charring, toasting (and recharring) refers to heating the inside of a wooden barrel prior to the addition of spirits or wine. Toasting is an original part of barrel making, the staves being hooped together at one end and then inverted over a low fire to relax the wood enough so that they can be bent together at the other end (now many barrels are steamed instead, although French wine barrels are still toasted). Deliberate charring dates to the end of the eighteenth century, when it was introduced as a way to keep casked water pure; by the 1820s, charred barrels were being used in America to age rye whisky. See whisky, rye.
The most efficient way to heat a barrel is to use a flame at various intensities. Heating barrels with an electric coil imparts less aroma from the heat source. Depending on its maturity, white oak, the standard material for whisky barrels, is made up of cellulose (40–53 percent), hemicellulose (15–30 percent), lignin (7–30 percent), and minor chemicals such as fats, oils, resins, proteins, and ash (5–11 percent). Cellulose and hemicellulose are rather large molecules that can be up to ten thousand sugar units in length. Lignin cements together wood’s cellulose and hemicellulose, much like mortar in a brick wall. It is the most abundant non-carbohydrate in the world and is made up of five thousand units of connected phenols. The primary purpose of heating a barrel is to break down the cellulose, hemicellulose, and lignin into their building blocks, termed “wood extracts.” A good analogy is the game Jenga, which involves a series of stacked blocks. Imagine each block piece represents either the sugar from cellulose, hemicellulose, or phenol from lignin. Each piece that is pulled out of the tower would represent a wood extract dissolving into the ethanol. Eventually the tower will crumble, and, as in real life, so does a barrel when used long enough.
Controlling the level of wood extracts from a cask can be difficult, especially when casks have been reused multiple times. How the barrel is heated is very important for creating flavors for the spirit or wine. There are two fundamental methods. The first method is charring: when a barrel is burnt with such intensity that it blackens the surface and transforms the hemicellulose, cellulose, and lignin to pure carbon. The second method is toasting: burning a barrel with a less intense heat, which turns the surface of the wood brown or red rather than the blackened surface of a char.
Wood flavor is controlled by temperature of the burn. When wood temperature reaches 100° C, its physical strength starts to change and its lipids, oils, lactones, and resins degrade. At 140–260° C hemicellulose breaks down into simple sugars and starts to brown (toasting), which creates caramel and toffee notes. Lignin degrades in the range of 150–300° C and creates vanilla and phenolic (smoky) notes. At 275° C wood catches fire, and the degradation of cellulose to wood sugars occurs. Cellulose breaks down from 275 to 350° C. Above 475° C all the components will degrade into pure black carbon, which absorbs any sulfur from the sprit or wine. There is a temperature gradient across the width of a burned barrel stave. It may be above 475° C at the surface and creating char, but several millimeters underneath there is a layer with a red hue that contains the wood extracts. When classifying barrels for purchase, specifications are based on millimeter of burn depth.
Barrels may also be heated to remove unwanted flavors from a previous barrel fill. Barrels act like a sponge, so that what was in the first fill of the barrel will diffuse into the second. The simplest way to remove unwanted flavor is to burn a barrel again—this is recharring. Sometimes blenders want to finish their product in a used barrel to create unique flavor combinations.
In the United States, where straight whiskies must be aged in new, charred oak barrels, there are four levels of char, of increasing thickness, culminating in the no. 4, or “alligator,” char. An important part of the American cellarmaster’s art is choosing the proper level of char for his or her whisky.
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By: Don Livermore