC. Fermentation led to alterations in fatty acid profile where oleic acid content increased by 1.22-fold, whereas linoleic and linolenic acid decreased by approximately 1-fold and 0.8-fold, respectively. On the other hand, a complete removal of hydrocyanic acid was observed with a 12% decrease in phytic acid. SEM micrographs confirmed surface degradation due to fermentation, and the peak shifts in the FTIR graph in the amide region (1200-1700 cm -1 ) revealed changes in functional groups. SDS-PAGE showed that the protein molecular weight decreased to approximately 25 kDa from 55 kDa, confirming the occurrence of proteolysis during fermentation. Functional properties such as water, oil holding, and swelling capacity were enhanced. SSF upcycles flaxseed oilcake by improving the nutritional and functional qualities, making it a promising nutrient-rich plant-based ingredient.