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Has anyone ever measured how the conductivity of fermenting liquids changes over time?

I have a hypothesis that it does change over time, and that as the fermentation rates and stages change, so will the capacitance of the wort. Also, that one will see more capacitance change during early fermentation, but it will settle down to a steady value as fermentation dies down, flocculation occurs, etc.

Why do I ask? I would like to digitally measure fermentation activity without directly sampling to measure specific gravity. Measuring SG would be great, but what I'm really interested in is activity level of fermentation. I'm not trying to calculate SG - I'll do that when I transfer the batch from primary -> secondary -> bottling. I'm thinking though that seeing that conductivity value level out is what's the signal for when to transfer.

Thoughts?

I've found online that seem to say this might be worth pursuing a bit. For example here is a paper titled, "Electrical conductivity as a tool for analysing fermentation processes for production of cheese starters" http://www.sciencedirect.com/science/article/pii/S0958694600000601

Another article at http://www.springerlink.com/content/9m42027266477mj2/ mentions, "We describe the interest of on-line monitoring electrical conductivity and his good correlation with pH during fermentation. The change in conductivity is primarily due to the assimilation of nitrogen (in particular, ammoniacal nitrogen), which is usually the limiting nutrient of the fermentation kinetics."

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Excellent question. If you have an Arduino (or similar), I think some experimentation is in order. If you do an experiment, let us know what you find out! –  Dustin Rasener May 23 '12 at 19:25
    
In fact, I just got one a few days ago with this in mind! Also probably have it do some air lock bubble counting as well as temperature monitoring. –  Tim Holt May 23 '12 at 19:28

1 Answer 1

I'm also researching the same thing, looking for traits in the wort that indicate how fermentation is progressing and when it's complete.

I don't have any figures, but I would be surprised if the electrical conductivity changes much during fermentation. Conductivity is mainly a result of ions in the wort, which come from the water, brewing salts we add to the mash, and the grist. The yeast extract nutrients from the wort including some of the ions present, which I would guess reduces conductivity to some degree, but I imagine the effect is very slight.

I would think a more profound change is permittivity, since there is considerably less substance at the end of fermentation that there is at the start. If the wort is used as the dielectric in a capacitor, then we can measure the capacitance which is linearly related to permittivity. The wort can be made into a capacitor by inserting two insulated metal plates - they have to be insulated since the wort is conductive.

You can measure capacitance by using a RC circuit. When applying a steady voltage V across the capacitor and resistor, the capacitor charges to 63 percent of the voltage V in RC time. We can continually measure the voltage, and when it reaches 63 percent of V we know RC. Since we also know R, the resistor value, we can deterimine C, the capacitance.

I'm building a capacitance meter for use with measuring keg volumes in my kegbot, using the arduino board. It would be interesting to try this also on fermenting beer.

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Would passing current through the wort affect fermentation? –  FrustratedWithFormsDesigner May 23 '12 at 20:56
    
Nice mdma. If you get around to trying this out before I do, let me know. I have a couple of new batches I'll be doing soon and hope I can setup something to test. I'm really trying to not fall into the "measure SG" trap - I just want something that changes nicely over time and then levels out when fermentation has completed. –  Tim Holt May 23 '12 at 20:56
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When measuring conductance, current is passed. To avoid any electrolysis affects, the current should be low voltage AC. I have some conductance triggered relays, which use 8v ac. When measuring capacitance, no current flows through the wort. –  mdma May 23 '12 at 21:06

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