When spirits are aged in barrels, the charring process plays a big part in the overall process as it allows the spirit to penetrate the oak pores and absorbs the sulfur compounds. A full depth charring of American oak barrels is legally stated to be 1/8 of an inch and even though too much charring can actually remove some of the flavors of the whiskey, American straight whiskey still uses full depth charring. In the US, it is illegal to age bourbon or other straight whiskey in anything other than American oak barrels that have been fully charged, although a lighter charring may well help to keep more of the flavor. Wood compounds that would be caramelized by a low or medium level char are burned up by heavy charring, and medium charring is needed simply to crack the wood of American oak barrels.
Rum, Canadian whiskey, Scotch, malt whiskey and other spirits that have a more delicate flavor have a much more balanced flavor profile because they are aged in bourbon barrels that have been used once. The flavors of these drinks would be potentially overpowered too much by the charring of new American oak barrels. The aging effects of American white oak barrels can be adjusted by adjusting the char from light, medium or heavy, and these will give a much more different congener profile as they provide various degrees of vanillin and lignin. Many bourbon companies in the US adopt this method to give their whiskey product a much more specific taste. This arguably gives their product a much better and smoother taste than they would otherwise have. To summarize, American oak barrels that have been charred definitely give the contents more of a flavor and the experienced whiskey drinker or taster can easily identify these flavors and determine that the charring process has been used. Charred American oak barrels produce more esters and a deeper color to the spirits. In addition the many other flavors, charring can mean the whiskey has more color, spice, honey and viscosity and the end result of this proven production process is a whiskey that goes down much more smoothly.