As a winemaker, much of my thoughts and actions are given to solving the problem of sugar in wine. Residual sugar in one wine can be the bane of a good vintage. But sweetness can also be another wine’s glory.
In my experience, the industry standard of fermenting wines to dryness is the imperative ninety-five percent of the time. To taste a Cabernet Sauvignon that you expected would be gum-tinglingly tannic and instead tastes like a chocolate covered raspberry would be off-putting to most because a sweet Cabernet Sauvignon is not the industry standard. The profile for many wines is dryness – in other words a lack of sweetness, the absence of sugar.
Stepping into the laboratory for a moment, let’s assume we have been fermenting the must* (crushed grapes) of the most recent vintage and our intention is to create a wine that has a profile that is lean and crisp and dry. According to our most recent test results the must has stopped fermenting before it has reached our intended goal of negative 1.5 brix*.
Now let’s assume we have 100,000 gallons of this wine. Our customers expect it to be lean and crisp and definitely not sweet. This is a problem. Houston, we have a ‘stuck fermentation.’ The sweetness needs to be removed. Your brand depends on it.
The most likely way we would try to remove the sugar would be to recreate the primary alcoholic fermentation process. To do this, you would need a yeast that is tolerant to high levels of alcohol. Alcohol kills yeast after all, and if you have a wine that is close to dryness but not quite dry, you will not only have high levels of alcohol but very little sugar with which to work.
For a re-fermentation to be successful the utmost care should be taken at each of the following steps.
First, the correct level of alcohol tolerant yeast, yeast nutrients (nitrogen-based), and corn sugar needs to be determined. Yeast needed should be about four pounds per every 1000 gallons of stuck must. Yeast nutrients can amount to about half of that. The amount of corn sugar is less important as it is only in play here to help build a healthy yeast population in the re-inoculate.
Corn sugar is primarily glucose and a better choice than regular white or brown sugar for the re-fermentation process. The corn sugar/glucose aids in initiating a second alcoholic fermentation in the stuck must without adding fructose, which is likely a portion of the perceptible sweetness in the stuck must. The corn sugar should be combined with a small amount of the stuck must to a sweetened concentrate of about 10° brix.
Next, hydrate the required amount of yeast in warm water. When the yeast is hydrated (minutes), add enough of the corn sugar and stuck must concentrate to help the yeast get started (minutes). When the surface of the yeast and stuck must becomes visibly petulant add enough of the corn sugar and must concentrate to the hydrated yeast to drop the temperature of the yeast five-to-ten degrees. This acclimates the yeast to the alcohol in the stuck must. It is at this point that the yeast nutrients should be introduced to the inoculate – before the inoculate drops too far below 10° brix.
More of the stuck must gets added only when the yeast have proven to be fermenting (visibly petulant). Ideally, a winemaker would want at least one third of the total volume that needs to be re-fermented fermenting before inoculating the total volume. Depending on the total volume that you are attempting to re-inoculate, this could take several days.
If all goes well, the remaining residual sugar that made your wine taste like a dessert rather than an entrée should dissipate, the wine will then be dry and ready for any additional treatments you wish to make.
On the off-chance that the re-fermentation does not take, there are treatments that could be useful in at least securing the wine’s palatial distribution before bottling. (See previous article Acid in Wine: A Tutorial)
Okay, now, let’s step out of the laboratory for a moment.
Let’s say you are not trying to make a dry wine. You are trying to make a sweet wine, a wine that tastes like honey, or chocolate, or butterscotch. How can this be accomplished? How do the finest Sauternes exude such glamour and finesse when in essence they are the antithesis of what most of the world drinks?
There are many ways to retain sweetness in a wine. A winemaker could add a bit of sulfur to a fermenting must to arrest the fermentation before the must is ‘dry.’ A winemaker could also add distilled spirits to the must to arrest the fermentation and fortify the wine. The level of sweetness in the final product is solely at the discretion of the winemaker, if he or she chooses to control the process at all.
If the winemaker wants to leave the sweetness in a wine to nature and the end goal is a sweet wine, they would likely focus their efforts in vineyard.
At their most basic level, sweet wines are made from grapes that have shriveled and/or dried either because they have been left on the vine for a long time, because they have been air-dried after being harvested, or because botrytis has infected the grape clusters and absorbed the water inside the grapes thus concentrating the amount of sugar.
For any winemaker, the easiest way to achieve additional sweetness within your grapes is to allow them to over ripen, either on the vine or in a drying room. What the drying process does is remove the water inside the grape by the process of evaporation. (See previous article How to Make Wine at Home: A Garage Wine Primer )
Imagine that the pH of a ripe grape is 3.5 and the brix is at 25. If that grape is dried after it is picked, the loss of water (which has a neutral pH of around 7) will begin to decrease the pH (toward 3) and increase the sugar content (toward 30). Simply put, the grape sugars become less dilute and the pH less basic. This is a good thing because sweetness and acid play off of each other really well.
Once the grapes are shriveled to perfection they are typically crushed and fermented in a traditional manner. A cool (50°F) fermentation is better than a warmer one. When the alcohol content reaches about fifteen percent of the total volume most of the yeast will begin to die off. When the wine stops fermenting, it should be racked from its lees and treated as though it were any other wine.
If the sugars were concentrated enough when the fermentation began, a pronounced sweetness will be present even though the wine has stopped fermenting. Withy any luck, the resulting wine will not only have the qualities of the variety of grape used to make the wine but a focused lush mouth feel, a burst of acidity, and a certain amount of balance on the palate.
With any sweet wine, or a wine with some residual sugar, there are dangers of the wine re-fermenting once it has been bottled (Have you ever opened a bottle of wine that was not supposed to be sparkling and it was?). A few ways to keep a wine from re-fermenting in the bottle are by using sulfur as an antiseptic and microbial barrier and filtering the wine before it is bottled.
In the end, it is at the discretion of the winemaker to assess each situation and make an educated decision about the wine they are making. When it comes to home winemaking, my advice is to try making a sweet wine some time, and to avoid sweetness in what would ordinarily be a dry wine.
* Must is any unfermented or fermenting amount of grape juice prior to its evolution into wine.
* Brix is a measurement of the concentration of sugar in a liquid. The brix will decrease as the must ferments. Dryness = negative 1.5 brix.