Several fierce objections to the idea that butter does’t have much flavor on its own. Being a lover of butter, I’m down with that. However I think it can be fairly said that a lot of factors influence the flavor of butter. In some circumstances butter has lots of flavor, in others it doesn’t.
For example, when it’s cold. A cold pat of butter, even the fiercest butter lover will concede, has less flavor than hot, melted butter. It still tastes like butter of course, and that’s due in large part to a specific compound, a ketone called diacetyl. Diacetyl is abundant in fermented foods, and ranks in the top five on my list of All-Time Yummiest Molecules. But diactyl isn’t the only ketone in cold butter. There are many others, but most of them are locked up inside larger organic acid molecules, and that prevents us from tasting them.
Until of course we heat the butter, at which point these acids — which go by the name of alkanoic acids — start breaking into pieces. When they do the result is a variety of methyl ketones and lactones which are responsible for the the rich aromas and flavors we associate with croissants and brown butter sauces. Of course the effects is heightened with actual browning, which brings all sorts of other flavors to the party.
But butter need not be browned to have its flavor enhanced by heat. If you’re wondering why some pastry makers out there prefer to use clarified butter instead of just plain soft butter, this is a big part of the reason. Butter that’s been heated to the boiling point simply has more flavor, because more of its flavor-producing ketones and lactones have been liberated.