What type of barrels must bourbon be aged in?

Bourbon must be aged in new oak barrels, specifically barrels that have never been used before for aging spirits. This requirement is one of the key legal definitions of bourbon, which contributes to its distinctive flavor profile. New charred oak barrels impart a variety of flavors to the bourbon, including vanilla, caramel, and spice, due to the caramelization of the wood sugars during the charring process. This aging process in fresh barrels is essential for developing the rich character that bourbon is known for.

The other options, while they refer to types of barrels that can be used for aging other spirits or wines, do not meet the specific legal requirement for bourbon. Used oak barrels are permitted for other types of spirits but cannot be used for bourbon aging. French oak barrels, often utilized in the aging of wines and some premium spirits, are not compliant with the bourbon regulations. Additionally, the term "all types of oak barrels" misses the crucial specification that bourbon must exclusively be aged in new barrels.