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A Technical Guide for Wine Producers
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WINE IN WOOD AND WOOD IN WINE
The use of heat and water in the construction of barrels
by Charl Theron, guest lecturer in Oenology, Tim Rypstra, professor in Wood Science and Jan Swart, director: Wood and Fibre Institute, Stellenbosch University
Traditionally the construction of barrels has always involved heat and water for practical reasons. In order to obtain the shape of the staves and to ensure different degrees of barrel toasting, it was necessary to take both these physical factors into account. In due course the suppliers of barrels and alternative wood products realised that being variables, these two factors presented different combination options. By making use of these possibilities, a greater variation of barrels or alternative wood products could be obtained.
Before the construction of barrels can take place, the wood is bent by heat to shape the staves. This can be done using heat and/or water. The use of heat is a decisive factor, however, to obtain specific flavour profiles.
Heat transfer can take place by means of conduction, radiation or convection. Conduction is the transfer of heat from a warmer part of fixed matter in a single unit, alternatively fixed matter that come into contact with each other. The distribution of heat through fixed matter is therefore conduction. Obviously different materials differ in their ability to conduct heat. Metals, for example, are good conductors while wood, glass, ceramics and fluids are poor conductors. The only instance where conductivity plays a role in barrel composition, is when a specific logo is burnt onto the barrel.
Radiation is the transfer of heat from a warm to a cooler substance through the release of particles or rays. This obviously includes radiation such as alpha-, beta-, gamma- and X-rays as well as micro- and radio waves. In the case of wood preparation, only thermal radiation is important. Other than with conduction, the various components do not have to be in physical contact with each other.
Convection is the heat transfer between liquids and gases. If one portion of gas or liquid is heated, a flow process ensues between the warmer and the colder parts. The result is that the warmer and the colder portions mix to result in an even temperature.
The traditional custom of constructing barrels around an open fire is based on the heat radiation of wood. Because heat transfer in this instance occurs in straight lines, the distance between the open fire and the inside of the barrel plays a very important role. In order to allow the toasting of the wood to occur, very high temperatures have to be obtained in the fire. This explains why traditional toasting only occurs to a depth of 4 to 5 mm. To obtain a deeper toasting, the temperature on the surface has to be high enough for charring or blistering to occur. The temperature difference between various degrees of toasting is therefore minimal. At the Demptos cooperage, for example, it is 131°F for light toasting and 176°F for heavy toasting with the various degrees of toasting, medium and medium plus, inbetween. If barrels are not toasted, however, the wood character would be too aggressive; the toasting of wood also brings about additional flavour components that will be imparted to the wine.
In view of the fact that the toasting of barrels results from a combination of temperature and time, the control thereof will differ, with varying results, from one cooperage to the next. Over the years the large cooperages have mechanised to a certain extent, but the role of individual coopers should not be underestimated. Wood being a natural product, the wood that is used for barrels will differ and individual cooperages must do their utmost to eliminate these differences between barrels.
One of the methods that may be employed to manage the toasting process more accurately, is the use of convection. This method was patented by Tonelería Nacional. In the case of convection heating temperatures never get high enough for charring or blistering of the wood to occur. Much deeper toasting can therefore be achieved. With the aid of computer programmes different temperature-time combinations can be determined and barrels toasted according to a recipe with specific objectives. Barrels toasted in this way using warm air differ from conventional toasting and it is possible to meet the specific demands of winemakers with greater accuracy. It is also possible, however, to obtain thermal radiation by other means than an open fire, for example by using heating elements, a process which was developed by World Cooperage. Obviously this can also be controlled better than traditional toasting (Philips, 2007).
Each cooperage will most probably continue to believe that its own method produces the best barrels.
Reference:
Philips, C. 2007. Fire, Water and Oak. How Barrels are Toasted. Wine Business Monthly, May 2007: 22 - 33.
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