RD80 Volatile Acidity Cash Still

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I recently acquired an RD80 volatile acidity Cash still. This is a famous winery tool produced by the scientific glass house, Adams & Chittenden. It is named after Lyman Cash of Gallo Wines who came up with the first version in the 1950’s, but also evolved from earlier designs. It has a very cool history. I knew of this tool from reading the Technology of Wine Making, but was told it may have value to heavy rum fermentation by a colleague who makes a lot of brandy, processing many unique lots of wine where knowledge of VA is important.

If heavy rum is a game of volatile acidity, shouldn’t we aim to measure VA? One thing becoming apparent is that for Jamaica process ferments, pH as a measure is subordinate to titrateable acidity and volatile fatty acids may create a lot of inhibition for both bacteria and yeast. We think of these ferments as being dirty and even saturated with bacteria, but VA may keep that in check. VA may also select for a fission yeast because of budding yeast inhibition. Fixed acids, like lactic, may also somewhat be junk, taking up space, and weighing down pH at the expense of aroma beneficial enzyme activity. Much could be said, but a lot of basic experiments still have to be performed to better frame these ferments in numbers while observing possible inhibition of bacteria and yeast.

To explain the still, it is a very quirky design aimed at productivity in the lab where a winery typically has many lots to process. Results have to be accurate enough to be actionable and ensure regulatory compliance. In general, a heavy rum ferment will have five times the volatile acidity of a wine that is close to the legal limit for VA so we are definitely stretching what the tool is primarily designed for. A winery lab lines up its samples and processes then in rapid succession. A feature of the still is its jacketed double boiler design as well as aspirator that can suck out the previous sample followed by rinses before a new sample is inputted. The product is ultimately titrated for acid determination.

The Cash still holds a reservoir of water large enough for many distillation batches before it needs replenished. The reservoir is depleted by steam entering the inner analyte chamber. The system will never be under pressure and there is even a hose barb under the reservoir to drain it. Tubing on the barb is simply pinched. Water should not be refilled until the device has cooled to reduce risk of the glass cracking so it needs a rest of roughly half an hour after every few batches. For wine analysis, they use a 10 ml analyte but collect 100 ml of product while for rum I use a 10 ml analyte but collect 150 ml of product. I can process three samples before the reservoir needs refilled. The heating element should never be run dry or it will likely break quickly.

The immersion heating element is designed to plug directly into a wall outlet and be functional, but I found the still shook too much for my comfort so I slowed it down to 75% power with a voltage regulator.

The device is filled via a dual channel stop cock. One orientation fills the water reservoir while the other fills the analyte chamber. Typically, you add 10 ml of analyte together with 100 ml of distilled water plus a drop of anti-foam. For the most part, I have been able to operate without anti-foam. The analyte sees serious agitation when boiling, but a trap prevents most all foam generated from entering the arm to the condenser.

After collection, spent analyte can be sucked out via an aspirator activated by another valve. This is very clever. Once the water leaves the condenser it goes back through the aspirator and when a valve is opened, suction is created. A minor thing this means is that waste analyte eventually contaminates the down stream condenser tubing. This only matters if you are trying to collect and reuse the water. It also makes it beneficial to be close to a sink which is typically always the case.

To optimize a method, winery labs always degas their sample and I use an ultra sonic bath. I tried increasing the analyte to 15 ml, but determined I was not collecting all the VA so I reduced it. When collecting only 100 ml, I determined I was still not collecting all the VA so I increased it to 150 ml. If you needed to be super accurate you could increase this further, but I find my result actionable.

Winery labs also typically add a drop of anti-foam and 30 micro liters (3 drops) of 30% hydrogen peroxide.

Some wine lab methods have added mercuric oxide to bind sulfur. I cannot remember exactly, but hydrogen peroxide may added to keep acetaldehyde from interfering in the result. Condensing water requires much more energy than ethanol so it is recommended to blanket thin condensing hoses with ice packs to maximize cooling.

A sound new wine should show less than 0.4 g/L of VA and a sound aged wine 0.7 g/L. The legal maximum is 1.1 g/L for whites and 1.2 g/L for reds according to the Technology of Wine Making. These small number mean you need a degree of precision higher than rum ferments which as previous stated, will always be many magnitudes higher.

Something I’ve noticed from photos is that my particular unit is larger than past examples. Old rules of thumb about levels for filling the reservoir don’t necessarily translate such as an inch above the coil. I fill my reservoir slightly higher than the inner level of a 10 ml analyte + 100 ml of distilled water. At this level, I get three batches before needing to refill the reservoir.

Possibly because of older designs, old methods tell you to prove the stopcock seal on the reservoir when boiling by filling the funnel. I have tried this as well as ignored it, because the teflon stopcock are of incredible quality. The thought is any leak would draw steam from entering the inner boiler, skewing results.

When you have multiple samples and time optimization is a factor, you can pre-warm your analyte and distilled water, but I find the process is rapid enough. It often only takes me 7 minutes to collect 150 ml of output.

This is a fairly large device, for clamping I use a ring support for the main bulb plus a chain clamp on the middle section. The condenser gets its own clamp.

I would recommend operators prove their method. Simply run a batch collecting 150 ml then collect another 50 ml in another vessel and titrate that to see what is left. Assess what is an acceptable result to make decisions upon.

Videos: 1, 2, 3, 4

Best setup video.

Wine industry data

Comparative methods for VA used in the wine industry.


Muck pit salvaged acids: pH 4.15 , VA 1.32 g/L

Dunder after multiple reuses in vinegar ferments: VA 3.66 g/L TA 20+ g/L

Many ferments I thought were VA 5.0 g/L were 8.0! These ferments started, even at SG 1.100, but eventually got stuck when the ABV crossed 3.0-4.0 and the gravity was only 1.070-1.060. What I hope is to set ferments at a more accurate 5.0 g/L and see how they perform before increasing any increments. I was concerned I had a non-fermentable sugars issue, but at the moment it looks like lack of control of VA inhibitory acids.

Arroyo described a complex between alcohol, temperature, and acidity and that may be in play here. We find levels were bacteria is inhibited by the complex and then different levels for both budding and fission yeasts. Every character seems to be incredibly tolerant of lactic acid, the predominant acid, which is among the reasons why pH becomes a subordinate concept.

I need to do a little more proving of my method. I’m thinking I may collect more acid if I start with 10 ml analyte + 50 ml of distilled water instead of 100 ml. This will still encounter plenty of steam.

Something I do not yet know is if I am breaking esters that have formed and then measuring them as VA. Was everything truly in the free state in the ferment? Likely not because adding so much water no doubt breaks esters.

Another experiment I’d like to try is running a 100 ml analyte to do a spot check on a ferment and see what that looked like. This could possibly give clues about rum oil formation.

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