One fermentation option available to winemakers is to allow a natural fermentation (native yeast) to occur. The juice or must is not inoculated with a yeast starter culture, but rather the natural yeast present on the grapes is allowed to develop and carry out the fermentation. Saccharomyces is so well adapted to grow in high sugar environments that it will naturally take over the fermentation even if no additional yeast is added. The difference between this method and the use of a starter culture is that the winemaker is at the mercy of the yeast already present. The fermentation may be dominated by a poor quality strain that does not ferment well. However, this method of fermentation is commonly practiced and many winemakers believe it produces a more complex flavor profile. Natural fermentations are usually slower to develop since smaller amounts of Saccharomyces are naturally present than the quantity typically added by inoculation and it takes longer for the natural yeast to develop an equivalent concentration in the juice. Extra attention should also be given to natural fermentations since the possibility for spoilage and problem fermentations is greater with this method. Here are some factors that increase this risk:

• Damaged fruit or fruit with any mold or rot is more susceptible to problematic fermentations.
• Juice with a higher pH – usually above 3.6 – has a greater chance of spoilage organisms growing in the wine.
• The first time a fermentor is used or if it has been aggressively cleaned, there is less natural microflora present. A greater chance exists of non-Saccharomyces yeast and bacteria growing in the juice before the reduced Saccharomyces population takes hold.