Whether the grain is open or closed depends on the size and abundance of the pores in a tree’s spring and summer wood – the ‘layers’ one sees in a tree’s cross section that are often described as ‘growth rings’. Furthermore, the thickness of these ‘rings’ and therefore the number that can be observed in a given length of radial cross-section determine if the oak should be considered as closed grained (many thinner layers closer together) or open (fewer, thicker layers, spaced apart). Open-grain oak has a higher proportion of larger pores, which are visible to the naked eye, whereas tight grain has smaller pores that often aren’t (or at least they are much harder to see).
Though oak from America is often characterised as closed grain and Europe as open, in truth, pore size and ring thickness is determined both by species and by the rate at which the tree grows, which in turn depends on various factors such as soil conditions, hours of sunlight and rainfall. This means that there can be significant variations in the pore size and grain of oak of the same species when grown in different areas.
American white oak (Quercus alba), for example, is harvested as far south as Georgia and Alabama, where a warmer, wetter climate promotes faster growth and consequently a more open grain. The furthest north is Michigan and Minnesota, where cooler, drier climates promote slower growth and a more closed grain. Between these two extremes, traditional sources of oak such as the Ozarks and Appalachian mountains have a more variable climate, which promotes a mix of open and closed grain.
Climate is, however, only one influence. How dense or spread out the trees are from one another in a forest is another significant factor, as this means more, or less, competition for water and nutrients from the soil. An inadequate water supply slows down the growth rate, though this in turn depends on the age of the tree. “A few years drought has a far greater impact on a younger tree, but little on older trees,” says Chris Morris, vice president, master distiller - whiskey innovation, Brown-Forman.
As trees grow, new sapwood forms beneath the bark, comprising active cells capable of transporting water and nutrients. Beyond the sapwood, deeper within the trunk, is heartwood, composed of dead cells. How trees program this change from living to dead cells remains a mystery, and there’s no formula to either, as this year’s sapwood doesn’t provide next year’s heartwood. A 100-year-old oak tree could contain 20 years’ worth of sapwood growth and 80 years of heartwood. The only certainty is that the percentage of heartwood continually increases, and only heartwood is used to make casks. The change from sapwood to heartwood begins when tyloses balloon out from cell walls to block water and nutrient-carrying vessels, though the size and number of tyloses varies depending on the oak species. Additionally, the level of lignin and tannin present in the sapwood increases significantly and is deposited on the ‘blockage’ making the wood more impervious. Meanwhile, tannins deter pests from devouring the wood by lending an unattractive bitterness.
Though these characteristics make heartwood vital to the strength and longevity of a tree, these changes also make it very attractive to whisky makers. Lignin is a pre-cursor and major source of vanillin, which gives vanilla and crème caramel notes to the maturing whisky.
Tannins are reputed to add ‘structure’ to whisky. The levels and ranges of tannins and other aromatic compounds present in oak have historically been tied to particular species. For example, Quercus alba is often described as relatively low in tannins, though this can be modified by the climate. Meanwhile, in Europe, Quercus robur (known as ‘English’ or pedunculate’ oak) is generally described as high in tannins, while Quercus petraea (sessile oak) is often characterised as sitting somewhere in between, but with higher levels of other aromatic compounds.
A recent study has shown that these classifications aren’t concrete, but that it is possible to identify an oak’s species with a decent degree of accuracy by testing for certain compounds which are more abundant in some species than in others. Another factor impacting tannin levels is sunlight, with abundant sunny days significantly energising trees. In some years, this energy is channelled into producing numerous acorns and results in lower tannin levels, while other years can see no acorns and higher tannin levels.
Typically, slower growth results in softer tannins, while faster growth promotes more assertive tannins. Neither is better or worse for whiskey makers – it depends on the influence most suitable for the house style and how the casks will be used. Higher tannin levels entail a risk of distinct dryness, but this can be balanced if there is sufficient sweetness in the distillery’s spirit.
So, there’s no easy guide to tannins or the range and level of other flavour compounds present in oak, which can differ among trees from the same species, the same forest, and even from different parts of the same tree.
Whatever the line-up, flavour compounds are ‘activated’ by toasting the oak. Different toasting ‘levels’ or intensities are often described as light, medium and heavy, which alludes to specific temperatures the oak is subjected to in a particular time-frame. Hypothetically, flavours are similar in open and closed-grain oak of the same species, but how these flavours present in maturing spirit will vary depending on how it’s toasted. Brown-Forman, for example, has 10 minutes toasting time and 36 seconds of charring, no matter where the wood comes from. And this is responsible for a significant variable: heat penetrates further into open grain, which means the same amount of heat toasts and chars open grain more deeply than closed grain. According to Chris Morris, open grain gives greater levels of vanillin and heavily caramelised notes compared to closed grain. However, comparing the two raises additional issues.
“Closed grain concentrates and increases the flavour-giving potential of the cask. However, with open grain there is a greater speed and greater depth of penetration than with closed grain, which means flavour extraction is more intense,” says Dr Bill Lumsden, Glenmorangie’s director of distilling, whisky creation and whisky stocks.
Individual time tables also include varying influences. Open grain makes a significant impact within the first four months and then the rate of extraction slows down, according to Alexandre Sakon, founder of ASC Barrels. “Closed grain impacts after four to eight months and then increases progressively, but it delivers a broader palette of flavours than open grain, and smaller pores also maximise oxidation which adds fruit notes, for example,” explains Sakon.
Confusing matters further, hypothetically, a barrel could consist entirely of closed grain or entirely of open grain, but in the majority of barrels a mix of both is most likely as both types will be arriving at cooperages due to local and regional climatic variations of the woodlands supplying sawmills. “Specifying staves with open or closed grain is a time-consuming proposition and very expensive. When making an everyday whisky, it’s not practical to do this,” says Chris Morris. This means one barrel can contain both open and closed-grain oak.
In the past two to three years, Sakon has had more distillers asking about the grain, though he says around 70 per cent of enquiries still focus on the species of the oak. A prime example of someone who stipulates everything is Dr Bill Lumsden, who determines toasting and charring levels, and whether the grain is open or closed for his ‘designer casks’. “Designer barrels give a smoother, creamier, sweeter and slightly fuller-bodied result,” concludes Lumsden. “The quantity of designer casks I purchase each year is increasing, and in an ideal world we would use all designer barrels, but this is not practical from the point of view of cost and supply.” Thus, the open case of oak grain’s effect on whisky is far from being closed.