Fermentation requires two things : sugars and yeasts. A ripe organic grape is full of natural sugars and there are wild yeasts living on its skin. As soon as the skin of the grape is broken, fermentation can begin. To make wine, all the winemaker has to do is collect his grapes and gently crush them, releasing the sugary juice and exposing it to the yeasts. Fermentation will continue until all the sugar has been turned into alcohol or the level of alcohol in the juice reaches around fifteen percent, whichever is sooner. At around fifteen percent alcohol, the yeasts will die naturally and any left over sugars will remain in the wine.
A natural wine is fermented only with the wild yeasts native to its terroir.
Yeast strains vary widely from place to place and contribute significantly to the odour of the finished wine. The yeasts indigenous to a particular area are an important part of what gives its wines their character.
Conventionally grown grapes have little or no wild yeast living on their skin.
The winemaker will kill whatever yeast remains with sulphur dioxide, and reseed the grapes with a single strain of commercially produced yeast.
Wines fermented in this way have less personality, all using the same few commercial yeast strains, and are less an expression of their terroir. This is one reason they taste so similar.
They are also less complex, as each of the many wild yeasts present on an organic grape will contribute something to the finished wine.
The level of alcohol in the finished wine is determined by the level of sugar in the grapes from which it is made.
More sugar means there is more for the yeast to convert into alcohol.
Grapes grown further north see less sun and therefore contain less stored sugar than those grown in the south. Traditionally, therefore, northern wines contain a lower level of alcohol.
Chaptalization is a way of boosting the level of alcohol in the finished wine by adding sugar to the juice during fermentation.The technique is named after Jean Antoine Chaptal, Napoleon’s minister for agriculture, who is said to have invented it.
A natural wine is fermented only with its own sugars.
Malolactic fermentation is a secondary process of bacterial conversion, which may follow or overlap with primary fermentation.
Harsher tasting malic acid is converted into softer, and less acidic, lactic acid. Carbon dioxide is also produced.
In practical terms this means a reduction in the acidity of the wine and an increase in its complexity. The level of alcohol is unaffected.
Like primary fermentation, malolactic fermentation can be induced by the introduction of cultured bacteria, or suppressed with sulphur dioxide.
If a wine is bottled quickly, it may take place inside the bottle. One reason SO2 is used at bottling is to prevent this.
A natural winemaker has to wait for the malo to finish naturally before he can bottle his wine.