Effect of Herbal Extracts on Lactic Acid Bacteria Growth, Acidification and Viability in Fermented Milk and Plant-Based Beverages

Fermented foods and beverages based on plant-derived ingredients are of growing technological interest, especially when they are designed as alternatives to conventional dairy products. This study evaluated the effects of herbal extracts from Verbascum thapsus L., Cnicus benedictus L., and Fumaria officinalis L. on lactic acid bacteria growth, acidification kinetics, and viable cell counts during the fermentation of organic milk, coconut beverage, and soy beverage. The extracts were characterized for extraction yield, total phenolic content, and antioxidant activity before use in fermentation trials. Mixtures of organic solvents and water produced extracts with higher total phenolic content and antioxidant activity than water alone. The highest values were obtained for F. officinalis extracts prepared with water and methanol or water and acetone, while for C. benedictus, the most effective solvents were water and acetone or water and ethanol. The agar well-diffusion assay showed no relevant antibacterial activity against the tested LAB strains under the applied conditions. No biologically relevant inhibition zones were observed in any of the 84 extract-strain combinations under the tested conditions. The only borderline response was observed for Lactobacillus acidophilus La-14 exposed to the 70% ethanolic extract of C. benedictus. The clear halo did not exceed 1.50 mm outside the 5 mm well and was treated as a weak, strain-specific screening result. Fermentation kinetics depended mainly on the food matrix. The coconut beverage acidified most rapidly, reaching pH 4.38 to 4.79 after 6 h, whereas the soy beverage required 24 h to reach pH 4.31 to 4.56. Organic milk showed the slowest acidification, and selected C. benedictus extracts delayed pH reduction. All analyzed fermented samples contained more than 7 log CFU/mL of viable LAB. These results indicate that selected herbal extracts can be used in fermented milk and plant-based beverages without reducing LAB survival, but their suitability should be assessed separately for each strain and matrix.