Wine microbiology covers the living organisms, mainly yeasts and bacteria, that turn grape juice into wine and that can also spoil it. A wine science curriculum treats this as foundational: fermentation, ageing and quality control all come down to managing microbial activity, deliberately or by accident.
This page introduces the core topics you will meet under this subject: the main fermentation organisms, the chemistry of ageing, the spoilage risks winemakers guard against, and the basic safety and quality-control concepts that connect microbiology to the finished bottle. It works alongside the other subjects listed on the modules overview, particularly where sensory outcomes and microbiological choices intersect.
Alcoholic fermentation: yeast
Saccharomyces cerevisiae carries out the primary fermentation in the great majority of commercial winemaking, converting grape sugars into ethanol and carbon dioxide. Winemakers either rely on the yeast populations naturally present on grape skins and in the winery ("wild" or spontaneous fermentation) or inoculate with a selected commercial strain chosen for predictable behaviour, specific aromatic byproducts, or tolerance of high sugar or alcohol levels. Non-Saccharomyces yeasts (such as Torulaspora or Metschnikowia species) are increasingly used in sequential or mixed inoculations to add aromatic complexity before Saccharomyces takes over the bulk of fermentation.
Malolactic fermentation: bacteria
Many red wines and some whites undergo a second, bacterial fermentation after alcoholic fermentation finishes. Oenococcus oeni is the bacterium most commonly responsible, converting the sharper malic acid into softer lactic acid. This lowers perceived acidity and can add buttery or creamy notes (from diacetyl) that are typical of many Chardonnays and red wines matured in barrel. Winemakers who want to keep a wine crisp, such as many aromatic whites, actively prevent malolactic fermentation through cold storage, sulfur dioxide additions or filtration.
Spoilage organisms and wine faults
Brettanomyces (often called "Brett") is the spoilage yeast winemakers worry about most. In small amounts it can contribute leather or spice notes some drinkers tolerate or even like; above a certain threshold it produces 4-ethylphenol and 4-ethylguaiacol, which read as barnyard, band-aid or medicinal off-notes and are considered a fault once they dominate the aroma. Acetic acid bacteria (Acetobacter, among others) can turn wine toward vinegar if oxygen exposure is poorly managed. Controlling both depends on hygiene in the winery, sulfur dioxide management and appropriate use of oxygen throughout handling.
Sulfur dioxide and other quality-control tools
Sulfur dioxide (SO2) remains the main chemical tool for controlling unwanted microbial activity and oxidation in wine, used at different stages from the crusher through bottling. Its antimicrobial and antioxidant effects are dose-dependent, which is why winemakers monitor free and total SO2 levels rather than adding it once and moving on. Regulatory limits on SO2 and labelling requirements vary by country and wine style, and "low-sulfite" or "no added sulfites" wines rely more heavily on hygiene, cold handling and sometimes alternative preservatives to manage the same microbial risks.
Ageing: oxidative and reductive pathways
Ageing is partly a microbiological story and partly a chemical one. Barrel ageing in oak allows slow, controlled oxygen exchange through the wood (sometimes supplemented deliberately through micro-oxygenation), which softens tannins and develops tertiary aromas. Reductive ageing, typical of wines matured in sealed stainless steel or glass, preserves primary fruit character but risks sulfide-related off-aromas if the wine is kept too far from oxygen for too long. Both pathways depend on the microbial stability established earlier in the process; an unstable wine will not age predictably regardless of the vessel used.
Checklist: core microbiology and safety concepts to master
- Role of Saccharomyces cerevisiae in alcoholic fermentation, and when winemakers choose wild versus inoculated fermentation
- Function of Oenococcus oeni in malolactic fermentation and its sensory effects
- Causes and sensory signs of Brettanomyces spoilage versus acetic acid spoilage
- How free versus total SO2 is measured and why both numbers matter
- Difference between oxidative (barrel) and reductive (tank/bottle) ageing pathways
- Basic hygiene and quality-control practices that prevent microbial faults before they start
Comparing the main microbial agents
| Organism | Role | Desired or undesired? |
|---|---|---|
| Saccharomyces cerevisiae | Alcoholic fermentation (sugar to ethanol) | Desired, central to winemaking |
| Oenococcus oeni | Malolactic fermentation (malic to lactic acid) | Desired for most reds, often avoided in crisp whites |
| Brettanomyces spp. | Produces volatile phenols during ageing/storage | Undesired past low thresholds; fault above that |
| Acetobacter and related bacteria | Oxidize ethanol toward acetic acid in presence of oxygen | Undesired; linked to poor oxygen/hygiene management |
For textbooks, journal sources and reference works that go deeper into these topics, see the module resources page. The sensory consequences of these microbiological choices, and how tasters and consumers actually perceive them, are covered separately under sensory analysis and consumer appreciation.
