Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
1 · 2 · 3 · 4 · Microbes Part 2
September/October 2009
Monitoring microbes during fermentation
If grapes arrive with mold and/or rot, take photos and save an uncrushed sample to freeze immediately as a record of the grapes’ condition. Test laccase (responsible for the browning of white wines and discoloration of red wines) and volatile acidity (VA), and consider rejecting fruit if VA is more than 0.1 g/L in juice. Note that crushing of grapes may be considered legal acceptance of the grapes “as is.”
After grapes that were not completely healthy are processed, take a sample from the tank and examine it under the phase-contrast microscope for bacteria and yeast. Fortunately, molds are very intolerant to alcohol, so they die as soon as fermentation begins. It is normal to see a few yeasts, but no more than about 5 per 40x field. There should be no bacteria, or at most one rod-shaped bacterium in several 40x fields. Even one Acetobacter/ Gluconobacter cell seen under the microscope is too many.
If any Lactobacilli are seen, add lysozyme to the must immediately, or continue to monitor the batch microscopically once per day, to see if the population increases. Also monitor growth of non-Saccharomyces yeasts, as discussed below.
Please see “Spoilage microbe population fluctuations during winemaking – causes, effects, solutions” by Robert Tracy, May-June 2009 PWV journal.
Lisa Van de Water,
Vinotec Napa
inemaking involves encouraging cooperation with the microbes that we want, and discouraging participation from microbes we do not want at all, or do not want at the time (for example, Saccharomyces in a bottled sweet wine). Levels of concern vary from wine to wine and from winery to winery, but there are some guidelines to start from.
Monitoring microbes effectively and efficiently during the winemaking process requires coordinating several different techniques. Examination with a phase-contrast microscope, culturing on various media, testing using polymerase chain reaction (PCR) including quantitative PCR and Scorpions™ genetic chemical tests, sensory cues, and other methods all have a place in today’s wine microbe detection protocol.
Especially during the harvest season, access to a good phase-contrast microscope is an extremely valuable tool, because samples
can be examined directly without staining. Many problems can be detected in a few minutes, so winemakers can respond immediately. A brightfield microscope can be used to examine yeasts after staining, but not to detect wine bacteria. Despite the cost (several thousand dollars), a high-level phase-contrast microscope is recommended for all wineries.
Even with such a microscope, the lower limit of detection by direct microscopic exam is around 2,000 cells/ml. Fortunately for the wine microbiologist, however, microbes involved in spoiling fermentations are present at higher levels – often much higher.
Microbes to watch out for: Non- Saccharomyces yeast species, lactic acid bacteria, acetic acid bacteria
Danger signals: Mold and/or rot, signs of spontaneous fermentation, acetic acid (vinegar) or ethyl acetate odor
Monitoring methods: Sensory cues, microscopic exam, PCR-based genetic testing.
Non-Saccharomyces yeast in stuck wine
Non-Saccharomyces yeast in stuck wine
Kloeckera apiculata
Non-Saccharomyces (small) and Saccharomyces in stuck Chardonnay (by Nicola Wilton)
Non-Saccharomyces (small) and Saccharomyces in stuck Chardonnay (by Nicola Wilton)
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