Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
email: Office@practicalwinery.com
1 · 2 · 3 · 4 · Microbes Part 2
September/October 2009
WINEMAKING
culprits that have died after causing, or contributed to causing, the stuck fermentation. Non-Saccharomyces yeast severely deplete nutrients and vitamins that Saccharomyces need, and they may also have some inhibitory activities.
Lactic acid bacteria
Stuck and sluggish wines are often attacked by Lactobacilli (as described above). Any CO2 production in a stuck or sluggish wine should be viewed with suspicion, for it could be caused by Lactobacilli (or Brettanomyces) rather than Saccharomyces resuming fermentation. Monitor stuck wines every day or two under the microscope while reinoculating with fresh yeast. In wines over 3.6 pH, it may be advisable to reduce pH or add lysozyme prophylactically, to reduce the chance of Lactobacillus spoilage.
Brettanomyces/Dekkera
Fortunately, very few musts become contaminated with Brettanomyces, and anyway, they tend to grow slowly compared to Saccharomyces. Stuck wines may become infected, especially if they are moved to wood to complete the fermentation. To detect Brettanomyces as soon as possible, culture any stuck red wine once per week, or check periodically by PCR. Production of 4-ethyl-phenol/4-ethylguaiacol (4-EP/ 4-EG) lags behind cell growth so this is not the best way to discover a new Brett infection in a stuck wine. Certainly, if apiculate yeasts are seen microscopically in a stuck wine, it should be tested for Brettanomyces immediately, but by that time the population may be already quite high.
Film on surface
Both Acetobacter and surface film yeasts (mainly Candida and Pichia) enjoy conditions that prevail in a stuck wine during reinoculation: warm, low SO2, not topped. Any film should be examined microscopically. To prevent film, the container should be brought as full as possible, and CO2 or other inert gas used liberally. Acetobacter can also be discouraged by lowering pH, but yeasts are not as sensitive to low pH as are bacteria.
Dry wines undergoing extended maceration are also at risk from Acetobacter, because there is not enough CO2 produced to prevent their growth.
ML FERMENTATION
Microbes to watch out for: Acetobacter, Lactobacillus, Pediococcus, Brettanomyces, Oenococcus, surface film yeasts
Danger signals: VArise (rise > 0.15 g/L, which is produced by Oenococcus from citric acid) in stuck wine, film on surface, off-odors or flavors
Monitoring methods: Sensory cues, microscopic exam, PCR
Commercial freeze-dried cultures are produced following extensive quality assurance procedures to be sure they are free from contaminants. Once they are put into must or wine, the resident bacterial flora may grow along with, or instead of, the inoculated strain(s).
Check wines during MLF for spoilage bacteria, both microscopically and by PCR methods, if possible. This is especially important if the culture is propagated before
adding it, allowing unwanted lactic acid bacteriamore opportunity to grow as well (or instead), and also giving aerobic yeasts and acetic bacteria a chance to spoil the starter. Acetobacter growth is particularly unfortunate in ML starters and during MLF. Microscopically, it is sometimes confused with Oenococcus.
If Lactobacillus and/or Pediococcus are a significant part of the wine’s bacterial flora, it may be best to kill them with lysozyme and reinoculate with a resistant Oenococcus strain. However, if the wine is dry, the winemaker may decide to accept the presence of some Lactobacilli during MLF, despite a tendency to introduce “pickle” or “sauerkraut” aromas.
Pediococci can make lactic acid from sugar but not acetic acid. They are considered undesirable mainly because some can continue growing after wine is dry and has finished MLF, and because of biogenic amine production, though off-flavors have been ascribed to some strains.
In cellars heavily infected with Brettanomyces, culturing once a week or testing with PCR during MLF is recommended.
   
[Lisa Van de Water, the “bad wine” lady, can be reached through the Vinotec group website, www.vinotecnapa.com, or by email: badwinelady@aol.com.]