Oak contains many flavor components. The primary compounds that may influence beer are tannins, which are a type of phenol. Although brewers generally seek to minimize the amount of tannins in their wort and beer, tannins may offer benefits to cask-aged beer. It is thought that tannins act as “body-builders,” contributing to a beer’s mouthfeel. Ellagitannin and gallotannin are known to clarify wine by reacting with proteins and precipitating them out of solution; it is likely that they have a similar effect on beer. The hydrolysis and oxidation of lignin, also found in white oak, produce vanillin syringaldehyde and other compounds in alcoholic beverages during cask aging.
The most significant non-volatile compounds in raw untreated oak include: cellulose and hemicellulose which together with lignin make up the complex, strong woody matrix of tree trunks; astringent and bitter-tasting hydrolyzable tannins; coumarins; gallic acid; and small concentrations of harsh condensed tannins.
Cellulose is a very large polymer of glucose, which gives wood its structural strength. Hemicellulose is a shorter polymer of glucose and many different sugar monomers form hydrogen bonds with cellulose. And lignin fills the spaces in the cell wall between cellulose, hemicellulose and pectin components. Cellulose undergoes relatively little change during seasoning and toasting and therefore has little impact on the final aging chemistry. But hemicellulose and lignin contents are reduced with increased levels of toasting.
Hydrolyzable tannins can be hydrolyzed, or split, into their gallic and/or ellagic acid and glucose components. Those tannins from gallic acid are known as gallotannins and those from ellagic acid as ellagitannins. The class of ellagitannins is the more significant of these two and, specifically, castalagin and vescalagin are the most important ellagitannins derived from oak wood.
Coumarins in oak wood include bitter-tasting scopoline and esculin, compounds which are found bound to sugar components but which then hydrolyze to the more neutral tasting scopoletin and esculetin compounds.
European oak is known to have higher concentrations of ellagitannins and coumarins compared to American oak. Harshness and bitterness are exacerbated by gallic acid (a phenolic acid) content, which contributes an acidic taste which enhances bitterness But hydrolyzable tannins and coumarins can impart that familiar oak sweetness because these compounds can hydrolyze and release sugar molecules.
Condensed tannins, also known as proanthocyanidins, are very large catechin and epicatechin polymers which are less astringent than hydrolyzable tannins and polymerize over long periods of time to give wine its stability.
The most significant volatile compounds include: long straight-chained and phenolic aldehydes; volatile phenols; and oak lactones. In raw oak wood, long straight-chained aldehydes include high concentrations of trans-oct-2-enal, trans-non-2-enal, and decanal, 8, 9 and 10-carbon aldehydes, which are responsible for the odor known as “plank smell.”
Phenolic aldehydes are characterized by a closed-ring chemical structure; the most significant is vanillaldehyde, commonly referred to as vanillin, which is responsible for imparting vanilla-like aromas. Phenolic aldehydes in wines affect taste, color and mouthfeel. Other less significant phenolic aldehydes include syringaldehyde, coniferaldehyde and sinapaldehyde; however, these will only play an important role in toasted oak.
Volatile phenols include compounds that are most often associated with toasted oak, but in raw wood, there is only eugenol, which is responsible for aromas of cloves, and to a lesser extent, phenol.
And then there are oak lactones, namely, methyloctalactone and its variants, which are found in higher concentrations in American oak. These are responsible for sweet, spicy, woody, fresh, leather, and coconut aromas.
There is obviously a lot that goes into barrel making and wood choice, and it’s important to note that the process of making barrels has not changed in hundreds of years. Squarrel is seeking to advance a traditional process; maintaining the strength of those customs while making them more modern and environmentally friendly.