Microorganisms in Wine-Making: The Role of Yeasts and Bacteria

Wine is the product of a complex microbial ecosystem. While grapes provide the raw materials – sugars, acids, and flavour precursors – it is the microorganisms living in and around the grape must that actually transform juice into wine. Yeasts drive the primary alcoholic fermentation, while bacteria carry out secondary processes that shape a wine’s acidity, stability, and sensory character. Understanding which microorganisms are involved, and how they interact, is essential for anyone studying or practising wine-making.

The central role of Saccharomyces cerevisiae 🔗

Saccharomyces cerevisiae is the single most important yeast species in wine production. It has been used as a starter culture for centuries because of its reliable and vigorous fermentation capabilities. In simple terms, this yeast consumes the glucose and fructose present in grape must and converts them into ethanol (alcohol) and carbon dioxide – the core biochemical reaction that turns juice into wine.

What makes S. cerevisiae so well suited to wine-making is a combination of stress-tolerance traits. It can tolerate relatively high concentrations of ethanol (typically up to 14-16 % v/v), function at the low pH values found in grape juice (generally between 2.8 and 4.0), withstand the antimicrobial effects of sulphur dioxide (SO₂) added during crushing, and remain active at the cool temperatures that winemakers often use to preserve delicate aromas. This combination of tolerances gives S. cerevisiae a competitive edge over most other microorganisms in the must.

Modern wineries typically use commercially produced dried yeast – selected strains of S. cerevisiae – to inoculate the must. Over 700 distinct strains have been catalogued, and individual strains can differ in fermentation speed, temperature preference, production of volatile sulphur compounds, and contribution to wine aroma. By choosing a specific strain, a winemaker gains predictability and consistency in the finished wine.

How S. cerevisiae dominates the fermentation 🔗

When grapes arrive at the winery after harvest, their skins are already carrying a diverse community of wild yeasts – mainly from the genera Kloeckera, Candida, and Hanseniaspora. These wild yeasts often kick off fermentation spontaneously as soon as the grapes are crushed, because they come into immediate contact with the sugar-rich must. However, most of them are sensitive to rising alcohol levels and the SO₂ that winemakers add early in the process. As the alcohol concentration climbs beyond roughly 5-6 %, their populations decline sharply.

This is where S. cerevisiae takes over. Its superior ethanol tolerance allows it to dominate the later stages of fermentation and drive the process to completion – a stage winemakers call dryness, meaning virtually all fermentable sugars have been consumed. Its vigorous fermentative capacity even in the presence of oxygen (known as the Crabtree effect) makes it an exceptionally efficient ethanol producer, which in turn suppresses competing microbes.

By-products beyond ethanol 🔗

Ethanol is not the only metabolite S. cerevisiae produces. During fermentation, the yeast also generates a range of secondary compounds that influence wine quality. Glycerol contributes to the body and mouthfeel of the wine. Esters – formed from the reaction of acids with alcohols – add fruity and floral notes. Higher alcohols (also called fusel alcohols) can lend complexity in moderate amounts but harshness when present in excess. Small quantities of acetic acid can also form; while traces add complexity, elevated levels cause an unpleasant vinegar-like character. The particular profile of these by-products varies from strain to strain, which is why strain selection is a strategic decision in commercial winemaking.

Other yeasts in the fermentation environment 🔗

Although S. cerevisiae dominates, it is rarely the only yeast present. In spontaneous (uninoculated) fermentations, the native microbial community on the grape skins initiates the process. Species like Hanseniaspora uvarum, Candida stellata, and Torulaspora delbrueckii contribute metabolites during the early phases that can add distinctive flavour complexity to the finished wine.

Some other Saccharomyces species are also relevant. Saccharomyces bayanus, for example, can tolerate alcohol levels as high as 17-20 % and is therefore used in the production of fortified wines like port and in vinifying grape varieties harvested at very high sugar levels.

Brettanomyces is another yeast genus that deserves mention – though its role is debated. In small amounts, Brettanomyces can add earthy or leathery complexity that some winemakers value. In higher populations, however, it produces volatile phenols that give the wine unpleasant barnyard or medicinal off-flavours. For this reason, Brettanomyces is generally classified as a spoilage organism, and winemakers go to considerable lengths to prevent its proliferation.

Some yeast species are termed “killer yeasts” because they secrete toxins that can inhibit or destroy sensitive strains of S. cerevisiae. This is a natural competitive mechanism, but in a winery it can cause stuck fermentations if a killer strain overwhelms the desired inoculated culture.

Lactic acid bacteria and malolactic fermentation 🔗

Once alcoholic fermentation is complete (or sometimes running alongside it), a second microbial transformation can take place: malolactic fermentation (MLF). Despite its name, MLF is not carried out by yeasts. It is driven by lactic acid bacteria (LAB), a group that includes species from the genera Oenococcus, Lactobacillus, Pediococcus, and Leuconostoc.

What happens during malolactic fermentation 🔗

In MLF, lactic acid bacteria convert L-malic acid – a naturally occurring grape acid with a sharp, green-apple taste – into L-lactic acid, which is softer and creamier, plus a small amount of carbon dioxide. The net result is a reduction in the wine’s total acidity and a slight increase in pH. This makes the wine taste rounder, smoother, and less tart.

MLF is considered essential for virtually all red wines and is also standard in the production of certain whites, most notably Chardonnay, where it contributes the well-known buttery character. That buttery flavour comes from diacetyl, a by-product of the bacterial metabolism of citric acid during MLF. At low concentrations diacetyl adds pleasant richness; at higher levels it can become overpowering.

Oenococcus oeni: the preferred MLF bacterium 🔗

Oenococcus oeni is the lactic acid bacterium best adapted to the harsh environment of wine. It can tolerate low pH, high ethanol, and the SO₂ levels typically present after alcoholic fermentation – conditions that inhibit most other LAB species. For this reason, commercial wineries usually inoculate with a selected strain of O. oeni rather than relying on indigenous bacteria. Controlled inoculation reduces the risk of off-flavour production and ensures MLF completes reliably.

When MLF occurs spontaneously – driven by whatever LAB happen to be present in the winery – the outcomes are less predictable. Species of Lactobacillus and Pediococcus may also participate, and while some strains contribute positively, others can produce biogenic amines and off-flavours that compromise wine quality.

Benefits and risks of MLF 🔗

Beyond deacidification and flavour modification, MLF confers microbiological stability. By consuming malic acid and other residual nutrients, the bacteria remove substrates that spoilage organisms could otherwise exploit. However, the rise in pH that accompanies MLF can also make the wine slightly more vulnerable to microbial spoilage if the pH climbs too high. Winemakers often address this by adding tartaric acid post-MLF to bring the pH back down to a safe range.

Acetic acid bacteria: the spoilage threat 🔗

Not all bacteria in wine are welcome. Acetic acid bacteria (AAB) are a group of strictly aerobic, Gram-negative organisms that can cause serious spoilage if they gain a foothold. The two genera most relevant to winemaking are Acetobacter and Gluconobacter.

How acetic acid bacteria cause spoilage 🔗

AAB oxidise ethanol to produce acetic acid – the compound that gives vinegar its sour taste. Elevated acetic acid in wine is measured as volatile acidity (VA), and even modest increases can ruin the sensory profile. Spoiled wines typically exhibit vinegar-like sourness, along with sherry, bruised-apple, or solvent-like off-aromas. AAB can also produce ethyl acetate (a nail-polish-remover smell) and acetaldehyde, further degrading wine quality.

Gluconobacter species are found mainly on grapes and in fresh must because they prefer sugar-rich, low-alcohol environments. They tend to disappear once alcoholic fermentation begins. Acetobacter species, on the other hand, are more ethanol-tolerant and can survive through fermentation and into the ageing and storage phases. Acetobacter pasteurianus is one of the most commonly isolated spoilage organisms in stored and bottled red wines.

Controlling acetic acid bacteria 🔗

Because AAB are obligate aerobes, the most effective control measure is to minimise the wine’s exposure to oxygen. This means keeping fermentation vessels and storage containers tightly sealed, topping up barrels to eliminate headspace, and being careful during operations like racking and pumping that can introduce air. Additional strategies include maintaining adequate SO₂ levels, keeping wines at low temperatures, and ensuring the must pH stays below 3.5.

On healthy grapes, AAB populations are typically very low – fewer than 100 cells per millilitre. But on damaged or Botrytis-infected fruit, counts can rise above 100,000 cells per millilitre, making careful grape sorting at harvest a critical first line of defence. Fruit flies (Drosophila) are another vector; they carry AAB on their bodies and can introduce the bacteria into the winery environment.

Interactions between microorganisms during wine-making 🔗

Wine fermentation is not a series of isolated events. Yeasts, LAB, and AAB coexist in the same environment and influence one another in complex ways.

During alcoholic fermentation, S. cerevisiae creates conditions – rising ethanol, nutrient depletion, CO₂ saturation – that suppress many competing organisms. This competitive dominance is one reason it is the industry standard. However, the metabolites yeasts leave behind, along with the nutrients released by dying yeast cells (the lees), provide a food source for bacteria that arrive afterwards. The timing and sequence of microbial activity is therefore critical.

If LAB begin growing too early – while residual sugar is still present – they may ferment sugars rather than malic acid, producing excessive volatile acidity. Similarly, if AAB find an opportunity to grow (due to air exposure, for instance), they can generate acetic acid levels that overwhelm the wine’s flavour profile. Winemakers manage these risks through temperature control, SO₂ additions, careful timing of MLF inoculation, and strict hygiene throughout the cellar.

Wild fermentations vs. inoculated fermentations 🔗

There is an ongoing debate in the wine industry about whether it is better to rely on indigenous microorganisms or use commercial cultures. Wild or spontaneous fermentations use whatever yeasts and bacteria are naturally present on the grapes and in the winery. Proponents argue that this approach produces wines with greater complexity and a stronger expression of terroir – the unique character linked to a specific vineyard and region.

Inoculated fermentations, by contrast, use selected strains of S. cerevisiae (and often O. oeni for MLF) to ensure predictability, consistency, and a lower risk of spoilage. Most large-scale commercial wineries favour this approach because it offers greater control over fermentation kinetics and the final sensory profile.

In practice, many winemakers use a hybrid approach. They may allow wild yeasts to initiate fermentation for the flavour complexity they contribute, then inoculate with S. cerevisiae partway through to ensure a clean, complete finish. Similarly, some will encourage spontaneous MLF in certain lots while inoculating others with O. oeni.

Key factors affecting microbial activity in wine 🔗

Several environmental parameters determine which microorganisms thrive and which are suppressed during wine-making.

Temperature is a major lever. Cooler fermentations (12-15 °C) slow yeast activity and preserve volatile aroma compounds, while warmer fermentations (25-30 °C) accelerate the process but risk producing harsher flavours. LAB generally require temperatures above 18-20 °C to conduct MLF effectively, and AAB grow best between 25 and 30 °C.

pH affects microbial survival directly. At wine pH values below 3.5, most spoilage bacteria – including undesirable Lactobacillus and Pediococcus species – are inhibited. O. oeni, however, has evolved to tolerate these conditions, which is why it predominates during MLF.

Sulphur dioxide is the winemaker’s principal antimicrobial tool. Added at crushing and at various stages thereafter, SO₂ selectively inhibits wild yeasts, LAB, and AAB while leaving the more tolerant S. cerevisiae to conduct fermentation. The molecular (active) form of SO₂ is the fraction that matters, and its effectiveness increases at lower pH.

Oxygen availability is the decisive factor for AAB. Without oxygen, these bacteria cannot grow. Keeping wine in a strictly anaerobic environment – topped-up barrels, sealed tanks, inert gas blankets – is the most reliable way to prevent acetic spoilage.

Why microbial management matters 🔗

Every decision a winemaker makes – from grape sorting to yeast selection, SO₂ dosing, fermentation temperature, and barrel management – is ultimately about managing the microbial ecosystem of the wine. A well-managed fermentation harnesses the beneficial activities of S. cerevisiae and O. oeni while suppressing spoilage organisms like Acetobacter and undesirable wild yeasts. The result is a stable, high-quality wine with the intended flavour profile.

Advances in molecular biology and genomics are now giving winemakers and researchers even deeper insight into these microbial communities. Techniques like real-time PCR allow rapid detection of spoilage bacteria at all stages of production, enabling corrective action before off-flavours develop. Meanwhile, ongoing research into non-Saccharomyces yeasts and alternative LAB species like Lactiplantibacillus plantarum is opening new possibilities for producing wines with lower alcohol, improved acidity balance, and novel sensory profiles.

What do you think? Given the trade-offs between predictability and complexity, would you prefer a wine made with carefully selected commercial cultures or one shaped by whatever wild microorganisms nature provides? And as climate change alters grape chemistry – higher sugars, lower acids – how important will it be for winemakers to find new microbial partners that can adapt to these shifting conditions?

References

1. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.01988/full
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC7565949/
3. https://www.frontiersin.org/articles/10.3389/fmicb.2017.02087/full
4. https://www.wienscellars.com/the-role-of-yeast-in-wine-fermentation-a-closer-look/
5. https://en.wikipedia.org/wiki/Yeast_in_winemaking
6. https://en.wikipedia.org/wiki/Malolactic_fermentation
7. https://extension.oregonstate.edu/catalog/em-9641-simultaneous-malolactic-fermentation-it-right-option-your-wine
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC9781322/
9. https://www.sciencedirect.com/science/article/abs/pii/S0740002023000576
10. https://wineserver.ucdavis.edu/industry-info/enology/wine-microbiology/bacteria/acetobacter-pasteurianus
11. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/acetic-acid-bacteria
12. https://pmc.ncbi.nlm.nih.gov/articles/PMC11720281/