Mixed Fermentation Using Saccharomyces cerevisiae var. boulardii and Kombucha Culture as a Strategy for the Production of Mead with Antimicrobial Activity

This study evaluated the antimicrobial activity of meads produced by mixed fermentation of Saccharomyces cerevisiae var. boulardii and a kombucha culture, using eucalyptus- and rosemary-monofloral honeys. Antimicrobial activity was investigated in vitro against pathogenic bacteria (Escherichia coli, Salmonella enterica, Listeria innocua, Staphylococcus aureus, and Bacillus cereus) and spoilage yeasts, over 28 days of storage. All tested matrices were autoclaved at 121 °C for 15 min before antimicrobial evaluation to eliminate viable fermentation microorganisms. Additionally, assays were conducted in simulated food systems at different substrate concentrations, characterizing organic acids, reducing sugars, and volatile compounds. Unfermented musts allowed survival of L. innocua and B. cereus, with rosemary honey showing stronger antimicrobial effects than eucalyptus. Kombucha exerted strong initial inhibition, possibly associated with its high organic acid concentrations, but did not completely inhibit the growth of Gram-positive bacteria. No pathogenic bacterial growth was detected in the fermented mead matrices, possibly due to reduced fermentable sugars, ethanol, organic acids, and other fermentation metabolites. Spoilage yeasts showed high tolerance to acid and osmotic stress, and efficacy decreased in substrate-rich systems. Importantly, the antimicrobial activity reported here refers specifically to autoclaved fermented matrices and does not necessarily apply to untreated fermented beverages.