Chardonnay wines have typically been aged in oak and undergo a secondary fermentation process known as malolactic fermentation (MLF).
MLF produces flavor and aroma compounds like diacetyl, acetaldehyde, and ethyl lactate, contributing to Chardonnay grapes' buttery characteristics.
Chardonnay made without oak barrels or the MLF process maintains its fruit-driven character and tends to be crisp and citrusy rather than buttery and creamy.
MLF can happen through the introduction of a specific bacteria. For Chardonnay, that bacteria is often Oenococcus oeni, which contributes to the buttery flavors and aromas.
The process of MLF typically occurs in Chardonnay over about two weeks, although this also depends on several factors. It happens more rapidly in oak barrels than tanks.