Wine Microbiology: Bacteria, Wild Yeast, and Spoilage Organisms

The Microbial World of Wine

Wine is a living ecosystem. From the moment grapes are crushed to the day the bottle is opened, microbial populations compete, cooperate, and transform grape juice into wine. Understanding which organisms are present, what they do, and how to manage them is a cornerstone of sound winemaking practice.

A single grape berry carries 10,000 to 10 million microbial cells on its surface, representing dozens of species of yeasts, bacteria, and molds. These populations shift dramatically during winemaking, with different organisms dominating at different stages. The winemaker's role is to encourage beneficial microbes while suppressing harmful ones.

The Microbial Succession

Wine production follows a predictable microbial succession. In freshly crushed must, the dominant organisms are non-Saccharomyces yeasts and various bacteria from the grape surface. As fermentation begins and ethanol accumulates, Saccharomyces cerevisiae takes over due to its superior ethanol tolerance. After alcoholic fermentation, lactic acid bacteria may dominate during malolactic fermentation. Throughout this succession, spoilage organisms wait in the wings, ready to proliferate if conditions favor them.

Beneficial Yeasts

Saccharomyces cerevisiae

Saccharomyces cerevisiae is the principal wine yeast, responsible for converting sugar to ethanol and CO₂ during primary fermentation. It dominates because of several key advantages: high ethanol tolerance (surviving 14-18% alcohol depending on strain), ability to ferment under anaerobic conditions, efficient sugar metabolism, and competitive production of ethanol that inhibits other organisms.

Commercial wine yeast strains are selected S. cerevisiae isolates that have been bred or identified for specific winemaking properties. Strain selection gives winemakers considerable control over fermentation character, including aroma profile, nutrient requirements, fermentation speed, and alcohol tolerance.

Non-Saccharomyces Yeasts

The term non-Saccharomyces yeasts encompasses a diverse group of species that are naturally present on grapes and in wineries. Once considered only spoilage organisms, many are now valued for their contributions to wine complexity. Important non-Saccharomyces genera include:

Torulaspora delbrueckii produces lower levels of volatile acidity and can reduce alcohol slightly compared to S. cerevisiae. It is often used in sequential inoculation protocols, where it is added first and then followed by S. cerevisiae after a few days.

Metschnikowia pulcherrima produces high levels of esters and can limit the growth of spoilage organisms by depleting iron through production of the pigment pulcherrimin.

Lachancea thermotolerans produces lactic acid during fermentation, which can naturally lower pH in high-pH musts without requiring acid additions. This property makes it valuable in warm-climate winemaking.

Pichia kluyveri enhances varietal thiol release, making it useful for aromatic varieties like Sauvignon Blanc.

Modern winemaking increasingly uses co-inoculation or sequential inoculation with non-Saccharomyces yeasts followed by S. cerevisiae, harnessing the aromatic and structural contributions of these diverse organisms while relying on S. cerevisiae to complete fermentation reliably.

Beneficial Bacteria

Lactic Acid Bacteria

Lactic acid bacteria (LAB) are the organisms responsible for malolactic fermentation (MLF). The most important species in winemaking is Oenococcus oeni, a small, Gram-positive coccus that is uniquely adapted to the harsh conditions of wine (low pH, high ethanol, presence of SO₂).

O. oeni converts malic acid to lactic acid, softening wine acidity and contributing to microbial stability (since malic acid is a carbon source for other bacteria, its removal reduces the risk of subsequent bacterial spoilage). Other LAB species that can conduct MLF include Lactobacillus and Pediococcus, though these are less desirable because they may produce biogenic amines and off-flavors.

Commercial MLF starter cultures of O. oeni allow winemakers to control the timing and outcome of malolactic fermentation. Direct inoculation after primary fermentation is the most reliable approach. Spontaneous MLF (relying on indigenous bacteria) is less predictable and carries greater risk of off-flavors.

Spoilage Organisms

Brettanomyces

Brettanomyces bruxellensis (commonly called Brett) is the most feared spoilage yeast in winemaking. It produces 4-ethylphenol (barnyard, band-aid, horse stable) and 4-ethylguaiacol (smoky, spicy, clove) from hydroxycinnamic acid precursors in wine. At low concentrations, these compounds may add complexity, but above threshold levels they are considered serious faults.

Brett thrives in wines with residual sugar or nutrients, elevated pH (above 3.6), and low free SO₂. It is extremely difficult to eradicate once established in a winery because it can survive in biofilms on barrel surfaces and other equipment. Prevention through rigorous sanitation, adequate SO₂ management, and careful monitoring is far more effective than treatment.

Acetobacter and Gluconobacter

Acetic acid bacteria (primarily Acetobacter and Gluconobacter) convert ethanol to acetic acid (vinegar) in the presence of oxygen. They are obligate aerobes, meaning they require oxygen to grow. This is why minimizing headspace, keeping containers full, and using airlocks are essential preventive measures.

Acetobacter also produces ethyl acetate at high levels, contributing a nail polish remover-like aroma. Wines with elevated volatile acidity from acetic acid bacteria are irreparably spoiled; there is no practical way to remove acetic acid once formed.

Pediococcus and Lactobacillus Spoilage

While some LAB species are beneficial, others cause serious spoilage. Pediococcus species can produce exopolysaccharides that make wine viscous and ropy (a condition called ropiness). They also produce diacetyl at concentrations far above the pleasant buttery threshold, creating an overwhelming butterscotch defect.

Certain Lactobacillus species produce biogenic amines (histamine, tyramine, putrescine) that can cause headaches and allergic-like reactions in sensitive individuals. They can also metabolize residual sugars and citric acid to produce excessive volatile acidity and mousy off-flavors.

Molds

Molds are primarily vineyard concerns rather than winery problems, but they can significantly affect must quality. Botrytis cinerea exists in two forms: noble rot (beneficial, producing the complex concentrated musts used for Sauternes and Tokaji) and gray rot (destructive, causing oxidation and off-flavors). Gray rot produces the enzyme laccase, which is highly resistant to SO₂ and causes severe browning.

Aspergillus and Penicillium species can contaminate corks, producing 2,4,6-trichloroanisole (TCA), the compound responsible for cork taint. TCA has an extraordinarily low detection threshold (about 1-2 parts per trillion) and produces musty, moldy, wet cardboard aromas that suppress fruit character.

Managing Microbial Populations

Sanitation

Sanitation is the first line of defense against unwanted microorganisms. Every surface that contacts wine, including fermenters, hoses, racking canes, bungs, and bottles, must be thoroughly cleaned and sanitized before use. Common sanitizers for home winemaking include potassium metabisulfite solution (2-3 tablespoons per gallon of water), Star San (phosphoric acid-based, no-rinse), and citric acid solution.

Barrels require special attention because their porous interior is impossible to sterilize completely. Burning sulfur wicks inside empty barrels and rinsing with hot water and citric acid solution are standard practices. Barrels with confirmed Brett contamination are very difficult to salvage and may need to be discarded.

Sulfur Dioxide Management

SO₂ is the most important chemical tool for controlling microbial populations in wine. Molecular SO₂ at concentrations of 0.5-0.8 mg/L inhibits most spoilage organisms. Because the proportion of molecular SO₂ depends on pH, maintaining low pH and adequate free SO₂ levels work together to provide microbial stability.

The timing of SO₂ additions is critical. An addition at crush (25-50 mg/L) suppresses indigenous microflora and gives inoculated yeast a competitive advantage. Post-MLF additions protect against Brett and bacteria during aging. Pre-bottling adjustments ensure the wine enters the bottle with adequate protection.

Temperature Control

Cold temperatures slow microbial growth, while warm temperatures accelerate it. Storing wine at 10-15°C (50-59°F) during aging significantly reduces the risk of spoilage compared to cellar temperatures of 18-22°C (64-72°F). Refrigeration is the simplest way for home winemakers to add an extra layer of microbial protection.

Sterile Filtration

Sterile filtration through a 0.45-micron membrane removes yeast and most bacteria from wine. This is the most reliable way to stabilize wines with residual sugar or wines intended for long-term storage. Home winemakers can achieve approximate sterile filtration using plate filters with tight pads, though absolute membrane filtration provides more certainty.

Frequently Asked Questions

How do I prevent Brettanomyces in my wine?

Prevention is the only reliable strategy. Maintain molecular SO₂ at 0.5-0.8 mg/L (which requires knowing your pH), practice rigorous sanitation of all equipment, keep wines at cool temperatures, minimize residual sugar, and monitor for early signs of Brett character. If you detect Brett aromas, increase free SO₂ immediately and consider sterile filtration. Barrels that have harbored Brett should be treated with hot water or ozone, or retired.

Is wild fermentation safe?

Wild (spontaneous) fermentation uses indigenous yeast populations from grapes and the winery environment. It can produce complex, terroir-expressive wines but carries higher risks than inoculated fermentation. The main dangers are stuck fermentation (if Saccharomyces populations are insufficient), volatile acidity production by apiculate yeasts, and spoilage by bacteria. Wild fermentation is safest with low-pH, healthy grapes and in wineries with established beneficial microbial populations.

What causes mousy off-flavors in wine?

Mousy off-flavor is caused by tetrahydropyridine compounds produced by certain lactic acid bacteria (particularly heterofermentative Lactobacillus species) and sometimes by Brettanomyces. It is perceived as a persistent, unpleasant mouse cage or cracker-like aftertaste. Mouse is most common in wines with low or no SO₂, making it a particular concern in the natural wine movement. There is no effective treatment once mousy compounds have formed.

Do I need to add malolactic bacteria?

For red wines, MLF is almost always desirable for acid balance and microbial stability. Inoculating with a commercial Oenococcus oeni culture is recommended for reliable, timely MLF. For white wines, MLF is a stylistic choice. If you want MLF, inoculate; if you want to preserve crisp acidity, prevent MLF by adding SO₂ (50 mg/L) immediately after primary fermentation and maintaining cool temperatures.

How do I know if my wine has a microbial problem?

Signs of microbial spoilage include a sudden increase in volatile acidity (vinegar smell), development of off-aromas (barnyard, mousy, ropy, medicinal), visible haze or film on the wine surface (indicating aerobic bacteria or film yeast), unexpected CO₂ production in a supposedly stable wine, and unusual viscosity. Regular sensory evaluation and SO₂ monitoring are the best early warning systems.