One doesn’t have to spend much time working in a Texas wine tasting room to learn that lots of folks enjoy sweeter wines. Wines with some residual sugar or sweetness often have bright fruit aromas and flavors, and are very easy to drink, either by themselves or with snacks and meals. The question is often asked how does one make a sweet wine. Many folks believe that certain grapes are just sweeter than others, or some grapes can get riper and thus have greater sugar content. That is the issue to explore in this edition of Carl’s Corner.
Every grape variety can be made into wine that varies from totally dry (no residual sugar or RS) to one that has semi-sweet or very sweet character. Before explaining, here are some commonly accepted definitions for various levels of sweetness. Wines with residual sugar levels ranging from essentially 0.0% to about 0.4-0.5% are considered to be dry. These are wines in which most, if not all, of the natural grape sugars have been converted during yeast fermentation into ethyl alcohol. The 0.4-0.5% range is considered the sugar threshold limit below which most tasters cannot actually detect sweetness in a wine. The small amount of sugar in this range may not taste sweet, but can impact the flavor profile by softening the perception of acidity on the palate (a technique often used by winemakers).
Wines with 0.4-0.5% up to 2% range of RS are considered semi-sweet. Many rosé and off-dry white wines currently produced fall into this category. Wines above the 2% level of RS are considered sweet. These can range all the way up to late harvest whites, like Riesling or Blanc du Bois wines with 5% or more RS, and will include most fortified wines like ports or sherries that may have higher sugar levels, 10% or more. A point to appreciate here is that 2% RS represents 20 grams of sugar per liter of wine (about 1 oz sugar per quart+ of wine). One of the keys to producing a pleasant, high-quality semi-sweet or sweet wine is to make sure the acidity, alcohol, and fruit flavors are balanced against the sweetness.
The winemaker can control the conditions and processes in the winery to affect the final sweetness in a wine. Sometimes the fermentation process can be stopped by additives or chilling cold enough that the yeasts stop working. This is generally a difficult thing to do and winemakers will need to be diligent in order to stop fermentation and capture natural RS in the wine. A second method to capture natural sugar is to add alcohol (typically above 17%) to a fermenting wine which will actually kill the fermentation yeasts and preserve sugar at the level when addition was done. This is the method used to produce classic port-style wines.
An easier method for producing sweeter wines is “back-sweetening” with sugar, honey, grape juice concentrate, etc., after the initial fermentation is completed. Most sweet wines produced in Texas, and in lots of other wine regions, are done this way. The winemaker will decide how sweet to make a wine based on the market for that wine, and what aroma and flavor attributes are in the available dry wine (acidity, fruit character, alcohol). Then trial tastings are done with various levels of added sweetener.
Once a wine is back-sweetened, the job is not done. Unless the alcohol level is high enough, like in a fortified sherry wine, a second fermentation in bottle can occur creating bubbles, sediment, and off-aromas and flavors. So, the winemaker needs to stabilize a sweetened wine by adding a controlled amount of potassium sorbate that will prevent any further yeast reproduction, and a level of sulfite that will prevent any yeast present from continuing fermentation. These additions are carefully measured and will, if done correctly, not impact the aroma or flavor of the wine.
However, one must be very careful with sorbate as under certain conditions, sorbic acid can be generated from the sorbate and metabolized into hexadienol, a compound that smells like rotting geraniums – something you definitely do NOT want in your wine. Sorry about this little bit of chemistry, but you know I love it!
Finally, a filtration process before bottling a sweetened wine can remove bacteria molecules and residual yeasts such that no further fermentation activity can occur once the wine is sealed in bottle.
Thus, one should appreciate the effort that winemakers undergo to produce wines with residual sugar levels for the pleasure of their customers. If you enjoy sweeter wines, make sure to tell your winemaker thank you, or ask your tasting room associate to convey your thanks. Enjoy!
The following reference provided helpful information for this posting.
“Beginner’s Block: Backsweetening” by Dave Green, WinemakerMag, Dec 2019-Jan 2020, p13.