The buttery character found in some white wines is not an oak effect and not a flavouring. It is a by-product of a bacterial process that winemakers choose to permit or prevent.
The conversion is bacterial, not a fermentation
Despite the common name, this stage is not a fermentation in the sense of yeast converting sugar. Lactic acid bacteria consume malic acid and convert it to lactic acid.
Malic acid is the sharp acid found in green apples, while lactic acid is the softer one associated with dairy. Replacing one with the other lowers perceived acidity noticeably.
The conversion also raises pH slightly, which affects colour stability and microbial risk. Winemakers therefore track it for reasons well beyond how the finished wine will taste.
Diacetyl is the compound doing the work
The bacteria produce diacetyl as an intermediate, and it is this molecule that reads as butter or cream on the nose. It is detectable at very low concentrations.
How much survives depends on timing, since yeast still present in the wine will metabolise diacetyl back into odourless compounds. Leaving the wine on its yeast lees reduces the buttery character.
This gives the winemaker a dial rather than a switch. The same conversion can finish with an obvious dairy note or with almost none, depending on how it is handled afterwards.
Why reds almost always go through it
Nearly all red wines undergo the conversion, because leaving malic acid in a red creates a microbial instability risk once the wine is bottled.
Bacteria that find malic acid in a sealed bottle will convert it there, producing carbon dioxide and haze in a wine that was supposed to be still and clear.
Completing the conversion deliberately in the cellar removes the food source before bottling. Stability rather than flavour is the primary motive in red winemaking.
Blocking it is a stylistic decision in whites
Crisp, mineral-styled whites are usually prevented from undergoing the conversion so their malic sharpness survives into the bottle.
Blocking is achieved by keeping the wine cold, filtering the bacteria out, or adding sulphur dioxide, all of which suppress bacterial activity.
Some producers put only a portion of a batch through the conversion and blend the results. That preserves freshness while adding a rounder texture underneath it.
What the drinker can detect
Beyond the aroma, the clearest sign is texture. A wine that has undergone the conversion feels broader and less angular across the middle of the palate.
The acidity may still measure high on paper while feeling gentler, because the shape of the acid has changed rather than the quantity.
Two wines from the same vineyard treated differently in this respect can taste like different varieties. It is one of the few cellar decisions with a signature obvious enough to identify.