I often see questions online from people wondering whether their hard cider is done fermenting. I have written several post about elements of fermentation and I am certain I will write more. But, I’ve never specifically talked about the point when fermentation is complete. Let’s first cover some terms that often get used when talking about fermentation and when it finishes.
- Specific Gravity: This is a measurement of the dissolved solids in the juice by measuring the density of the liquid and comparing it to a reference density, which is water. For hard cider makers, this represents the amount of fermentable sugar in the juice. This can also be defined in Brix and other units of measurement depending on what is standard for your country or region.
- Dry: When a wine or hard cider is dry, it means that all the fermentable sugars have been converted to ethanol.
- Non-Fermentable Sugar: Your juice can naturally have sugars in it that won’t ferment with common yeast strains. It can also produce some of these sugars during the fermentation process. These are usually sugar alcohols like sorbitol, which is more prevalent in pears than apples, or erythritol, but others can also be present. These are usually very small or trace amounts.
- Stuck Fermentation: If your hard cider stops fermenting and still has fermentable sugars, it is considered a stuck fermentation.
Cider has three main sugars: fructose, sucrose, and glucose. These are all relatively easy for the yeast most commonly used in wine and beer to ferment. There can be small amounts of other sugars but unlike beer, hard cider doesn’t contain complex malt sugars. The attenuation numbers found on beer yeast are a reference to its ability to ferment these complex malt sugars. For hard cider, most yeast being used will take a hard cider to dry. If that doesn’t happen, it is most likely because of a stuck fermentation. Either the juice ran out of key nutrients like nitrogen or the alcohol tolerance of the yeast was reached causing it to stop fermenting.
Running out of nutrients is most likely to occur when you have chaptalized your juice by adding large amounts of sugar to it. In effect, this creates an apple wine versus a hard cider. But it also means you are most likely to have residual sugars. Let’s assume you didn’t add a lot of sugar or at least kept the addition to a small amount so your level of specific gravity is in the 1.040-1.065 range. So, when is your hard cider done fermenting?
As I am prone to say, it depends. As your cider ferments and creates ethanol alcohol, you should be aware that the density of ethanol is 0.79 versus water, which is 1.00. As your ethanol level increases, the density of your cider drops. Expecting a density of 1.000 to be the completion of your cider, may be a bad assumption. You might have up to 0.004 points of fermentable sugar hiding in your hard cider. A 6% ABV hard cider, should have a specific gravity of 0.987 if it was pure. However, these are all estimates with some margin of error. Remember, you might have some non-fermentable sugars and there are also some other solids suspended in your cider. This is where time is your friend. Letting a hard cider sit at room temperature for several weeks and seeing a stable specific gravity in the 0.996-1.001 range is a good indicator that your hard cider has completed fermentation. I usually recommend this as part of a secondary aging process.
The reason why this is important is carbon dioxide (CO2). The volume of CO2 produced during fermentation is huge relative to the volume of juice. You can produce over 30-40 times the volume of the juice you are fermenting. Each 0.001 point of specific gravity will produce over 0.6 volumes CO2. That means a 1 liter or 1 gallon container with 0.001 point of specific gravity will create 0.6 liters or 0.6 gallons of CO2. You can do the math but a few points of specific gravity will quickly turn into a large volume of CO2 or carbonation. Understanding how much fermentable sugar is in your hard cider is important if you are bottling and not stabilizing the yeast through pasteurization or another process like filtering.
Ultimately, the answer to when a hard cider is done fermenting is when it’s done. However, that should be when your specific gravity has stabilized at room temperature. This could take days, weeks, or months. Normally, that would be when your specific gravity is somewhere between 0.996 – 1.001.
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