An anatomy of the double retort pot still
We reproduce, here translated from French, an article that appeared in issue 86 of Whisky Magazine, courtesy of Christine Lambert.
Photo credits: Fredi Marcarini.
With its characteristic outline, it’s the symbol of Jamaica's distillation heritage. But how does this still with two additional boilers mounted in sequence work?
The double retort still first appeared in Jamaica in the 19th century, perhaps as a variant of the distillation equipment originally devised by Frenchman Jean-Edouard Adam at the beginning of the century - man has never spared himself in his pursuit of the quintessence of the 5th element. But, unlike the evolution of rum tradition in other areas of the Caribbean like the French Antilles, at Jam Rock column stills have never replaced pot stills and continuous distillation didn't appear before the late 1950s.
The first column still was installed at Long Pond around 1958 at the instigation of the then-owners, the alcohol giant Seagram, but the technology only took hold on the island in the mid-1960s. Today, column stills are nowhere to be found in half of all Jamaica's distilleries (Hampden, Worthy Park and Long Pond) while double retort stills are still in use - probably because they're required for producing high-ester rums, something Jamaica has been specializing in since the nineteenth century.
But why has the pot still evolved so much?
“For efficiency,” replies Master Distiller Vivian Wisdom, who has worked for New Yarmouth, Clarendon and Hampden and now works at West Indies in Barbados, where a few smaller double-retort pot stills can still be seen.
“With just a single distillation, these stills can produce an 87% ABV spirit, something a pot still with no retort can only do in no less than two steps, heating twice, loading twice, etc. The spread of single pot stills allowed us to produce larger volumes more quickly.”
And volume, like time, is money, especially when a distillery’s business model relies almost entirely on wholesale.
Like triple distillation
But let's not lose sight of our main question: how does this diabolical and wonderful machine work? Before starting the distillation process, the retorts need to be loaded - small vessels (like Jamaican stills!) connected in sequence to the main still. The first retort contains low wines with an alcoholic strength (ABV) of about 30%, while the second contains high wines with an ABV of about 70% - with subtle differences depending on the distillery. What about quantities? In the first retort, low wines make up 12 to 15% of the still’s total volume, compared to 10% for high wines in the second. Again, quantities may vary, as long as enough space is left in the retorts for the steam to rise. Can you see?
Distillation can now begin following the traditional process: the wash inside the main still is heated to boiling but the steam, instead of flowing towards the condenser, escapes through a tube that extends through the first retort almost down to the bottom. The steam makes its way up into the low wines, where it dissolves and gets loaded with alcohol, going from 30% to about 60% ABV. The heat then vaporizes it again and it flows into the next retort, this time bubbling in the high wines (with a higher ABV, to compensate for the loss of heat). There, it gets loaded with alcohol again, returns to a gaseous state and reaches the condenser, which produces liquid spirit.
The distillation heads are only a small amount and are quickly discarded, while the heart – that is, the rum - flows from an ABV of 89-90% down to 85%, which is where most distilleries make the cut. The heart is followed by high and low wines - most the liquid coming out of the still - with a decreasing alcohol strength. These are sorted by volume, not by ABV, each fraction going into a specific tank. The result is similar to a triple distillation, with each retort acting a bit like a small still, but all in a single step. This process is cheaper, less time- and labor-intensive as well as less accurate than traditional double distillation, as some distillers quietly admit.
Kevin Barnett, plant manager at Long Pond, denies this: “What we are trying to collect comes out of the still at 85% ABV, which means our method is effective.” In particular, what they look for is ethyl acetate, an ester responsible for the fruity notes in rum which is found in the highest concentrations at the beginning of distillation.
The shape, size (2000 to 2500 l on average) and load of the stills, as well as the distillation speed (a standard 4 to 5 h) can vary and cause more or less reflux - considering that some distilleries like Worthy Park also have a disengageable rectifier. Also, in some Blair stills the tube connecting the three vessels doesn't run into the retorts from above but connects in an S-shape from below. Overall, however, the process is not very different from one distillery to another.
“This goes to shows that in Jamaica it's fermentation that creates rum as opposed to distillation, where we all do pretty much the same thing,” sums up Vivian Wisdom. But “pretty much” makes all the difference.
The production of high-ester and dunder-style rum rapidly degrading the stills and causing retorts to literally fall apart is a sure sign of an increased localized acidification, which occurs during distillation. And depending on whether it's the first or second retort, the know-how is different. But that's another story.