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
first day of fermentation, a large inoculumof a vigorous active-dry yeastmay be required (even if no yeast addition was originally planned). If non-Saccharomyces yeasts reach 106/ml or more, the populationmay need to be reduced by centrifuging or filtering, before inoculating with properly rehydrated dry yeast.
Nutrient additions to uninoculated fermentations should be delayed until Saccharomyces is recognized under the microscope, or until a clean-smelling fermentation is under way. Fermentations with more than around 10% non- Saccharomyces yeasts should be considered at high risk and carefully watched for signs of sluggishness.
Lactic acid bacteria
Spontaneous growth of lactic acid bacteria during fermentation can sometimes be a positive occurrence, if the bacterium is Oenococcus oeni, MLF is desired, and no trouble in finishing the yeast fermentation is expected. If MLF is not desired, or if the yeast fermentation could stick late in fermentation, even Oenococcus can be a problem. Never inoculate a struggling fermentation with ML bacteria.
But if certain heterofermentative Lactobacillus species (called “ferocious” by Dr. Ralph Kunkee) grow when there is fermentable sugar, the result can be a disaster, traditionally called “piqure lactique.” This is more common in musts with higher pH (>3.6). Besides converting malic to lactic acid, they metabolize glucose and fructose to acetic acid, raising VA to 1 g/L or higher. Acetic acid, along with toxins that at least some species produce, is inhibitory to Saccharomyces, and the fermentation may stick. While Oenococcus is heterofermentative and thus theoretically could cause this type of spoilage, in fact Lactobacillus is overwhelmingly the bacterium responsible.
Lactobacillus spoilage may go undetected until too late. Because Lactobacilli do not make ethyl acetate, there is no telltale odor, and sugar can mask the acetic “bite” at the finish. Culturing is not recommended because
wine Lactobacilli grow very poorly in culture, and often do not grow at all. PCR based tests detect certain ranges of lactic acid bacteria, but not necessarily all wine Lactobacilli. For example, at present the Scorpion test detects only three wine species (L. plantarum, hilgardii, and brevis), though others may be responsible instead.
Microscopic examination easily determines whether Lactobacilli of any species are growing. If there are enough cells to cause a problem, there will be a large enough population to see. Chemical tests can alert the winemaker to check for Lactobacillus: pH rise, drop in malic or rise in lactic acid, and rise in VA/acetic acid.
Microbes to watch out for: Lactobacilli, Pediococcus, Acetobacter, Brettanomyces, surface film yeast
Danger signals: CO2 production, spontaneous MLF, VA rise, film on surface
Monitoring methods: Visual, microscopic
exam, PCR, culturing, chemical tests
Stuck wines are extremely susceptible to spoilage. The sugar offers a substrate for a number of microbes, and any efforts at encouraging completion of fermentation, including warming, aerating, and nutrient addition, will also encourage spoilage microbes. To determine whether non- Saccharomyces yeasts helped cause a stuck ferment, take an unstirred sample and then a stirred one. In the settled lees, there may be large numbers of odd-shaped yeasts,
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