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This research was focused on whey ultrafiltration permeate (UF) fermentation by genetically unmodified (wild) organisms. We chose five classical Propionibacterium (Prb) strains that are attributed to the dairy subclass. The optimal temperature for Prb growth is around 28-30°C, however, for energy saving purposes the aim of this study was to observe the organic acid, especially propionic acid (PA) and biomass production of the chosen Prb (DSM 20273 Acidipropionibacterium acidipropionici = A, DSM 20272 Acidipropionibacterium acidipropionici = B; DSM 4902 Prb. freudenreichii = E; DSM 20535 Acidipropionibacterium jensenii = J; DSM 20276 Acidipropionibacterium thoenii = T) at the room temperature (22±0.5°C). As expected, all the strains were able to convert lactose into acid, however, with different efficiency, thus the production of PA by Prb is strain -dependent. The higher productivity of PA was observed in samples with strains T, J, and B in a 26-day period, however, it should be noted that also the strain E showed high efficiency in a shorter (6-day) period. The highest concentration of PA was produced by strain T, and it reached 3.7±0.13 g L-1 within a 26-day period. The concentration of the PA produced depends on the duration of fermentation, and on the proportions of other acids produced as well. An individual strain initially can produce PA more slowly than others, but over time can be effective, which may be related to adaptation to environmental conditions. We also observed the presence of background microflora in whey permeates – most likely lactic acid bacteria. Lactic acid was produced most rapidly and was the most abundant acid produced from whey UF. [ABSTRACT FROM AUTHOR]