Sensitivity of Wine-associated Acetic Acid Bacteria to Sulfur Dioxide and Their Effects on Yeast Fermentation during Winemakin

Contamination by bacteria species capable of producing acetic acid are particularly dangerous to successful primary fermentation during the winemaking process. These acetic acid bacteria consume nutrients that yeast cells need to survive, and they produce organic acids that stress the yeast cells, which are therefore prevented from optimally metabolizing fermentable sugars into ethanol. This can result in stuck or sluggish fermentations. Before they inoculate with yeast, winemakers usually add the antimicrobial compound sulfur dioxide to suppress the growth of these bacteria, which originate from vineyard grapes and winery surfaces. This experiment tested the effect of sulfur dioxide treatment on eight bacteria species isolated from stuck wine fermentations; and it also tested the effect of sulfur dioxide on fermentation progress in grape juice batches inoculated with yeast and contaminated with one of each of these bacteria. Of the tested species, only Lactobacillus kunkeei strain UCD 26 was completely inhibited at any sulfur dioxide concentration up to 100 mg/L in both synthetic and real grape juice, while Acetobacter orientalis strain UCD 267 was completely inhibited only in synthetic juice. Other species were not completely inhibited at any sulfur dioxide concentration up to 100 mg/L. Of the species that were culturable in the synthetic juice, Gluconobacter strains G. cerinus UCD 281, G. japonicus UCD 583, and G. frateurii UCD 587 appeared to consistently and completely arrest fermentation under high-sulfur conditions. This is likely due to the bacteria producing enough gluconic acid to bind free sulfur dioxide and proliferate to levels that in combination with sulfur dioxide stress on the yeast causes fermentation arrest.