Increasing interest in functional non-dairy beverages has led to the development of fermented cereal-based products with improved bioactive properties. This study evaluated the influences of lemon, orange, and mandarin essential oils (0.01–0.05%) on fermentation, phenolic contents, antioxidant activities, and sensory properties of wort-based beverages fermented with Lacticaseibacillus casei ssp. rhamnosus Oly. The applied concentrations did not suppress bacterial growth, although fermentation proceeded slightly more slowly. In all samples, viable cell counts remained above 11 log CFU/mL. Fermentation contributed to the preservation or even enhancement of phenolic compounds at the lowest and highest oil concentrations and generally increased antioxidant activity, as determined by the FRAP assay. CUPRAC, DPPH, and ABTS results also indicated changes associated with microbial transformation of phenolic constituents and their interactions with citrus essential oils. Sensory assessment demonstrated improved aroma and taste compared with the control, and the highest sensory ratings were obtained for beverages supplemented with 0.04% lemon, 0.02% orange, and 0.02% mandarin essential oil. Among the tested formulations, the beverage containing 0.02% orange essential oil provided the most favorable combination of sensory quality and functional characteristics.

The growing demand for functional non-dairy beverages has stimulated interest in cereal-based fermented products with enhanced biological activity. This study investigated the effect of addition of lemon, mandarin, and orange essential oils (0.01–0.05%) on the fermentation performance, phenolic composition, antioxidant capacity, and sensory characteristics of wort-based lactic acid beverages fermented with Lacticaseibacillus casei ssp. rhamnosus Oly. The tested concentrations did not inhibit bacterial growth, although a slight delay in fermentation was observed. Nevertheless, all variants reached viable cell counts above 10¹¹ CFU/mL, confirming their probiotic potential. Total phenolic content was determined by the Folin–Ciocalteu method, revealing that fermentation promoted the retention or increase of phenolic compounds. Fermentation generally enhanced the antioxidant capacity of the beverages, with the most pronounced increases observed by the FRAP assay. Depending on the formulation, CUPRAC, DPPH, and ABTS analyses also revealed changes in antioxidant potential associated with microbial biotransformation of phenolic compounds and interactions with citrus oil constituents. Sensory evaluation demonstrated improved aroma and flavor, compared with the control, with the beverages containing 0.04% lemon essential oil. 0.02% orange essential oil, and 0.02% mandarin essential oil receiving the highest sensory score. However, only the beverage with 0.02% orange essential oil showed high biological value.