What is the natural flavoring that's added to Challenge butter?

The natural flavoring that is used as an ingredient in Challenge Unsalted Butter, Challenge Unsalted Whipped Butter and Challenge Unsalted European Style Butter, is a natural flavor distilled from fermented milk. It is added to the cream prior to churning and produces flavor compounds that give our unsalted butter a distinctive, pleasing taste. It is an all natural ingredient that is approved by the USDA and the FDA.