[This post is part of a series of background tests that work in conjunction with the birectifier]
[Something I have found about the sulfuric acid test is that if after letting it sit for a few days it doesn’t now seem too organoleptically exciting, 5ml of water from a pipetter can wake it up. This only applies to rums that are positives for rum oil. Often you may need a few experiences to confirm your judgement.]
In my post on Wüstenfeld’s exhaustive test I also briefly described two other minor German tests for spirits, especially rum. The first is the sulfuric acid test and the second is the tasting glass evaporation test. I did not exactly nail down modern protocols for these tests, but I made a lot of progress that hopefully some readers will help me take further.
I was first introduced to the sulfuric acid test by Arroyo in Studies on Rum:
10 ml of the sample are run into a 25 ml test tube. Three to five milliliters of strong sulphuric acid are mixed with the liquid in the test tube, and is left standing overnight. the next morning the mixture is tested organoleptically for residual aroma. Total disappearance of the aroma indicates that the distillate lacked rum oil as an ingredient. A faint survival of aroma indicates a low rum oil content; and strong aroma retention indicates a good product. Sometimes the original aroma changes to a disagreeable odour, and this may be caused by the presence of sulphur compounds in large quantities in the molasses used to produce the rum.
I had procrastinated performing this test for a while because had no practical means of procuring the concentrated acid and did not want to pay way too much to have it shipped. Inspired by MacGyver, I went out on a limb and purchased an old fashioned sulfuric acid drain cleaner which Youtube University revealed (part I, part II) to be of remarkable quality and concentration. If you use it, make sure you watch both parts of the video and understand the dangers of concentrated sulfuric acid and observe all safe handling conventions (keep baking soda around to neutralize spills). This discovery of affordable, easily accessible sulfuric acid is exciting because it will also be used to power the sulfuric acid desiccator in the Seagram’s gin botanical assay procedure.
What the sulfuric acid does is rapidly oxidize all the aldehydes, fatty acids, esters, and possibly fusel oils which sort of subtracts them. What is left intact are congeners associated with rum oil which somehow have a resilience to the effect of the acid. It would probably be smart to write out all the reactions, but to be honest, I do not understand chemistry well enough. Any volunteers?
For the test, I selected 11 rums and a vodka to see what happened. There were a few rums selected that I thought would definitely be positives so I would get the spectrum of experiences. Everything was prepared in 50 ml Erlenmeyer flasks which were a good choice. I used 10 ml of rum with 5 ml sulfuric acid. I do not want to name names yet until I perform this a few more times and better understand what I’m doing.
The results were no miracle. I feel I did find some affirmatives such as the 24 year Jamaican rum (a gimme) while an overwhelming amount of the samples had varying levels of brimstone aroma possibly cause by excessive quantities of sulfur compounds in the molasses used to produced the rum. [What I have found is that the test benefits from sitting around for many days if you can spare the glassware. A sample of Hine Cognac improved greatly and revealed its Cognac oil aroma after several days.]
Roast me in sulphur!
Wash me in steep-down gulfs of liquid ﬁre!
—William Shakespeare, 1564-1616
Before we speculate too wildly, Tom Mansell’s article, Brimstone in the Bottle: Sulfur Compounds in Wine, should be a must read.
We could implicate non-ideal molasses or we could implicate poor yeast nutrient use or we could implicate aroma-beneficial aroma compounds with low thresholds of perception altered by their sulfur bath and oxidized to gnarly brimstone. Another great article I stole the Shakespeare quote from is High Impact Aroma Chemicals Part III Fire and brimstone: sulfur aroma chemicals.
If you want to jump into molasses-isn’t-the-same-anymore conspiracies, it is worth Ctrl-F-ing for sulfur through Olbrich’s Molasses text. The most interesting stuff is on page 64 of the PDF.
To resolve what we can on the molasses-isn’t-the-same front, we should be collectively trying this 10 ml test on vintage rums we come across and share reliable observations. Possibly also on other beverages that also likely share minor sulfur derived aroma compounds (cough, cough, cognac).
To evolve the protocol in the future, I would explore doing an array of one strong positive and cutting the sulfuric acid such as 10 ml of rum and 3,3½,4,½, and 5 ml of acid. This would give an understanding of what varying the acid did, if anything.
Next I would scale it up such as 20 and 30 ml of rum to see if it made anything more salient. Arroyo said 10ml and he no doubt got his protocol from Dr. Kurt Brauer (1922):
The examination by concentrated Sulfuric acid by adding 4 cc concentrated Sulfuric acid to 10 cc of rum showed that this German rum still showed a clear aroma after 24 h, whereby it differs from artificial products and equals the real rum.
As always, my motto is systematically try everything. Sadly, Brauer never mentions the brimstone aroma as an aspect of the test.
The next test is the tasting glass evaporation test which is sometimes known colloquially as the Cognac bedtime test. If you finish your glass of Cognac before bed, and leave it on the bedside table, when you go to clear it in the morning, the residue in the glass still smells heavenly! A noble rum should aspire to the same. How do we arrive at that conclusion in a controlled experiment? I botched this a bit and need to revisit it. For starters, I should have used a Cognac as a positive to know I was doing it correctly.
We can find some advice in the old literature:
Prinsen Geerligs from The Quality of the Java Arrak (1905):
This is in complete accordance with the fact that in the saponified arrak, where therefore the acids are neutralized and the esters are dissected, the smell has remained unadulterated and rash with the experience, which teaches, that a glass with arrak, that one stands up and from which a part of the contents evaporates, preserves its smell, which would not be the case when that smell was caused by the so volatile esters.
Dr. Luckow from For the Qualitative Assessment of Noble Brandy, in Particular Rum, Arrak and Kirschwasser (1933):
Finally, we have to report on a method of examination, which we also always apply when assessing kirschwasser. This is the so-called evaporation test. It is common practice to prove the authenticity, in particular the test for a possible addition of plum eau-de-vie by allowing the Kirschwasser residue in a tasting glass to evaporate, and then the next day odor detect the remaining flavor. We designed this method in such a way that we empty the glasses at the end of the tasting and then leave them covered overnight with a watch glass. The alcohol as well as the more volatile kirsch bouquets evaporate, while the watery constituents as well as certain volatile fumes remain and are expressed in a certain mixed odor. It emerged in our experiments that the bitter almond aroma could never be detected. Also, the actual Kirschwasser bouquet was lost mostly, while in other cases, especially in high-quality products, still a clear bouquet was expressed. Naturally, the trailing character already mentioned in the tasting was very noticeable here, often particularly striking. This can be clearly seen in the smell test of plum eau-de-vie by a plum-like aroma, while in pure Kirsch, water reminds one only of cherries, but also does not come to light at all. We agree with the view often held that the connoisseur should in any case consult the evaporation test. It can also be observed here that certain strange nuances, such as e.g. barrel smell, impure fermentation, foreign flavorings and the like, are clearer to recognize than in the original. In cases where the evaporation sample appears perfectly pure and neutral in the odor, one can almost certainly conclude that it is pure, not mixed with other fruit brandy.
I finally acquired watch glasses that match my tasting glasses. The plan is to find a standard amount of spirit to add to the glass such as 1.0 or 1.5 ml. To roll the glass massively increasing the surface area of the liquid then to leave uncovered for a set amount of time. This may be one hour or even a fraction of an hour then covering with the watch glass.
It would be great to see this test explored by bars that have vast arrays of spirits and possibly multiple kirschwassers. They could also adulterate a small sample of kirschwasser with slivovitz and explore Dr. Luckow’s methodology.
We can start speculating before we even nail this test. Sugar has fixative properties that reduce volatility and a traditionally sugared spirit, such as Cognac, may survive the evaporation test in better shape than a holier-than-thou rum that goes un-sugared. We may be able to design some experiments to explore the subtle fixation of added sugar.
This at the moment is an incomplete test, but if you have insights or a successful protocol, please comment.
3 thoughts on “Ye devils… Roast me in sulphur! The last two German tests”
I imagine the sulfuric acid is doing a few things here.
1) Esters: promoting quick hydrolysis of abundant esters to generate their parent acids and alcohols.
2) Alcohols: higher-substitution alcohols that have heavy, lasting aromas will dehydrate easily with exposure to concentrated sulfuric. For example, the pungent amyl alcohol should dehydrate to isobutene in these conditions, which likely just evaporates away.
3) Acids: it is trickier to fathom how these are suppressed since sulfuric acid is not the strongest oxidant of organic molecules by itself and the acidic medium will ensure the volatility of organic acids. However I recall a paper on the coordination of organic acids and sulfuric acid in solution, which in a concentrated solution could act to bind up native acids in a way that minimizes their volatility and perception.
4) Aldehydes: possibly promoting acid-catalyzed aldol condensation where high-volatility, low-threshold compounds like acetaldehyde lose their olfactory-oomph by gaining molecular weight and threshold of perception.
The density of free H+ ions in something like this will be really high — high enough to possibly dissuade most native oxygenated molecules from filling the headspace at all due to the positive charge instilled on them by the sulfuric. The only things that are likely to avoid this effect will be hydrocarbons, like TDN, etc…
Hi Matt. I was hoping you may comment. I was definitely in over my head. A friend explained a few potential reactions very similarly but the elusive one was the higher alcohol fraction. I wonder if understanding it may open the door to new methods of processes and recycling tales fractions for heavy rum production.
Any thoughts on the brim stone aromas? I just tested another batch of distillates (but no whiskeys yet) and found no brimstone on a Cognac, but a notable amount in a fresh cane juice rum I actually made myself many years ago. I suspect its presence cannot tell us much, but possibly may correlate to yeast nutrient use? I am looking forward to trying it on the various fractions of 8 stage Micko distillation.