## 5 Conclusions  
This study showed that the selection of lactic acid bacteria cultures significantly affected the quality of fermented dairy, dairy-oat, and oat beverages, with effects depending on the base material. Yogurt cultures (_Streptococcus thermophilus_ and _Lactobacillus delbrueckii_ subsp. _bulgaricus_) improved structural stability and reduced syneresis, especially in dairy and dairy-oat beverages. Adding _Lactiplantiobacillus plantarum_ 299v increased gel hardness compared to using only the yogurt culture. The lowest pH values occurred in samples with _Lactobacillus acidophilus_ La-5, indicating strong acidification. Oat-based beverages maintained more stable pH during storage than dairy-based ones, showing resistance to further fermentation. Lactose breakdown and galactose changes were greater in samples with _L. acidophilus_ La-5 and _L. plantarum_ 299v, confirming active carbohydrate metabolism. Su-crose content in dairy-oat beverages decreased over time, suggesting enzymatic hydrolysis. _S. thermophilus_ remained viable (>7 log CFU/g) in most samples, while the viability of _L. acidophilus_ La-5 declined more in oat-based beverages. In conclusion, the bacterial culture and base material both shaped beverage quality. Dairy and dairy-oat drinks with yogurt cultures had better stability, while _L. plantarum_ 299v and _L. acidophilus_ La-5 enhanced acidification and sugar metabolism. These results suggest that bacterial culture combinations can be optimized to improve fermented beverage stability, texture, and taste.  
A limitation of the study was the assessment of only selected quality parameters, excluding factors like flavor and aroma. The fermentation period may have been too short to fully evaluate the fermentative capacity of _L. plantarum_. Future research should include more lactic acid bacteria strains and their combinations to identify optimal cultures for various beverages. It is also important to conduct studies under industrial-like conditions to better understand how processing affects beverage quality and stability and to analyze the impact of fermentation on sensory attributes (taste, aroma, and texture) and consumer preferences. Optimizing bacterial culture combinations could enhance product stability and sensory qualities, offering valuable insights for developing high-quality fermented beverages.  
Conceptualization, M.Z. and D.Z.; methodology, M.Z., E.K. and T.F.; validation, M.Z. and D.Z.; formal analysis, M.Z., E.K. and T.F.; investigation, M.Z., E.K. and T.F.; resources, M.Z. and D.Z.; data curation, M.Z. and D.Z.; writing--original draft preparation, M.Z., D.Z., E.K. and T.F.; writing--review and editing, M.Z., D.Z., E.K. and T.F.; visualization, M.Z. and D.Z.; supervision, M.Z.; project administration, M.Z.; funding acquisition, M.Z. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
The authors gratefully acknowledge the Institute of Food Sciences of Warsaw University of Life Sciences WULS-SGGW for supporting and providing necessary infrastructure and research materials.  
The authors declare no conflicts of interest.