Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
1 · 2 · 3 · 4 · 5 · Microbes Part 1
January/February 2010
Microbes to watch out for: Dekkera/ Brettanomyces, lactic acid bacteria; if residual sugar is present, Saccharomyces and Zygosaccharomyces Danger signals: Change in color, aroma, or flavor, CO2 production, clouding Monitoring methods: Sensory evaluation, microscopic exam, culturing, PCR-based genetic techniques.
To assess microbial stability before bottling, there are four main questions to ask: 1. Does the wine have residual fermentable sugar (over 0.5 g/L glucose + fructose)?
2. Does the wine have residual malic acid if in pH range for MLF (this pH will vary)?
3. Does the wine have Pediococcus?
4. Does the wine have Brettanomyces?
If the answer to ALL four questions is no, the wine is likely to be microbially stable even if not sterile-filtered to remove all microbes. Surface film yeasts can grow in the bottle for only a few days, until dissolved oxygen is depleted, and Acetobacter cannot grow unless the closure is compromised.
1. Residual fermentable sugar trumps everything else.
Most wine microbes will use residual fermentable sugar if it is present. Fructose can be an exception; Saccharomyces is reluctant to use fructose if the glucose/fructose ratio is less than 1:6 to 1:10. Although it is risky to count on it, and other yeasts and bacteria (including Zygosaccharomyces) can use fructose, Saccharomyces refermentation is less likely in stuck wines that have been depleted of glucose.
Traditionally, the lowest levels of concern for Saccharomyces are 5 to 10 cells per BOTTLE (not per ml). Wines with only 6 cells per bottle have fermented, but other wines escaped problems with 15 cells per bottle. These are very low numbers, however, and methods that do not detect as few as 3 cells per bottle are not sensitive enough.
Filtering 250 ml of wine for yeast and 100 ml for bacteria, and culturing the membranes, is the preferred method for checking bottle sterility. Genetic tests that require 10 cells/ml for detection are not suitable even if an entire bottle is filtered to concentrate the cells.
PCR tests can help monitor bottled wines for microbes that are at high enough levels, and viable but non-culturable (VBNC) microbes or
as David Mills (UC Davis) calls them, “non immediately culturable cells.” Current plating methods do not detect microbial cells in this state.
Levels of Zygosaccharomyces for inbottle fermentation are not known, but because of the aggressive and tenacious nature of this yeast, even one cell is considered a serious problem in a wine with residual sugar. Zygosaccharomyces can take up residence in a bottling line for months or even years, surviving normal sanitizing procedures (even hot water), and infecting wines sporadically.
Certain lactobacilli can cause problems in the bottle by metabolizing residual sugar and producing acetic acid (but no ethyl acetate so there is no smell).
2. Leftover malic acid
Any of the lactic acid bacteria can cause trouble in the bottle if they metabolize residual malic acid. Cloudiness, a fine sediment, CO2, and a rise in pH (which can lead to protein or tartrate instability) are all possible consequences.
How much malic acid is too much depends on the wine; haziness is more noticeable in whites than in reds. In reds, a drop of around 0.5 g/L malic acid causes a slight spritz and, if the wine is light in color, a perceptible haze. Most wines at pH 3.4 or higher with residual malic acid should be filtered with a 0.45 μ membrane.
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