A search for something to help a blog reader prompted me to take a trip back through the databases. More and more literature is digitized every year or has its copyright expired.
This paper on Whiskey Verdigris was a fun one for me because I love looks at distillation phenomena that are seldom explored. If you encountered a still puking verdigris as a first time distiller it may be a ‘wtf?, that’s not in the text books!’, but it is a phenomenon understood to be normal by experienced commercial scale distillers. My very first explanation of the phenomenon was back in 2014, A Still Operation Phenomenon Explained.
The paper is from 1937 and the experiments were conducted down in Kentucky from a sample of whiskey verdigris secured for the authors by a former University of Kentucky alum. This is all pre chromatography era so they explore and torture their sample MacGyver style to elucidate what the hell it is and how the hell it got there.
Copper is reactive and distillation is all about concentration so waves of reactive compounds move through the still. The end of a spirits run also has a unique relationship with the beginning of the next as we learned in Demisting and the Spirits Safe. Stuff at the end of the run, distilling primarily with water vapor (but not necessarily water soluble), has a tendency to be sticky. This stuff often gets stuck in the condenser, affixing to the copper, but is liberated by the next run where it is soluble in the very high alcohol content of the heads fraction. Chemical reactions happen with the copper producing a colored patina that takes the name whiskey verdigris though it is chemically different from the classic verdigris of the decorative arts (but no less beautiful!). Another strange phenomenon can also come along depending on how a distillery preps its beer (they usually try to avoid this). If the beer has not been de-gassed and liberated of its CO2, as it heats, it will have a tendency to belch. The liberated CO2 has both a corrosive action and a force that can scour the inside of the still and puke out whiskey verdigris. If you have this going on you may want to figure out how to de-gas your beer because the raw copper revealed can negatively impact your flavor.
The short paper is worth a read. These chemists were brilliant and it is fun to try and keep up with an understanding of what the hell they are doing. Among the many parts of their experiment, they are making whiskey soap and getting to smell isolated fractions that few of us will ever get to experience. Wonderful stuff.
If you have some, either send in a photo or mail me a canning jar of the stuff. I will turn it into paint and create a neo-pointalist self portrait.
As the concentration of alcohol falls in the doubler a white, insoluble, fatlike material appears in the trial box. Although most of this goes back into the singling tank, some collects in the condenser and is partially dissolved and washed out by the higher alcoholic content of the next distillation. This appears in the heads or foreshots of the next distillation and is colored a distinct green. This part of the insoluble material goes directly into the whisky well and dissolves in the strong alcohol present. Thus a part of the original volatile fatty material collects in the singling tank, and part finds its way into the whisky. The trade calls this material “verdigris” which is an unfortunate name since it has no connection with the verdigris of commerce.
The amount of this material is small in comparison to the volume of alcohol produced. Probably 250 grams per 30,000 liters of high wines would be a fair approximation, although no exact figures are available and would be very difficult to obtain.
UNSAPONIFIABLE MATTER. The ether extracts of the soap solutions upon evaporation yielded 1.4 grams of a viscous oil having somewhat the odor of corn.
The green solid when leached with hot alcohol was dissolved, leaving a brown solid. Upon filtering and cooling, the alcohol solution deposited green crystals; hence the palmitic acid is considered to be held as a cupric salt.
The higher fatty acids and their derivatives found in whisky verdigris without doubt originate mainly in the corn (3) which makes up from 60 to 89 per cent of the total grain used in making Bourbon whisky from which the sample was obtained. The corn oil alone does not offer an explanation of the presence of laurate and caprate esters, although Hilger (6) reported the free acids to be present in fusel oil. The occurrence of the various fatty acids and their derivatives in the beer is easily understood, but their presence in the distillate is more difficult to explain. Although it is known that the higher fatty acids are volatile in steam, or at least volatile in steam containing the vapors of more volatile acids, it must be remembered that this is not purely a steam distillation.
It is possible that the acids distill and cling to the copper condenser, and that partial salt formation (11) and esterification take place there. The majority of the esters are probably formed in the beer, and many other possibilities are obvious although none appears to explain satisfactorily the absence of stearic acid or its derivatives. Although this acid has been reported in fusel oil (6), the writers were unable to find any indication of its presence in whisky verdigris.
Whisky verdigris has a strong odor of green whisky and may be said to be yeasty: although none of the substances mentioned by Hochwalt and others (7) were found, their hydrogenation process may owe part of its effectiveness to the reduction of the unsaturated derivatives which otherwise become rancid.
These two photos come from rum distiller James Copeland christening a new still.
An accumulation of beautiful whiskey verdigris.
Insoluble flecks collected in a low wines receiver.
Whiskey verdigris can even end up as a precipitate in the tales fraction.
Insoluble flecks can be collected in cheese cloth suspected over the low wines receiver.
The last four photos were courtesy the wonderful Kings County Distillery which primarily produces a bourbon.
This last photo is from the Auchentoshan distillery in Scotland. Courtesy an astute reader with an eye that doesn’t miss much.
Feel free to write in and add to the photos. They can be attributed or submitted anonymously with the type of spirit distilled.