The History and Science of the Barrel- Lesson 8: Toasting and Charring

Lesson 8: Toasting and Charring

Nothing has a greater impact on oak’s chemistry than toasting. The chemical bonds between the three major polymeric building blocks (cellulose, hemicellulose and lignin) are disrupted, especially the hemicellulose and lignin. There are major changes happening to the structure as well as to the number of tannins as they are destroyed by heat.

There are several factors involved, such as toasting temperature and total duration that have an effect on which flavor compounds will form and at what depth they are maximized. It is during this process that one needs to keep in mind the density of the oak since American oak is denser than French oak. So the rate of heat penetration will be different.

Also, during the process of toasting, tannins are substantially destroyed by the heat. You can still get tannins about one-third of the depth from American oak staves, but they are almost completely depleted in the French oak. The barrel’s shape will also limit the toasting temperature and duration. Since all of the heat is applied to the inside surface of the barrel, there is a natural temperature gradient that varies throughout the depth of the stave. Say the visual penetration of the toast goes in about a quarter inch into the wood, the chemical changes are occurring at a much deeper level. Hence, the extent to which flavor chemicals are formed and the precise depth at which they are maximized depends greatly on the amount of heat applied and the length of toasting time and temperature.

Traditionally the terms light, medium and heavy toast are used based on visual appearance of the inside of the barrel. Light toast implies only a mild darkening of the staves; a medium toast has the same appearance as toasted bread; and a heavy toast is very dark, almost like a dark chocolate. As the wood is heated during toasting, the flavor compounds begin to break down releasing certain colors, aromas and flavors. For example, hemicellulose will release simple sugars, such as glucose, xylose, rhamnose and arabinose. Lignin, on the other hand, releases vanillin when heated. The process for toasting can take about 12 minutes for a medium toast and 18 minutes for a heavy toast.

Charring on the other hand is the process of “burning” the staves. This method acts like an activated-carbon filter to adsorb sulfur compounds. Charring, with the help of temperature, produces a deeper colored rum and higher production of esters, but over-charring the barrels can actually destroy some of the flavor chemicals that are needed to obtain a well-balanced aged rum.

Our special thanks to Independent Stave Company for their support in our research of oak.