1. Fermentation Starts on the Grape
Grape skins carry a population of wild yeasts and bacteria. When grapes are crushed and left to ferment without added cultures, these organisms begin to multiply in the juice. This is spontaneous or indigenous fermentation, and its course depends on the vineyard, the season and the cellar.
2. The Succession
Fermentation proceeds through a sequence. Early on, non-Saccharomyces yeasts such as Hanseniaspora and Metschnikowia dominate, because they tolerate the high sugar and low alcohol of fresh juice. As alcohol rises, these yeasts decline, and Saccharomyces cerevisiae, the species most used in commercial fermentation, takes over and completes the fermentation.
Typical succession in spontaneous fermentation (qualitative)
| Stage | Dominant organisms | Condition |
|---|---|---|
| Early | Non-Saccharomyces yeasts (for example Hanseniaspora) | High sugar, low alcohol |
| Middle | Mixed populations, with Metschnikowia and others | Alcohol rising, nutrients being used |
| Late | Saccharomyces cerevisiae | Higher alcohol, lower nutrients |
3. Why This Matters for Style
Early yeasts produce different aromatic compounds from Saccharomyces, so the succession can affect the character of the wine. Winemakers who ferment with indigenous yeasts accept a less predictable result in exchange for a wider range of character. Those who prefer control often inoculate with a selected strain at the start.
4. Trade-offs
Spontaneous fermentation can stall, produce off-flavours or vary from batch to batch. Cellar hygiene, temperature control and monitoring of sugar and alcohol are needed to manage these risks. Each approach has its own discipline, and neither is automatically better.
5. Monitoring the Ferment
Track sugar levels, temperature and microbial populations through the ferment, and act early if the fermentation slows. Records from each vintage help winemakers decide whether to rely on indigenous yeasts again.
6. Ester Production
Esters are aromatic compounds that give many fermented products their fruity notes. Both non-Saccharomyces yeasts and Saccharomyces produce esters, but the amounts and types differ. Fermentation temperature and the availability of nitrogen both influence ester levels, so cellar conditions can shape the aroma as well as the fermentation pace.
7. Fermentation Risks
Spontaneous fermentations can stall before all the sugar is used, a condition known as a stuck fermentation. They can also produce high levels of volatile acidity or allow unwanted microbes to grow. Monitoring sugar and temperature, keeping equipment clean and acting early when the ferment slows are the standard controls.