Have any Questions?


Jun 08, 2022 View:

proofing with obscuration

I have read some of the threads on proofing liqueurs and so forth and thought I had everything straight, but upon watching the TTB webinar (in tools/tutorials section of ttb website) on lab distillation I am confused. To give a quick rundown, they want you to fill a volumetric flask at 60 degress. Pour this into your lab still and rinse it with 50% the volume with distilled water. Proceed to distill up to 96% the original volume. Add the remaining 4% as distilled water and then calculate your proof of obscuration as the difference between this and your original reading.

Before watching this video, I would have planned to take a known quantity of distillate by volume and distilled all of it. At this point, add water to bring it back to the original volume at the original recorded temp (no real need to be 60 degrees technically...) and take the new proof. The difference between the original proof and the proof after distillation is your proof of obscuration. This means your original proof is your recorded plus the proof of obscuration.

If anyone can explain the TTB version I would appreciate it. Was my original way of thinking correct or not? I don't understand why one would stop at 96% yield in the TTB's example as you would be leaving both sugars and alcohol in the lab still. All help is appreciated.

This is for a Old Tom Gin that is being sweetened with maple syrup. As a little add on question, does anyone know what a normal range is for sugar quantity in Old Tom Gin? I took some off the shelf that show obscuration of 14 proof and one that was .5 proof. They both showed the same brix reading (but of course alcohol is throwing that way off on my refractometer reading). Rather I should say, what the upper limit is for added sugar, as I know you can get by adding none just increasing sweeter botanicals.

Thanks and cheers!


Volumetric flask is at 60 degrees, because you are measuring alcohol by volume at 60 degrees. Likely, if you do the procedure at another temperature, the ratio will be close to the same, but no guarantee, so if you don't do it at 60 degrees or with temperature compensation in the measurement, you have a small but unknown error, because the temperature expansion ratios of the liquid before and after may not be the same.

The 4% works because by the time you distill down to 4%, if you rinsed with 50% of the volume, you have distilled off almost 2/3 of the augmented volume, almost all of the alcohol will be gone, and any residual liquid captured with salts and organics will be water. By making that 4% up as distilled water, you will have a final liquid that will give you a reasonable measure of proof.


Ethanol, water and sugar all have different rates of thermal expansion. The net effect of the lab distillation is that you are replacing the volume of the sugar with water. If the before and after volumes are not done at 60°F then when corrected to 60°F the volumes would not be exactly the same. The differences are very small, but measurable.

If you distill down to 4% of the original volume you can be very sure the alcohol is all gone. Evaprating to dryness would just make the cleaning of the lab equipment more difficult.

Some cane molasses based distilleries put the fermented beer through a multistage evaporator before feeding it to the stripping column to avoid having solids in the column. In a 4 or 5 effect evaporator the water and solids stream is reduced to 25% of the original volume and contains a commercially negligible quantity of alcohol. If you go to 4% you are very safe. Note that a 4 effect evaporator is very different from a 4 plate column because there is no reflux.


If you distill down to 4% of the original volume you can be very sure the alcohol is all gone.


Jeff, sorry that I got the volumes wrong. I should have watched the video before commenting. I have now watched the video, but I'm not sure if I watched the same one as you because it spoke of measuring the volumes before and after at 20°C and not 60°F. I have not checked the difference that those temperatures would make, but I am confident they would be negligible.

I do not have the means to model this distillation with the sugar present, but it is easy to do if we regard the spirit as water and ethanol only. Assuming 100 ml of 80 proof to start with and then using 50 ml of pure water to rinse and distilling 96 ml over into the collection flask gives me a residual alcohol in the boiling flask of 0.2 proof. If I was able to take the sugar into account it would result in a lower proof left in the boiling flask, so I guess this method gives sufficient accuracy. It would be easy to check in practice by running the pure water/ethanol distillation I have modeled in parallel with a sample that does contain dissolved solids. If you have no solids to start with then the proof of the heel can be checked with the usual methods.


In the TTB webinar, they take 100ml sample, add 50 ml water, and then collect 96ml, so there is way more than a 4% left on the still. I imagine that the amount that is alcohol is quite low, but I would think it would be outside the TTB bottling tolerance. Perhaps I heard something wrong and should watch it again...