The smell and, to a certain extent, the taste of butter is present in many wines. How did it get there you ask? In two ways, I’ll tell you.
The reason we smell and taste butteriness in a wine is because there are perceptible amounts of chemicals that give butter its aroma: diacetyl. In most cases, this diacetyl is a by-product of malolactic fermentation: a process whereby the sharp malic acid of grapes is converted into the creamier lactic acid of dairy products.
Another way for that butter smell to get into your wine is via barrel ageing. Most wines spend time ageing on oak barrels. This allows the wine to soften and integrate. In order to bend the wood into the barrel shape, the staves are heated over a fire. The exposure to that fire toasts the inside of the barrel, creating many complex compounds that add flavors, such as a buttery note, to some wines.