Issue link: http://winesandvines.uberflip.com/i/131907
winemaking Figure 1—Some vineyard sprays for powdery mildew contain Bacillus subtilis; the spores can persist in cellared wine for many months. How much microbiology do winery personnel need? BY Lisa Van de Water, Vinotec Napa E very winery has the goal to produce a consistent, stable product that satisfies its customers. Microbial stability is an important part of achieving this goal. The question addressed below is: How much should winery workers know about microbial stability at bottling? Because of its alcohol content and low pH, wine is an inhospitable medium for most microbes. Of the thousands of microbes in the world, only a few grow in table wine (11%+ alcohol, as opposed to low-alcohol wine products at pH 3.1 and above. Perhaps 50 species or so are wine-tolerant, and no confirmed human pathogens have been found to date growing in wine. Many more species found on grapes and in wine cellars are often incidental contaminants, particularly in newly bottled wine, but will not grow in wine. Wine-tolerant microbes Fermentation yeasts: Saccharomyces cerevisiae and its "evil twin" Zygosaccharomyces bailii, which is insensitive to sorbate or normal levels of SO2 . Spoilage yeast: Brettanomyces/Dekkera bruxellensis, found in red and some white wines all over the world, grows mostly in barrel-aged wines and in infected wines bottled without very tight filtration. There are very significant strain differences in their toler58 p r acti c al w i ne ry & v i ne yard J U NE 20 13 ances (alcohol, SO 2 ) and metabolism (what substrates can be used, and what compounds are produced). [Conterno, L., C.M.L. Joseph, T. Arvik, T. HenickKling, T., and L. Bisson. 2006 "Genetic and Physiological Characterization of Brettanomyces bruxellensis Strains isolated from Wines." Am. J. Enol. Vitic. 57: 2, 139–147.] Aerobic yeasts: certain surface-film yeasts (mostly Pichia and Candida, but not all species of those genera); these create wrinkly surface growths on wines cellared with headspace. Lactic acid bacteria: Oenococcus oeni, Pediococcus parvulus and damnosus, some species of Lactobacillus (around a dozen have been identified in wine but there may be more). Most of these bacteria can perform malolactic fermentation (MLF) in wine, though some Pediococci cannot (though they may grow in dry wine that is finished with MLF). Lactic acid bacteria require higher wine pH than yeasts (3.2–3.3 pH, often higher). Aerobic bacteria: Acetobacter spp. are aerobic bacteria that form a slippery film on the surface of cellared wine exposed to air. They have been found growing in bottles, but only when the closure was compromised or when bottles with natural corks were stored upright, allowing diffusion of air through the cork. [Bartowsky, E.J., P.A. Henschke. "Acetic acid bacteria spoilage of bottled red wine—a review." Int. J. Food Microbiol. June 30, 2008; 125(1): 60–70. doi: 10.1016/j. ijfoodmicro.2007.10.016. Epub Dec. 23, 2007.] Not all of these microbes can grow in any given wine. Saccharomyces and Zygosaccharomyces, for example, cannot affect dry wines. A very low-pH wine (below around pH 3.2–3.3) usually will not allow the growth of lactic acid bacteria, though occasionally Oenococcus oeni grow in wines below 3.2 pH. Wine microbiologists should know which microbes are likely to pose a threat to each wine at each stage in its development and which do not. They also should, however, watch for a previously undescribed microbe, or for microbes that hardly ever grow in wine but could possibly be a problem. For instance, while Bacillus spp. almost never grow in wine (although the spores can survive in wine for months to years), a few events of clouding or H2S in wine from Bacillus (Lucy Joseph, U.C. Davis; personal experience) have occurred in the past 35 years or so. Thus, when wine microbiologists evaluate cultures or other lab results from wine, especially bottled wines (in any retail package), they can tell (or can Figure 2—Candida cantarelli colonies resemble Brettanomyces colonies, but C. cantarelli cells (top) are round, not apiculate like Brettanomyces (bottom).