Copper, a metal long valuable in the construction of breweries, but now largely replaced by stainless steel. Copper is malleable and can be formed into sheets, which allowed the engineering of large brewing vessels. The sheets were easily joined to produce leak-proof seals. Copper is also an excellent conductor of heat, which made it the metal of choice in the days when direct firing of kettles was common. See direct firing. In Britain, brewing kettles are still sometimes referred to as “coppers.” Copper is also, in trace amounts, a nutrient needed by yeast. Wort is mildly acidic and dissolves the small amounts of copper necessary for good yeast health. In beer fermentations, copper acts to reduce concentrations of hydrogen sulfide, a gas with an aroma reminiscent of rotten eggs.

However, copper does have its drawbacks. It is difficult to clean—much more difficult than stainless steel. Although it is malleable, it is also relatively soft and does not have steel’s structural strength. Although brewhouses are still made with copper cladding for aesthetic reasons, the interiors of the vessels are almost always stainless steel.

Humans, like yeast, require trace copper (1–2 mg per day in a healthy diet) for normal cellular function. In recent years, much attention has been focused on possible copper toxicity and its implications for brewers. Whether this is a valid concern remains highly debated. The U.S. Environmental Protection Agency limits copper in drinking water to 1.3 mg/l. Copper in high concentrations is surely toxic, both for humans and for many microbes, including yeast. Although the debate continues, it might be noted that many of the world’s most beautiful brewhouses, from the Czech Pilsner Urquell to the Belgian Trappist Rochefort, are constructed of copper, and this appears to have had no negative effects on either people or yeast.