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This is part 2 of our article series on how to tell when fermentation is complete. Click here to check out How to Know When Fermentation is Finished - Part 1
Monitoring Fermentation - The Scientific Way
So you followed our recommendations on the equipment distillation guide and you purchased yourself a beer hydrometer, a hydrometer test jar, and a beer sampler. You now are looking at your new equipment with a puzzled look; as it looks like nothing you've seen before. This simple guide will go through each step of the process of using a hydrometer. We will teach you how to determine the alcohol content of your mash, as well as help you know when fermentation is finished.
The specific gravity of water is 1.000. When you take a specific gravity reading of your mash it will have a higher density compared to water because of the sugars present in mash. During fermentation these sugars will consumed by yeast causing the density and therefore specific gravity to lower. Pre-fermentation will have the high number and the number will be the lowest at the end of fermentation.
Equipment Needed to Use a Hydrometer
Hydrometer Test Jar- You can use plastic or glass for the mash- only use glass when you are distilling as the plastic could crack.
Beer Sampler- You will need a turkey baster or wine thief to take a sample of the mash for the test jar.
The Basic Process
Take a post-fermentation specific gravity when you think the mash has finished fermenting.
- If the post-fermentation gravity reading is at 1.000 or less, it is definitely done.
- If the post-fermentation gravity reading is 1.020 or higher, you may want to wait a day or two and then take another reading
- Keep taking readings (if needed) until the gravity stops dropping (which means that fermentation is complete).
The only true way to know if fermentation is finished is to take a gravity reading. A good rule of thumb is if the gravity of the wash has not changed over the course of 3 days then the mash is done fermenting. We know you are anxious to distill your mash but you don't want to run it prematurely as you lose out on on precious alcohol. After the airlock slows down and you are not getting much activity take a sample in your test jar and take a gravity reading. Once the gravity remains the same for 3 days in a row, the yeast is most likely done with fermentation. The specific gravity at the end of fermentation is called FG or Final Gravity.
For information on using a proofing hydrometer check out our article "Moonshine Alcohol Content"