Sustainable Use of By-Products (Okara and Linseed Cake) in Tempeh Fermentation: Effects on Isoflavones and Lignans

Okara and linseed cake are nutrient-rich by-products containing phytoestrogens, which are underutilized in food production. The aim of this study was to optimize hydrothermal treatment and fermentation time using common starters for tempeh fermentation. Changes in isoflavones and secoisolariciresinol diglucoside (SDG) were analyzed using the HPLC-PDA method. Okara and linseed cake were steamed for 5 to 40 min, followed by fermentation with Rhizopus oligosporus at 30 °C for 48 h. The optimal steaming times were 10 min for okara and 20 min for linseed cake. Isoflavone analysis revealed a change in total levels depending on the microflora, with a significant increase in aglycones during fermentation. SDG levels did not show a significant reduction. In 24-h fermented tempeh, the isoflavone acetylglycosides content increased after frying. These results suggest that steam treatment is an effective hydrothermal method, offering energy and water savings. Additionally, the ability to modulate isoflavone composition through fermentation creates new opportunities for pro-healthy food development. This study concludes that okara and linseed cake can be successfully used for tempeh production, providing a sustainable alternative to conventional tempeh substrates. By utilizing these by-products, this research promotes the reduction of food waste and supports the development of environmentally friendly food production practices.