Ask Wine Wizard

I added grape concentrate to boost the sweetness of a dry wine at bottling and added sorbate. The flavor is good, but there is some kind of gas in the wine. Any thoughts on what happened?



Dear Wine Wizard,

I made a Muscadine wine this year that ended up very dry with harsh flavor. I wanted it to have more of a grape flavor so I added some grape concentrate to it. I achieved the flavor that I wanted, but there is some kind of gas in the wine (you can see it fizzing, like a soda). What have I done? By the way, I used potassium sorbate (1/4 tsp per gallon) before I added the grape concentrate.

Ann Cherry
Via email

Wine Wizard replies: Kudos to you for being creative enough to take the plunge and really tweak your wine the way you like it.

Blending and amelioration (adding non-grape or wine adjuncts to a wine) are sound, time-tested arts that winemakers throughout history have employed — when not prohibited by law. Winemakers have been known to blend vastly differing wines and to add sugar, juice, herbs, spices and flower extracts. These practices are intended to introduce, mask or bring out certain characteristics in a finished product. Even home winemaking purists, those who believe that something cannot be called wine if it’s had non-grape material added to it, are known to do things like add a bit of brightly-colored Syrah to their White Zinfandel to punch up the color.

Let it be known, however, that any time you introduce a foreign substance to a wine (or blend two wines together) you need to take a moment to think about the potential consequences. Two products that may be considered stable on their own could potentially yield a very different stripe when combined. Because each wine, juice or concentrate is made up of such an immense number of sugars, proteins, polysaccharides, amino acids and aromatic compounds, it is nearly impossible to predict all of the interactions that two components might have.

However, there are some wine chemistry relationships that can be anticipated. Tannins, for example, can precipitate proteins in wine. For example, if you add an infusion of black tea to a wine in order to boost your tannin content and mouthfeel, you may find that you get an unsightly microlayer of protein at the bottom of your carboy. Similarly, adding a wine high in tartaric acid to a wine that is high in potassium, may result in a subsequent precipitation of potassium tartrate crystals (if the pH conditions are right). It is important for the winemaker to keep these kinds of things in mind when blending or making additions.

As if that weren’t enough to keep track of, the winemaker must also take into account the ubiquity of microbial life (bacteria, yeast, fungi, etc.) that can survive and even thrive in wine and grape matter. Many wine microbes are surprisingly hardy. Even when an initial yeast fermentation is complete and a wine is dry, there are many microscopic flora and fauna that can survive and reproduce in the seemingly non-fermenting environment. A microbial population that may be inhibited or dying out in one environment (like your dry muscadine), may get a new lease on life once another source of nutrients and sugars (for example, grape concentrate) is introduced.

It is almost certain that this is what led to the fizziness and activity you mention in your question. When the original yeast that conducted your fermentation came into contact with the new sugar in the grape concentrate, they woke up, began to metabolize sugars and release carbon dioxide gas into the wine (again). Even if you rack the dry wine off its primary lees (the sediment that collects on the bottom of containers after a wine finishes fermenting) before adding the concentrate, you still cannot avoid carrying at least some of the yeast cells into the new container. You could also be experiencing secondary activity by wine-loving bacteria like Lactobacillus or Pediococcus, which do not need sugar to survive and aren’t inhibitedby the potassium sorbate that you mentioned.

This brings us to a discussion of sorbate. Sorbic acid, sometimes sold in powder form as potassium sorbate, is authorized for winemaking use in almost every country (notable exceptions being Switzerland and Austria). It is fungistatic (inhibitory to yeast) and even fungicidal (kills yeast) in higher concentrations.

Sorbic acid is not very soluble in water, has a slightly acidic taste and does not have any effect on bacteria, as mentioned above. Roughly, 1/4 teaspoon of potassium sorbate translates to 2 grams, of which 75% is pure sorbic acid in solution. Unfortunately, this isn’t enough sorbic acid to inhibit a re-fermentation from occurring.

One thing you should be concerned with is a stuck fermentation; if you added a lot of sugar, it is likely that the yeast from the first fermentation (as weak as they are) might not be able to handle all of the sugar you’ve thrown at them. There’s not much you can do except keep the fermentation in a relatively warm place (around room temperature is fine) and not in a cold basement — this would slow the yeast down too much. You could try to add about 100 mg/L of a proprietary yeast nutrient like Superfood or Fermaid K to make sure the yeast have all of the nutrients they need to complete the fermentation.

If you’re lucky, once the sugar is fermented, the wine will be relatively stable again — that is, if you add approximately 50 ppm sulfur dioxide in an attempt to keep the malolactic bacteria from starting a secondary malolactic fermentation. The bottom line is this: Whenever blending anything into a wine, try to remember that there are all sorts of instabilities, re-fermentations and precipitations that may occur. It’s best to trial-blend a small portion initially and see what happens on a small scale before treating your entire lot. This may be tedious, but it’s often the best way that home winemakers have to avoid blending problems.

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