The Surface Tension of Rum.

This is part of my year long commitment to building a physical laboratory to advance the new distilling scene. There is lots of stuff to tackle. We need an analytical balance that can measure 0.1 mg. An automated titrator to tackle Δ delta acidity (among other measures). A serious micro distilling column to tackle 8 fraction analysis, etc. I am also recreating Seagram’s botanical assay lab for essential oil yield calculation to scale botanical charges. If you have any leads on corporate partnerships for equipment, please send them my way.

My first introduction to the idea that the surface tension of a distillate could tell you anything meaningful was in Arroyo’s Studies on Rum. He uses a drop weight method (and here is a better version with math!) using a calibrated burrette and creates a proprietary measure instead of dynes/cm. He doesn’t dwell on this measure much, but runs a bunch of competitor analysis through it while surveying all the rums that are on the market.

Arroyo likely got the idea from either Dr. Brauer or his clique who were all reading each other. Brauer points us to Traube’s Rule and we see what it is all about.

Traube’s Rule is a relationship between hydrocarbon chain length and surfactant activity. It states that for every extra CH2 group in a surfactant molecule, the surface activity approximately triples.

So, shorter chain length molecules have less surface activity than higher chain length molecules. In terms of cleansing or emulsification that means you can get away with using a much lower concentration of a longer chain length surfactant.

It is mostly used in the realm of cosmetic and detergent science. All the long chain funky stuff in rum should/could measurably change the surface tension. Relatively low surface tension measures could be like seeing spikes in GCMS data.

This is sounding promising, but is it useful or actionable? Congeners have powerful influence on the perception of spirits, but they are present in very minute quantities relative to ethanol and water. Certain types of GCMS analysis even requires extraction of the congeners from the ethanol/water matrix with pentanes. So how promising is surface tension for the distiller?

If a distillate had a little more long chain stuff than another, could the instrument tell you? Or could it just tell you the difference between light rum and heavy rum? Would any inaccuracy in the proofing of the spirits you are comparing throw off anything meaningful in the surface tension you are trying to attribute to long chain fatty acids and their esters?

Could the technique also help to compare distillates like gin? I just put a CSC-DuNouy style tensiometer with platinum ring in the mail. If it fails, the instrument is cool enough for my book shelf.

There is tons of help on the internet for operating and understanding tensiometers, but not for predicting whether they will be useful in our context. There seems to be no literature on them for distillers besides that of Brauer and Arroyo, possibly because of expense (new units are $4k and they didn’t have the eBay salvage market back then) and because the instruments were designed alongside spectroscopy and chromatography which supplanted them for big budget research projects. They may still be relevant to the scrappy clever distiller.

There are three major methods for what could be useful to a distiller:

Arroyo used a drop weight method likely because of what he read and what he could do on the fly with common equipment. He calibrated a burrette to drop a set volume of water over a duration of time and compared how long it would take for distillates of a standardized proof to do the same.

Others use a capillary tube method where the glass tubes suck up liquid to a certain height. This can be done with precision which involves Japanese capillary tubes ($600) with known internal radii measured by lasers. Or you can go MacGyver and buy ebay bundles of glass capillary tubes and create your own proprietary measures using sharpies and a ruler to measure the differences. Some very clever people use what is called a travelling microscope to measure all the dimensions and add math to it.

I opted for the drop ring method which seems accurate and has lots of surplus equipment because they used to be used by the Department Of The Interior for soil science. However, they are usually sold without their precision platinum ring which if not in perfect condition can mess with your measurement. I paid $170 shipped for the instrument and $130 for a ring I think is in usable condition.

There are more advanced modern methods for measuring surface tension, but they get startlingly advanced.

A little more Traube theory.
Surface tension theory and math.
Capillary rise method math.
Calibrate CSC ring tensiometer. The swatch of paper is so create a platform and you do not scratch the platinum ring.
CSC Accudyne data tables.
Surface tension ethanol water matrixes.
Surface tension was important for augmenting head retention in beer. It would also be important for stabilizing foam in culinary applications.
Surface tension by stalagmometer. The lab in this video is wild and it makes you think of the great lost rums of the world.

This paper is extremely cool and someone should probably try and apply this apparatus to spirits.

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