KEEPING A BEADY EYE ON YOUR LIQUOR
Written by Paul Hughes, Ph.D. and Grady Szuch
F
or those of us that have seen some of the moonshine TV programs, the idea of shaking new-make spirit in a Mason jar and correlating observation of the bubbles (so-called “beads”) with a good estimate of proof is unlikely to be news of the day. The concept is attractive, though. There is no requirement for anything more than a Mason
jar and an eye that isn’t too bleary! But how reliable is it? We certainly couldn’t envisage the TTB adopting it as a primary method, but nonetheless as a quick-and-dirty method it seems to have merit. At the risk of being proposed for an IgNobel award, and as part of our thinking for a $1000 QA/QC lab for distilleries, we
Fig. 1. Relative surface tension of water – alcohol mixtures at 20°C. For higher proof spirits the dependence of surface tension on %ABV becomes progressively less than at lower proofs. 1.0 0.9 0.8
Relative surface tension
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0
10
20
30
40
50
60
Alcohol by volume
118
70
80
90
100
wanted to delve a little deeper. A proposed rationale for the correlation between proof and beads is that ethanol and ethanol/water mixtures have lower surface tensions than water (Fig. 1) and that this lower surface tension reduces the stability of the air/water interface and results in an increase in the mean size of bubbles. To explore this further we should explain what is meant by surface tension, which requires a segue into a brief description of the phenomena of intermolecular interactions, particularly in the liquid state. From fundamental principles, the relative mass of a molecule influences its volatility. As an example, the simplest alkane methane, CH4 , with a molecular mass of around 16, is more volatile than ethane (C2H6 , molecular mass of 30), which boils at temperatures of more than 70 degrees C higher, at atmospheric pressure. This increase in boiling point continues as we add carbons and is a good demonstration of how molecular mass influences boiling point. However, not all molecules are so well-behaved. The forces between adjacent molecules in alkanes are weak and so there is relatively little interaction between them. This is in contrast with many other chemical entities where intermolecular W W W . ARTISANSPIRITMAG . C O M