The Basics of Wood Maturing

It's no secret that beer used to be matured in wooden barrels, or even that wood contributed to the flavour of the finished product. There's a lot of history that I'm going to skip because you can find it elsewhere and I assume you're looking for information about wood and beer because you already know the history. There's also a lot of technical information that I'm not going to bother with because it's largely stuff you can't control unless you're in a position to be making demands of cooperages. 

Let's start with the three main effects you're looking for out of wood. These are:

  • Additive - the wood contributes flavour (also whatever was in the barrel previously)
  • Transformative - the wood takes a chemical from your beer and turns it into something else
  • Subtractive - the wood removes a chemical from your beer

Subtractive is the easiest. If your barrel has been charred on the inside, like most barrels, there is a layer of active carbon. This will remove sulphur compounds, such as H2S and S02 from your beer. It works essentially the same way carbon filters for tap water work. Theoretically, your beer shouldn't have a lot of sulphur in it, but it's good to keep in mind, especially for yeasts that tend to put off a bunch of it. Note that not all barrels and few chips/staves/cubes will have char. 

Additive is where the level of toast comes into play. Chemical compounds which arise from heat applied to the wood leech into the beer. In general, the less toasting, the more wood flavour - but different compounds come about at different temperatures. In brief:

  • Light toast - vanilla, coconut, caramel, clove cinnamon
  • Medium toast - vanilla, honey, caramel, toast, coffee, cocoa
  • Heavy toast - vanilla, espresso, smoke, creme brulee, butterscotch, toffee, molasses

Most brewers won't have a say beyond these three levels of toast and the origin. Speaking again in general terms, American oak will give more vanilla, coconut and honey, while French will give more clove and cinnamon with Hungarian oak being a bit of a mix between the two. 

Lastly is transformative. This is where higher alcohols are esterified. To explain this, I need to explain what an ester is and where it comes from. During yeast growth, carbon is diverted from ethanol (alcohol) to making lipids and sterols in order to build cell mass and drive reproduction. The step from acetaldehyde to ethanol, however, is a balancing reaction and the cell needs to do that balancing somewhere else. Without going into too much detail, higher alcohols are the result. These can give the beer a harsh, biting flavour and aroma. Depending on the level of oxygen, some of these can be taken care of in the fermenter by joining with acyls to become esters, though the amount and type is strain dependent. With wood, aldehydes are reduced to acyls and then joined with higher alcohols in a micro-oxygenation environment.  

Now that you know about the effects of wood, we're ready to plan. Rather than simply dumping oak chips into the fermenter, start by asking yourself a few questions. 

  • What flavours do I want to remove?
  • What flavours do I want to gain from the wood? 
  • Will this beer stand up to ageing or does it need to be done quickly? 

If there are no sulphur flavours or aromas, the char layer isn't needed. It may add a bit of colour, but that's it. If there are sulphur flavours that you want to reduce or remove, adding uncharred wood will reduce them, but much slower than charred wood. 

Once we start discussing flavours, you need to ask yourself a few other questions.

  • Is the wood old or new?
  • What was in the barrel previously? 
  • What level of toast? 
  • Where does the wood come from? 

Old wood will contribute less flavour than new wood, though it's previous contents will contribute much more than the wood itself, regardless of age. If you're using chips/cubes/staves, it's likely that the wood is new unless you can see a stain from previous contents on the inner surface. 

Lastly, whether or not the beer will stand up to ageing depends largely on its alcohol content. Barrel aged beers are usually high alcohol and heavy, but that's not the only beer to be wood aged. Up until recently, beers as light as Budweiser were wood aged (albeit quickly) over beech wood and Augusteiner's Edelstoff is a barrel aged Helles. My most requested beer was a Vienna Lager aged in an oak cask and it was sublime. 

Many of the finer points of wood ageing simply need to be done through experimentation. There's nothing to say you can't fill a growler, add some wood and see what happens in a day. Or a week. Try toasting the wood in the oven at different temperatures (not chips unless you want a house fire). Try charring the outside a little. Sometimes wood will remove compounds that were contributing to harshness you didn't know was there. Although oak is traditional, try different types of wood. Chestnut, beech, and alder have all been used and contribute different flavours, but there's nothing that says pine or aspen can't be used as cubes or chips (they would leak as barrels) if you're looking for a unique resiny flavour.