We have recently experienced some problems when using the standard TTB procedure for determining ABV by lab distillation of liqueurs containing above 30% solids, especially if we have milk solids present. If we start with 100 ml of sample, add 50 ml of rinsing water and then distill off the recommended 96 ml what is left in the boiling flask is so thick that it is impossible to deal with.
The actual ABV of the sample is known because we know how much neutral spirit had been added and we know the total volume (from the total mass and measured density). This corresponded very closely with the ABV calculated by AlcoDens LQ, but the lab distillation always gave low results. I suspect this was due to carry over of solids. Even commercial lab results were quite far from the known ABV.
We modified the lab distillation procedure and are now getting much more accurate and consistent results, and I would really appreciate your comments if you have experienced similar problems.
In our modified procedure we still start with a 100 ml sample but we add 200 ml of water and then distill off 196 ml (which we make up to 200 ml), leaving approximately 100 ml in the boiling flask. This means that the initial solids are still in 100 ml and remain nicely in solution. My calculations show that for a sample containing 15 ABV this procedure will recover +99.99% of the alcohol.
Since the alcohol that was initially in 100 ml is now in 200 ml the measured ABV has to be doubled. We understand that this halves the precision of the measurement but the results are so much closer to the known value and are much more consistent - and the glassware is much easier to clean afterwards!
How have you gotten around this problem?
How much lower were your results at 50mL rinse water?
@teh_pitts With a true ABV of around 13 % the lab distillations with 50 ml rinse water indicated 0.9 to 1.2 ABV % low.
Your findings do not surprise me at all. I have often measured lower than expected ABV and suspected heating the flask contents once most of the alcohol and water have gone would most likely boil off solids, which will then cause obscuration of the collected distillate. I have been tempted to re-distill the distillate to see if this was happening. Your method would be easier and probably more accurate.
I had a call yesterday from a friend who was finding the opposite of what he expected after lab distillation.
He makes absinthe.
Normally with a spirit containing sugar a hydrometer will float higher and indicate a lower ABV because of the higher density of the sugar. Lab distillation removes this sugar-solids-obscuration and the hydrometer will show a higher ABV
He measured the absinthe directly with a hydrometer and calculated the ABV
Then did a lab distillation and got a lower reading. I am sure he did the distillation and calculations correctly.
My feeling is the substances in absinthe that are causing obscuration have a lower density than ethanol. (oils?)
I believe that your friend is analyzing his absinthe correctly in accordance with the TTB guidelines, but the TTB procedure is not valid for absinthe in my opinion. If he follows my revised method outlined above he should get the true values. Let me explain my thinking.
As far as I know, absinthe contains little or no added sugar. An hydrometer reading on the original absinthe should therefore suffer very little obscuration and the apparent ABV will be very close to the true ABV. The flavorings are present in trace quantities and won't have a material effect.
But absinthe typically contains very high levels of alcohol and the TTB procedure will not transfer all the alcohol from the sample to the distillate when using the recommended quantities. It is therefore reasonable that the distillate ABV will be lower than that of the original sample - some of the alcohol is still in the boiling flask.
I ran some calculations assuming that the absinthe contains 55 %abv. If the standard 100 ml sample, 50 ml of rinse water and 96 ml of recovered distillate is used I estimate that only about 85% of the original alcohol will report in the distillate. Changing the procedure to my 100 ml sample, 200 ml added water and 196 ml distillate increases the recovery to +99.9%. But remember that the measured abv of the distillate must be doubled to get the original sample abv.
There are two reasons why the high alcohol levels cause problems. Firstly there is simply more alcohol in the sample so it is not as easy to get it out. Secondly, the volatility of alcohol relative to water decreases rapidly as the abv increases. We all know that it is much easier to double the strength from 5 abv to 10 abv than it is to go from 45 abv to 90 abv. When the alcohol is dilute it literally leaps out of the water. At high abv you have to really squeeze it. Part of my reasoning in using the higher quantity of added water is to dilute the alcohol and increase its volatility.
If you would like to send me the actual sugar and alcohol levels in the absinthe I will rerun the numbers to see how close the calculations come to the measured values.
Thanks meerkat, I have forwarded this discussion to John, not sure if he is a member but he can at least read the thread.