Regardless of the number of years the spirit is left in the bourbon barrels for the second or third fill, it does not provide the same flavor profile. In actuality, there is a variation of flavors instead of subtler flavors wit subsequent fills. This can be advantageous, for example with a light unpeated malt that is aged in a second fill bourbon barrel to ensure it is not predominated by the oak influence.
The first-fill of a Spanish oak sherry cask result in a dark mahogany whisky offering woody spices such as ginger, cloves, toffee and dried fruits. When the barrel is refilled, the oak is less robust and the new character includes more apples, floral notes and esters, while the tannins result in a lighter spice note, such as cinnamon instead of oak.
The bourbon barrel reacts in a similar way. The first fill bourbon barrel gives a different influence than one that is filled down the line. A first fill bourbon barrel provides a chance for the oxidation products to shine through giving an additional floral top note, but greater oxidation also reflects more moisture during aging at the traditional warehouse.
The spirit in a first fill bourbon barrel has optimum contact with the char and although its active influence diminishes during maturation as well as physically breaking down the bourbon barrel. Thus when the spirit is initially emptied from the first fill, some charcoal will leave the bourbon barrel reducing the char in the bourbon barrel for the second fill. The reduction of char in the bourbon barrel reduces the vanilla and the burnt heather notes, while increasing the level of influence in the oak itself. The bourbon barrel has lower levels of wood extractive liquid in the staves as well.
The greatest amount of wood extractive liquid in a bourbon barrel is found in the first fill and released within a few years. Some remain for the second fill of the bourbon barrel, but very little remains for the third. While this was once considered the most important element, it is not known the nature of the oak, European or American is the primary influence in maturation in a bourbon barrel.
The influence of a bourbon barrel can be divided further according to the amount of charring that took place. The scale, ranging from 1-4 begins with the burnt toast effect and culminates in the alligator char. Alligator char indicates a burning of the staves to the point they resemble alligator hide. Various distilleries have an inventories of these barrels that range all through the char levels with the heavier char emphasizing the vanilla, toastiness, crème caramel and hint of smoke as well as the intensity of the color. The milder char; however, may promote more of the vanilla and honey with an overall sweetness that endows the spirit with additional body from the reaction with the bourbon barrel.
The number of times the bourbon barrel has been filled with a spirit and allowed to age is another consideration, with many manufacturers using a recipe that includes barrels of different fills to create consistency or a consistent flavor profile. Each time the bourbon barrel is used will influence the degree of spirit diminishing until it is coming to be seen as exhausted. Each reuse of a bourbon barrel decreases the percentage of character it provides to the spirit as well as the amount of time required for the product to complete its subtractive maturation. This means an spirit in s second or third fill might require ten to fifteen years to reach the same maturity of one aged only seven to eight in the original fill of the bourbon barrel.