Copyright 1998-2021 by Bryan Fazekas, all rights reserved. This document may be freely distributed by any means, electronic or physical, as long as:
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This document is not guaranteed to be exhaustive or correct. It is based upon my personal experiences in winemaking since 1981, and upon research that I have done. Research includes professional books and articles, and information from fellow winemakers that I consider reliable. Topics I have personally verified will be noted as such.
If anyone sees anything that is incorrect, please contact me with the correction.
Table of Contents
Note: at the time this was written, Usenet was the most common medium for discussing winemaking. References to rec.crafts.winemaking refer to the most popular Usenet group for winemaking.
This document started as a quick write-up of how I use potassium sorbate to prevent a renewed fermentation when sweetening a wine. Part way through the first draft I realized I was missing a lot of things, so I expanded the scope to generally cover sweetening of wine and/or preventing a renewed fermentation.
Most of the description to follow is based upon grape wine making. Please note that although some terminology used is for grape wines, the methods described work equally well for non-grape wines.
There have been quite a few posts in rec.crafts.winemaking requesting information regarding how to safely sweeten a wine so that a renewed fermentation in the bottle will NOT take place. To the best of my knowledge there are at least 5 ways to produce a sweet wine:
- Sweeten the wine with a substance that the yeast cannot consume.
- Start with enough sugar that the yeast produces too much alcohol for the yeast to survive in.
- Add enough of a spirit (vodka, brandy, etc) to produce a high enough alcohol environment so that the yeast cannot survive.
- Remove all yeast from the wine to prevent additional fermentation that will consume the sugar.
- Stabilize the wine so that any yeast is not able to begin additional fermentation.
There may be other methods of preventing a renewed fermentation, but the above are easily available for home winemakers. Following is a detailed description of each method, including some perceived benefits and drawbacks.
Fermentation is the process of yeast consuming sugar, emitting alcohol and CO2 as byproducts. The CO2 bubbles off (assuming the container is not sealed) while the alcohol remains. However, yeast is capable of surviving only certain concentrations of alcohol, and this limit varies by yeast strain. In effect, the yeast is capable of producing an environment that it cannot survive in, e.g., it poisons its own environment.
The bottom line is that fermentation continues until either all sugar is gone or until the yeast produces enough alcohol to create an environment in which it can no longer survive. The issue with sweetening a wine is to prevent the yeast from initiating a new fermentation.
Wines can be sweetened with a number of substances. Probably the easiest is to use processed sugar, e.g., table sugar or corn sugar. Another method is to reserve unfermented juice or concentrate under refrigeration so that it doesn’t ferment itself, and add that to the wine. Or the wine can be sweetened with other natural sweeteners, e.g., honey, maple sap, or whatever. Finally, wine can be sweetened with artificial sweeteners.
Certain sugars are non-fermentable — for chemical reasons yeast cannot use them for food. In fact, “dry” wines, e.g., wines with no sweetness, actually contain 0.25% to 0.75% residual unfermentable sugar. I don’t believe there is actually any grape wine with 0% residual sugar. Please note that this distinction is minor.
Artificial sweeteners (aspartame, etc.) are similar to the non-fermentable sugars in that they taste sweet to humans, but are not digestible by yeast.
So one option in preventing a renewed fermentation is to sweeten the wine with one of the above substances. This has a couple of drawbacks. First, I don’t know of any source for unfermentable sugars. Second, the taste of artificial sweeteners is different from natural sugars, so the taste of the sweetened wine will be difference.
However, in the case of artificial sweeteners, the wines should be safe for those with medical conditions that limit or prohibit sugar while not prohibiting alcohol. NOTE: Consult a doctor before trying this.
Since yeast will keep producing alcohol until it creates an environment it can no longer survive it, another way of leaving residual sugar in a wine is to start with enough sugar (or add it later on) so that the yeast cannot consume it all. Although this is probably the simplest and most natural method of producing a sweet wine, it has 2 drawbacks:
First, depending upon the yeast used, the alcohol level of the wine may be very high, in the range of 14% – 18%.
Second, there may be a question as to whether the yeast has killed itself off, or if it is merely in a “stuck” mode and may at some point begin fermentation again. I’ve had one instance of a renewed fermentation in the bottle, which merely pushed some corks out a bit. Others have told me horror stories of entire batches (sometimes hundreds of bottles) all blowing their corks and the wine spilling out onto the floor.
However, if the specific gravity calculations indicate that the final alcohol is at about the level indicated as the max for the strain of yeast, it’s probably safe. Or you could bottle a couple of bottles and put them someplace warm to test for problems.
This is how port and madeira are made – brandy or a neutral spirit is added to an active fermentation, raising the alcohol level to a point where the yeast cannot survive. Typically this is 18%-21% alcohol. Again, this produces a wine with a very high alcohol content, although there is little question as to whether a renewed fermentation will take place.
Note: Some commercial yeasts survive alcohol concentrations up to 18%, while others die off in much lower concentrations. Check with the manufacturer for the survivability of the yeast.
While I’ve seen various formulas for calculating the amount of a spirit, they all use a Pearson’s Square for determining the calculation. One formula I found recently is published by the government of Australia — it appears authoritative so I chose to use it.
For those who are not mathematically inclined, I’ve skipped the background and gone straight to the simple formula.
Factor = (Strength Desired – Strength of Wine) / (Strength of Spirit – Strength Desired)
where “Factor” is a number I multiply by the quantity of wine to determine how much spirit to use.
Let’s assume I have a wine that is 12% alcohol, I want to fortify it to 18% using a 80 proof (40%) brandy. Substituting these values in my formula is:
Factor = (18 – 12) / (40 – 18) = 6 / 22 = 0.27
If I have 5 gallons of wine, multiplying 5 * 0.27 tells me that I need 1.35 gallons of brandy to fortify the wine. Note: While I used gallons in my example, liters can be used as well, e.g., 19 liters of wine multiplied by 0.27 indicates I need 5.13 liters of brandy.
Suggestion: sugar is cheaper than brandy so feed the fermenting wine sugar until the yeast dies of alcohol poisoning. In the above example if the wine was fermented to 16% alcohol the formula becomes
Factor = (18 – 16) / (40 – 18) = 2 / 22 = 0.09
meaning I need only 0.45 gallons of brandy to fortify the wine.
If the yeast is completely removed from the wine there is no chance of a renewed fermentation. Since yeast is approximately 0.8 microns in size, it requires a filter that is capable of stripping out such small particles. I believe “sterile” filters, available for most common models of wine filters, strip out particles as small as 0.45 microns.
Note: There is some controversy surrounding how good the sterile filters are, and if they actually remove all the yeast. However, I know of at least two commercial wineries that successfully use this method for their late harvest wines.
This method has two advantages: First, the alcohol level of the wine can be “normal”, e.g., a high alcohol level is not required. Second, no additional chemicals are required to stabilize the wine.
The primary drawback is that taste and smell constituents in wine are actually particles, and sterile filtering does strip some of these out.
The final method described here is to use potassium sorbate to prevent a renewed fermentation in a wine that currently has not activity.
This method is actually 2 different methods. Method “A” is the method I have used successfully since I started winemaking and it is the method I am most familiar with.
Method “A”: Let the wine ferment out dry.
There should be no residual sugar (specific gravity less than 1.000) and I typically let the wine set for 3 to 10 months, so that it is clear and all sediment has precipitated. Then add 1/2 tsp potassium sorbate per gallon of wine and 50 to 60 ppm SO2 [1/4 tsp potassium metabisulfite per 5 gallons, or 1 Campden tablet (crushed) per gallon]. Sweeten to taste and bottle.
The advantage of this method is that it allows the winemaker full control of how sweet the wine will be.
The big disadvantage is that it requires the addition of another chemical, potassium sorbate.
Method “B”: Stop an active fermentation.
There are two ways to do this. One method is to filter the yeast out of the wine, as described above. The other is to use cold to stop the yeast from producing.
To stop the yeast from producing, chill the wine down to approximately 30 degrees F and keep it there for at least a week. This may cause additional sediment to precipitate. It might be beneficial to add a fining agent prior to chilling the wine. Rack the wine without letting it warm up. Then add potassium sorbate and potassium metabisulfite, exactly as for method “A”, and bottle.
This has the advantage of not requiring the addition of any sugars, e.g., all sugars are natural grape sugars.
It has a number of disadvantages. First, it does require the addition of another chemical. Second, the winemaker must stop fermentation at exactly the right time to get the desired residual sugar. And finally, the fermentation must be completely ceased, or the sorbate will not work.
Note: I have not used method “B” — the description above is from a fellow winemaker who has used it successfully.
The home winemaker has a number of methods available for sweetening a wine. Each method has advantages and disadvantages, so the winemaker must review his or her options and make their best choice.
Copyright 1998-2021 Bryan Fazekas
This page last updated: 18 April 2006