Brewer's spent grain (BSG) is an abundant brewing by-product, but a short shelf life limits its use. A rotary drum press can separate BSG into a solid and a liquid fraction (BSG liquid) that contains sugars. Propionic acid producing bacteria (PAB) strains from the family Propionibacteriaceae (DSM 20270, 20272, 20274, 106790) were screened for organic acids formation from BSG liquid in three fermentation setups: A single-step fermentation, a two-step fermentation with Lactiplantibacillus plantarum (NFICC 27) followed by PAB, and a co-culture of PAB with L. plantarum. The two-step fermentation produced 9.5 g/L propionic acid and 5.4 g/L acetic acid. GC-MS analysis of the fermented BSG liquid showed a decrease in off-flavours and an increase in compounds with fruity, chocolate and caramel notes. The fermented BSG liquid also had antimicrobial activity. Fermentation of BSG liquid may therefore be a suitable means of creating a clean label food ingredient with long shelf life.