Rot your timbers

We see, and use, the term ‘traditional’ extensively in beer descriptions, typically with reference to ingredients and occasionally to processes and values. To be truly traditional is, of course, an impossible achievement – we can’t reverse time. Also, because we don’t have the same equipment and, being considerate to our staff, because past working conditions would seriously infringe today’s employment and safety regulations.

What though could be included to more authenticate the term than just using heritage malt and hops? If we go back to the golden age of brewing – arguably from 1860 to 1880 (or possibly 1914) – we would be running multiple mashes, boiling wort with coal and delivering casks by dray horse. Go back further and bottles would be leather, pipework raw lead and recipes including condiments to enhance intoxication.

One feature, however, which contemporary brewing has successfully revived from history is the use of wooden casks. Cask-aged ale has an enhanced marketing image and a higher premium return. True, it requires wooden casks and extended storage but certainly has an impact on beer flavour. All these lend towards single variety production and enhanced demand. What though is the chemistry behind brewing in wood and its future potential?

Thankfully the chemistry of wood impact on beverages is well known due to investigations by the spirit and wine industries, whisky and brandy particularly. These show the flavour contributions from wood when degraded by charring or toasting during cask construction. Hemicellulose and lignin polymers degrade to smaller, flavour active compounds with vanillin, aldehydes and phenols commonly extracted. Colour can increase if oxidation occurs. Charring of the wood also removes potential contaminants leaving a layer of charcoal which can adsorb undesirable flavours from a spirit. Finally, some oxidation of compounds is likely as air permeates into the cask.

All this takes time, at least three years in the case of whisky but often 10 or more. But what of beer which can’t afford such a maturation?

Studies with beer show that wood cask-ageing does not deteriorate beer quality but can change flavour through the release of the compounds mentioned previously. Vanillin is particularly important providing sweet, spicy and candy aromas with a low flavour threshold of 200 µg per litre. Phenol extraction can provide smoky, phenolic and burnt character while eugenol with a low threshold of 6 µg per litre contributes clove and balsamic aromas.

All these flavours will complicate the intrinsic flavours of your beer and it is advised to manage cask maturation to prevent them dominating and distracting excessively. To do this look at the effects of alcohol, time, temperature and the degree and type of wood contact. Each of these will enhance the rate of extraction so high alcohol beers (say over 7% abv) kept for 12 months in oak casks at room temperature will develop a more pungent and unique character compared to a bottled version. Change these factors and the intensity can be controlled. Casks themselves aren’t the only means of wood aging but are certainly the most expensive and difficult to manage. Adding wood chips to casks is a common alternative allowing large surface areas to contact beer and accelerate flavour extraction. Studies show similar effects to wooden casks under identical conditions although permeation of air and oxygenation will be absent. Chips may also reduce cask to cask variation which can be up to 50% in flavour levels.

Incubation time has an effect with different phases identified in the movement of flavour initially from the surface of wood and progressively from the interior. These depths will have different structures and progressively release different flavours. That said some flavours may rise then fall as further chemical changes occur in the beer.

One feature deserving further study is the effect of different woods. Oak is a typical choice providing extensive tannic content. Other woods are possible although not particularly used in wine or spirit maturation and, of course, some such as yew and mulga are toxic while others can induce allergies. Nevertheless, with caution, this could be a consideration and trials with beers worth initiating. The geographic origin of barrel wood is well noted to vary the flavour impact and, in effect an example of terroir. With more breweries investing in hop gardens will we see woodlands planted for future brewery use?

A final and more speculative element of wood aging is the effect of microbial contamination. This is a recognised hazard particularly from

Brettanomyces fungi but also from bacteria. While this may be a variable feature it is likely to have been a common contribution to historic stock ales resulting in the specific character of these beers highly regarded by Victorian brewers. Brettanomyces and other fungal species have been identified in the DNA analysis of historic Victorian beers by Brewlab. Brettanomyces was often present at high levels but timber saprophytes including Stereum, Hyphodontia and Trechispora were also evident.

The effects of Sterium growth are also recorded in the brewing literature and may have been more prevalent than currently known. It is unlikely that such saprophytes are hazardous beyond flavour spoilage. In contrast, some timber fungi are increasingly promoted for health benefits. The turkey tail fungus, Trametes versicolor, is one example, cultivated and sold for its bioactive components. With the potential for breweries to develop biotechnology applications wood chip inoculation could be an interesting opportunity to rot your timbers and diversify your brewing.