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So you have 5 gallons of delicious corn mash fermenting in your basement and your Clawhammer moonshine still has been assembled and cleaned. You already read our article Bourbon, Whiskey, Vodka and Moonshine - How Much Yeast? and you added the correct amount of healthy yeast. Now you need to know how to tell when it's done fermenting.
There are a few variables that can impact how long fermentation will take.
The type of yeast you use will play a large role in the amount of time it takes for a mash to ferment. Turbo yeasts will finish fermenting a lot faster than a bread yeast will.
The temperature of the room the mash is fermented at will have a big impact on how long it takes to finish. A mash fermenting at 80 degrees will ferment a lot quicker than a mash fermenting at 55 degrees.
The amount of sugar is the mash will also play a big role in the amount of time needed for fermentation to finish. The more sugar in the mash the longer the fermentation will take. If you have an 8% ABV mash it is going to ferment a lot quicker than a 20% ABV mash.
Measuring Fermentation - the Easy Way
We like to call this the "set it and forget it" method. It applies if you have your mash fermenting in a carboy or a bucket with an airlock.
Make sure 18-48 hours after you add the yeast to the carboy/bucket that you see movement in the airlock. You should see the airlock bubble a few times a minute. If you see activity in the airlock it means that the yeast is working and you're good to go. Let the mash sit for 14 days. If you still see bubbles in the airlock after 14 days let it sit for another few days, or at least until you see no bubbling for at least a minute or two. Once there is no activity in the airlock, the mash is ready to run. This is a non scientific method but has been pretty reliable in terms of judging when fermentation has finished.
What if I don't see any activity in air lock after 18-24 hours?
If you don't see any activity after 18-24 hours make sure that the lid and airlock are tight. If both are tight then gently swirl the bucket/carboy- you just want to mix everything together. Check back after 12 hours if you still don't see any activity then add more yeast from a new yeast starter. After you add more yeast check back after 18-48 hours and you should hopefully see some activity. You should also try and keep the bread yeast happy -they seem to thrive between 74-78 degrees.
Monitoring Fermentation - the Scientific Way
The most accurate way to monitor the fermentation process when making moonshine is to use a hydrometer. Taking specific gravity measurements with a hydrometer not only lets you know that it has completely finished fermentation, it also allows you to determine the potential alcohol of your mash and the actual alcohol content of the wash. These numbers can be used for all sorts of things and we discuss all of this in our article on monitoring fermentation (the scientific way).