When is Fermentation finished?
by Shea Comfort
In about two weeks most of the sugar will have been consumed by the yeast and fermentation will slow, making it easier to keep track of the falling sugar level of your wine. You want to be aware of your sugar levels because they will give you an overview of how the ferment has been progressing. You may wish to stop the fermentation early and leave a bit of residual sugar in your wine.
Note: This time frame is dependent on yeast strain selection, the starting ºBrix and fermentation temperatures. Just like people, yeast are more active when they are warm. So, if you are fermenting at 65º F the sugars will drop much sooner that if you are fermenting at 55º F. Each wine's rate of fermentation will be different so you will need to check it throughout the fermentation to monitor the progress.
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When is the Fermentation Over?
The fermentation is considered done when you either reach your desired sugar level or go "dry" at 0° Brix. A wine with 0.2% residual sugar contains two grams of sugar in a liter of wine. Dry wines are typically in the 0.2%-0.3% range, off-dry wines in the 1.0%-5.0% range, and sweet dessert wines are normally 5.0%-10%. However this can be a little subjective on taste and a wine with .5% or 5 g/L may taste completely dry depending on the wine. In the end there is no "correct" sugar level for your wine, it just comes down to your personal preference.
Creating a dry wine
Fermenting to dryness simply involves letting the yeast continue the fermentation until all of the sugars have been consumed. If a secondary malolactic fermentation (MLF) is desired, no SO2 is added and ML bacteria are added to the wine (see our Guide to Malolactic Fermentation). If no MLF is desired then the wine is immediately sulfited (with a thorough stirring) and we proceed to the ageing period (see our Guide to Tasting and Adjusting during Ageing).
Creating a wine with residual sugar
Finished wines with residual sugar can be made in one of two ways; by either fermenting to dryness then sweetening at bottling, or stopping fermentation before reaching dryness so that some residual sugar remains in the wine. The techniques for each are as follows:
Ferment to dryness, sweeten later: Prior to starting fermentation, a small percentage of the refined and sulfited must can be set aside and put in the freezer (A zip-lock type freezer bag works great for this- remember to squeeze all the air out before sealing it to limit oxidation). This reserved must will be used to sweeten the wine before bottling and is called the "sweet reserve". The rest of the wine is fermented to dryness. Whenever the wine is to be bottled, the sweet reserve is taken out and added to the dry wine until the desired level of residual sugar has been achieved. A bench trial will help determine the ideal ratios to add (see our Guide to Bench Trials). The wine is then filtered and bottled (see our Guides to Filtering and Bottling).
Note: household sugar can also be used to sweeten the wine, but depending on the amount used the flavor in the final wine will not be as rich as if you used the original juice.
Stopping fermentation before dryness: Once the desired sugar level has been reached the wine is sulfited in the fermenter (with a final stir to distribute the SO2) and immediately chilled to 40° F or below. Depending on how exact you want to be with your chosen RS% level, you may want to start the cooling a little earlier than right when the must is at the desired sugar level. Yeast will still be consuming sugars as they are being chilled. When they finally do get cold enough to stop being active, you may find you have a lower °Brix level than you wanted. To avoid this scenario start cooling 1-2° Brix higher than where you want to end up. Whenever the wine is deemed ready it is filtered and bottled.
Note: An active fermentation can also be stopped by adding spirits to the wine, as in Port winemaking. However, unless you are after this kind of specialized winemaking, the added alcohol will make your wine very out of balance and this technique is not recommended for making non-fortified wines with residual sugars.
A word about Potassium Sorbate
Potassium sorbate is used to help stabilize a wine that contains residual sugar. It inhibits yeast reproduction and will stop a renewed fermentation from taking place. However, it will not stop an active fermentation.
Add at the rate of .5 to .75 grams per gallon (125-200ppm) in conjunction with .3 grams of meta-bisulphite (50ppm) per gallon. Use the higher end of the range (200 ppm) as the wine's pH approaches or exceeds 3.5 or when the alcohol content of the wine is below 10%.
Note: Potassium sorbate should never be used in a wine that has undergone MLF because the bacteria will metabolize the sorbate and create an odor of rotting geraniums in the wine!
Once the primary, alcoholic fermentation is over it is time to take a look at malolactic fermentation (if we haven't already) and the ageing period!
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