No doubt about it: Distillation equipment can be beautiful. Glistening stainless and polished, hand-hammered copper—or perhaps covered in a patina to indicate age and experience—can offer a striking, aesthetically pleasing visual experience.
It’s not all about looks, though. The materials that go into still construction are well-suited for the job and serve particular purposes.
Stainless steel is a modern workhorse. Invented just over a century ago, its usage has ballooned through a variety of industries, including distilling. Distilleries use it for tanks, mash cookers, pot and column stills, condensers, fittings, and all variety of other parts.
Stainless steel’s resistance to corrosion from ethanol and acidity make it an ideal material for the distillery, in comparison to metals such as aluminum, brass, zinc, and iron, which react with ethanol, acidity, or both. Stainless can also be cleaned with strong chemicals, such as caustic soda (sodium hydroxide).
Copper, on the other hand, is one of the oldest metals known to be used by humans—the Copper Age, after all, happened more than 5,000 years ago, between the Stone and Bronze ages.
Copper is soft and reactive, a far cry from the strength and durability of stainless. Before stainless steel, copper was the metal of choice for still manufacturing. Its malleability makes it easy to shape and work with, and copper offers excellent heat transference. For pre-modern stills that relied on direct-fire heating—a method that a small number of craft distillers still use—it meant a more efficient heating of the distilling media.
Stainless steel offers lower thermal conductivity than copper. But when using a modern still with an external layer of insulation and a steam jacket in the interior wall of the still, that’s less of a concern.
However, copper is still a valuable—in many cases, essential—part of still manufacturing. This is due to the ways in which it reacts with distillate and helps to improve the spirit that results. For all its durability, stainless steel is merely a conduit allowing safe passage for mash, wash, vapor, and distillate on its transformative journey.
Cleanup Crew
Copper, on the other hand, improves the spirit in a couple of ways. The most commonly discussed avenue is through bonding with sulfur compounds. There are a variety of potent sulfur-based chemical compounds that can form during fermentation, from the rotten egg of hydrogen sulfide to the lit-match aroma of sulfur dioxide or the canned/cooked vegetable note of dimethyl sulfide. Some craft brewers even pursue a certain group of compounds known as mercaptans—responsible for strong cabbage or skunky aromas. These compounds are also known as thiols, and—because of the bright tropical-fruit notes they can also produce, among others—there are yeasts on the market that have been engineered to increase their presence.
All of these strongly aromatic chemical compounds, however, are susceptible to exposure to copper, which reacts with them and binds them, removing them from the spirit. Copper essentially scrubs these off-flavors from the final spirit.
That aspect may not be as valuable for a distiller sourcing bulk neutral spirit and redistilling it to create gin, liqueurs, or other botanical spirits, but it’s very valuable when distilling fermented wash or mash, which contains much higher levels of potential off-flavors.
Copper as Catalyst
Beyond that cleanup work, copper also helps to catalyze new flavors in the spirit. Copper helps to support the formation of ester compounds, which lend primarily fruity or floral notes to the spirit.
The bond of an alcohol and an acid creates esters. The most common alcohol in spirits is, naturally, ethyl alcohol or ethanol, and the primary acid tends to be acetic acid. As a result, the most common ester formed in spirits is ethyl acetate, which lends a solventy, nail-polish-remover character to the spirit—although, at very low levels, it can be pleasant and fruity.
Fermentation processes can create a wide range of acids and alcohols, even if they exist only in trace amounts. Each alcohol-acid pairing offers its own unique flavor profile; other common ethanol-based esters include butttery ethyl lactate (ethanol and lactic acid), tropical ethyl butyrate (ethanol and the terrible-smelling butyric acid), and the apple-y ethyl hexanoate (ethanol and hexanoic acid).
An acid, likewise, can combine with any number of alcohols to create different esters. A common secondary ester created with acetic acid is isoamyl acetate; acetic acid and isoamyl alcohol combine to produce the banana-y ester that gives certain beers a strong banana flavor.
While these esters can also come about while aging in a barrel, the presence of copper helps to facilitate more rapid ester production during distillation, providing an important avenue to flavor development.
Copper also helps to catalyze the formation of ethyl carbamate—which can be a double-edged sword, depending on the design of the still. For whiskey distillers who use barley malt that contains glycosidic nitrile (GN), a precursor to the carcinogen ethyl carbamate, still design and the location of copper is an important consideration. Copper is commonly used for the helmet, but it can also be part of the pot, lyne, arm, and even condenser, so the distiller should be aware of whether they are using a GN-positive or GN-zero barley when considering how to design their still and where to use copper versus stainless.
When heated, GN produces hydrogen cyanide, an intermediary compound on the path to ethyl carbamate. Copper catalyzes hydrogen cyanide into ethyl carbamate.
Ethyl carbamate, once catalyzed, is heavier than ethanol, so it will fall back down into the still if it is created above the still or on an upward-sloping lyne arm. However, hydrogen cyanide is lighter than ethanol vapor, so if it remains uncatalyzed through any upward vapor path and encounters copper on a transverse or downward-sloping lyne arm, or in the condenser, it can result in ethyl carbamate that rolls downhill and winds up in the final spirit.
Copper Exposure
All of the binding and catalyzation reactions discussed here rely on clean, chemically available copper. Copper that has been used a long time will eventually reach a point where it can no longer react with the spirit because it has formed a surface layer of compounds left over from previous reactions.
As a result, it is important to clean and acid-wash copper periodically to ensure its efficacy in the distilling process. While caustic soda is corrosive to soft metals such as copper, a milder alkaline cleaner such as PBW is effective for removing surface soils. After an alkaline cleaning and a rinse, the distiller should build a mild acid solution using one to two ounces of powdered citric acid per gallon of hot water and running a CIP or otherwise circulating the solution over the copper.
This process will remove the outermost layer of copper, at a molecular level, that has reacted with and bound up sulfur compounds, exposing a clean surface that will be chemically active and available. Because acid cleaning removes minute amounts of copper, it will slowly erode the copper over many years, eventually leading to a need to replace the pot, helmet, or other copper parts. This should not be considered a deterrent to incorporating copper, however; it is merely a cost of doing business.
Copper and stainless both have a place in still design and operation. No other metal can match the combination of strength and resistance of stainless steel, while the presence of copper is an essential element for the production of many spirit types.