What does “buttery” mean when talking about Chardonnay?

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Dear Dr. Vinny,
What does “buttery” mean when talking about Chardonnay?
—James, Sacramento, Calif.
Dear James,
I mostly see the term “buttery” in reference to Chardonnay, but other wines can also have buttery flavors, aromas or textures. This note typically comes from a compound called diacetyl, which is a natural byproduct of fermentation. In fact, some beers also have diacetyl. Buttery is one way to describe it, but it can also come across as butterscotch or the note of butter flavoring on popcorn. In fact, diacetyl is added to foods like popcorn, crackers and margarine for its flavoring.
Even though it occurs naturally, winemakers will encourage further production of diacetyl by making their wine go through malolactic conversion, by which a wine’s tart malic acid is converted into softer, creamier lactic acid, like the kind found in milk. Exposing wine to oak barrels can further emphasize buttery notes—both by adding toasty notes and by softening a wine’s texture.
"Buttery" is neither a positive nor a negative term, but I should point out that buttery Chardonnays were all the rage in the 1990s and early 2000s. These days it’s more fashionable to make versions that show more restraint than “butter bombs,” so it might be considered a negative descriptor among some.
—Dr. Vinny