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A Whisky Beginner’s Guide to Sherry Casks

What is Sherry?

Like Scotch for whiskies produced in Scotland, Sherry refers to aged wines produced in the wine-growing region of Jerez, Spain. It is worth noting that not all the wines produced in this region can be labelled as Sherry. The production has to follow the specific processes set by Consejo Regulador (Regulatory Council). In addition, with the latest regulatory update, Sherry no longer has to be fortified (adding a distilled spirit into the wine). However, it still has to contain a minimum alcohol strength of 15 degrees.


When you dine at a restaurant and see labels such as Jerez (the Spanish word for Sherry) and Xérès (the French equivalent), rest assured that they all refer to Sherry. Just a friendly tip: don’t ask for “a glass of Sherry.” That’s where I got it wrong! Sherry is, in fact, a collective term for 8 different wine styles: Fino, Manzanilla, Amontillado, Palo Cortado, Oloroso, Pedro Ximénez, Moscatel, and Cream (which can be further categorized into Medium, Pale Cream, and Cream Sherry). These styles mainly differ in: (a) production location (Fino vs Manzanilla), (b) grape varieties (traditionally Palomino Fino, Pedro Ximénez, or Moscatel. The new regulation has included six more varieties), and (c) aging process (biological aging and/or oxidative aging). Of all the Sherry wine styles, six are most relevant to whisky. Therefore, I’ve made a cheat sheet for the characteristics of these six Sherry wine styles.


Sherry Wines


The Real Sherry Casks

To understand why people lament the quality of Sherry casks used in whisky maturation nowadays, let’s take a step back and talk about the OG. What are Sherry casks, really?

American white oak is often used to make barrels for the aging of Sherry, because it is widely available, provides good breathability, and is low in tannins. Now comes the surprise (at least to me)! Although new oak barrels might be used at the fermentation stage (a practice that has mostly been replaced by steel tanks), Sherry is actually aged in old, inactive oak barrels.


Unlike other wine makers who try to “arrange a perfect marriage” between wood and fermented grape liquid, Sherry makers want the oak barrels without the wood impact. Why is that so? It is said that wood tannins tend to inhibit flor during the aging process. Also, Sherry makers simply don’t want wood flavors and tannins in Sherry wines. To avoid that, Sherry makers commonly use 600-liter oak barrels (three times bigger than standard American Bourbon barrels) for a lower wine-to-wood ratio.


In addition, before an oak barrel is “qualified” for the job of Sherry maturation, it has to be first used to ferment grape musts or to age young fortified wines for a minimum of 3 years by law (but often much longer in practice). That means that by the time an oak barrel is used to age Sherry, the impact of wood has more or less been exhausted. In fact, Sherry doesn’t get flavors from oak barrels. Instead, it gets flavors mainly through oxidization. It might also soak up some aromas from the young fortified wines previously aged in these oak barrels.


Sherry Casks in Whisky Maturation

Now you know the specific requirements of oak barrels for Sherry maturation. You might think, like Bourbon casks, the Sherry casks used in whisky maturation are the oak barrels retired from Sherry maturation, right? Nope! Not even in the good old days! This was another revelation to me.


In the good old days, the Sherry casks used in whisky maturation were oak barrels used to transport aged Sherry from Spain to UK. Originally made from European oak, these transport casks had a capacity of 500 liters. Similar to the oak barrels for Sherry maturation, these transport casks were first used for fermentation or short periods of maturation, so as to reduce wood impact during transportation. Then, they were filled with aged Sherry and shipped to UK. Until the aged Sherry was bottled, it could stay in these transport casks for up to several months. During this process, a good amount of aged Sherry would seep into the wood pores. As you may guess, it didn’t make economic sense to ship these empty transport casks back to Spain. So, they were sold to the whisky industry for re-use.


Sadly, in 1986, the Spanish introduced a law dictating that all Sherry wines shall be bottled in Spain, effectively putting an end to transportation and transport casks. I should mention that, before this law, there were already practices to make barrels that mimic the effects of those Sherry-seasoned transport casks. But this law has, unintentionally, turned these practices into large-scale business operations involving three parties. Whisky distilleries specify their cask requirements – the wood type, the toasting level, the type of Sherry used for seasoning, and etc. Spanish cooperages produce new oak casks accordingly. Once they’re done, the new oak casks are sent to Spanish bodegas for Sherry seasoning.


The Certified “Sherry Cask” Guarantee Label

In 2015, Consejo Regulador registered the “Sherry Cask” brand and drafted a document to regulate the production of Sherry-seasoned casks for quality control. To obtain the “Sherry Cask” guarantee label, the production of Sherry-seasoned casks should meet the following criteria:

  • The cask has to be filled to at least 85% of the total volume, with a certified Sherry wine made by bodegas registered with Consejo Regulador.

  • The cask has to continuously hold the wine at the required fill level throughout the entire seasoning process. This means bodegas can’t empty the cask and re-fill it with other wines in between.

  • The minimum seasoning period is one year.


Well, we all can read regulations. What comes now is me reading between the lines. There are three factors that will affect the quality of a Sherry cask but are not specified by Consejo Regulador:

  • Age of Sherry: Although the wine has to be a certified Sherry, there’s no regulation about the wine age. Word has it that producers typically use 2-year-old Sherry wines.

  • Re-use of Sherry: Furthermore, there’s no regulation about how many times the same wine can be re-used for seasoning! In practice, it is often re-used for several times before being discarded or being used to distill Sherry brandy or Sherry vinegar. Theoretically, a cask seasoned with a “virgin” wine can be quite different from a cask seasoned with a re-used wine, as a re-used wine contains more wood tannins.

  • Transportation: There’s no regulation about how the casks should be transported, either. Should they be transported dry? Should they be transported with Sherry? How much Sherry should these casks contain during transportation? These are, again, up to whisky distilleries.

Whisky distilleries typically have a say in these factors when making an order. Or, at least, they know what casks they are paying for. But I guess such information is too wordy for the aesthetics of their fancy product packaging; and it takes them too much effort to put it online.


An Overview and Comparison of Sherry Casks

The Real Sherry Cask for Sherry Maturation

Transport Cask for Sherry Transportation

Today’s Certified Sherry Cask for Whisky Maturation

Wood

American oak

European oak

Both, although American oak is commonly used now

Volume

600 liters

500 liters

Typically 250 liters (aka hogsheads)

Treatment before Its Intended Use

Used for least 3 years to ferment grape musts or age young fortified wines

Used to ferment grape musts or age young fortified wines for a short period of time

Seasoned with a certified Sherry wine (often 2 year old) for at least one year

Implications for Whisky Maturation

These casks are rarely used in whisky maturation. Just forget it!

Aged Sherry is absorbed into wood pores

Young Sherry is absorbed into wood pores

* The original article was first published in Malt on December 3, 2021.

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