“So ya shake it, and if y’all see them large bubbles burst quickly then it’s real god-damn moonshine.” said a particularly hand-wavy historian talking about the Prohibition in America in the 20s.
Becoming somewhat of an occupational hazard, I feel the need to expand upon any slightly dodgy statements regarding bubbles, of course from the comfort of a glass of a non-foaming spirit (which subtly rhymes with orca)…
So, I was watching a random youtube video about moonshine distilleries during the Prohibition (as you do) and at one point they started to shake those bottles up and down for no discernable reason. It was only later that I found out the point of it was to get the moonshine to foam in order to detect its alcohol content, and thus its authenticity.
Is that right? Well, somewhat ish, verging kind of to a degree of rather moderately sort of.
Assuming the historian isn’t aware of Newtonian fluid dynamics at low Deborah numbers (interesting story the Deborah number, it was named by the Israli professor Markus Reiner to describe the timescale of a solid exhibiting fluid patterns, so for example, glass is technically a fluid with an insanely large Deborah number. The inspiration here comes from the Bible verse ‘The mountains flowed before the Lord.’ in a song by the prophetess Deborah.. and who says there isn’t room for religion in science?!)
Anyway, this method of shaking derives from the fact that pure things do not tend to foam whereas impure substances do. Going further into the science, this is due to the ‘Marangoni’ surface tension gradients caused by the impurities which affect the fluid flow on a bubble. For a pure substance, there is no surface tension gradient (or less gradient) and so gravity drains the fluid in the bubble membrane causing it to collapse and thus popping the bubble. The presence of the surface gradient creates a flow countering gravity, so the membrane and therefore the bubble persists for much longer and so you’ll see a foam. [To read more on this, see Who burst your bubble?]
tl;dr: roughly speaking, pure -> no foam, impure -> foam
It is by pure (no pun intended) luck that the purity is somewhat of an indication of the authenticity of the whiskey and so the shake test has survived the test of time. But, due to the incredibly complex process that is foaming (one could even say there is a PhD in that :) ), the previous paragraph is a rough guide at best until the advent of a modern foaming theory taking into account of the multitudes of chemical effects of the additives within a rather self-consistent fluid formulation. So, the conclusion for now is that: bubbles in a whiskey indicates the presence of impurities (hence could be dodgy), on the other hand, the lack of bubbles only indicates that the mixture is pure but not necessarily authentic (since tap water also doesn’t foam up!).
Moral of the story:
Do say: That thing foamed, so it’s a fake.
Don’t say: That thing didn’t foam, so it’s real.