July 7, 2014

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Potassium Sorbate as a Wine Preservative

Potassium Sorbate (K-sorbate) is a relatively recent wine additive (it only first started to be used about 50 years ago), used primarily as a preservative to help prevent re-fermentation of sweet or semi-sweet wines. It is widely used in many types of foods ranging from cheese and yogurt to dried fruit and meat. It is even used in cosmetics to help give them a stable shelf life. It is generally considered as safe, having about the same toxicity as table salt.
To be honest, I hadn’t heard much of wineries using K-sorbate before returning to Minnesota, and had never used it in off-dry or sweet wines in either Alsace or Australia, and now I get questions about it on a regular basis. So, I’ve been doing a bit of research on it lately and figured I’d share with you what I have learned.
When added to water, K-sorbate breaks down into sorbic acid (sorbate) and ionic potassium (K). It is the sorbic acid that is active as an anti-microbial. It doesn’t kill yeast cells, but only prevents them from growing and being active. It has no effect on lactic acid and acetic acid bacteria at the amounts added in wine. Therefore, it should only be added to wine that is already stable via its pH and free sulfur. Another important point to remember is that just as sulfur dioxide (SO2) is more active at lower pH, so is sorbate.
Here are some more important points to know:
  • Like SO2, sorbic acid is detectable in wines when it is added above certain levels, though the reported values vary from 135 to 400 mg/L. * 
  • Over time, sorbic acid will be reduced to form ethyl sorbate, which has been described as having pineapple and celery aromas. You cannot prevent this from happening in wine, as this reduction occurs naturally with the presence of ethanol. While these aromas aren’t inherently objectionable, they will mask other fruity aromas in your wine. Some people may consider it a flaw. The concentration of ethyl sorbate will continue to rise over time, and is dependent on your initial sorbic acid concentration. Therefore, sorbate is generally added to wines which aren’t destined to be aged.
  • It will not inhibit bacterial activity. If you add it to wine before bulk storage or bottling, you need to be absolutely certain that the wine is stable. If there is any lactic acid bacteria present in your wine, it will still be there after sorbate is added. Potassium Sorbate must ALWAYS be used in conjunction with proper SO2 addition.
  • If lactic acid bacteria is present in the wine, it will metabolize sorbic acid and produce a chemical that has a strong odor of Geranium leaves and is considered a major wine flaw. 
  • The amount of sugar in the wine has no effect on the amount of sorbate needed. The only concerns are pH, alcohol, and the initial population of yeast cells (which should be less than 100/mL – make sure the wine is very clear before adding K-sorbate).
  • When adding K-Sorbate to wine, remember that it contains about 75% sorbic acid by weight (100 mg of K-sorbate contains 75 mg of sorbic acid).
  • The BATF limits sorbic acid addition to wines to 300 mg/L (the European Union regulations limit its addition to 200 mg/L).
  • Sorbate SHOULD NOT be added to dry red or white wines. There is no risk of refermentation when there is no sugar present. You are only adding the risk of off-odors from ethyl sorbate as well as risking the production of geranium taint.
  • Sorbic Acid is not very soluble in water. Precautions need to be made when adding it to wine to ensure that it is properly dissolved in the wine.
  • Sorbate is not allowed as an additive in production of organic wine
  • Certain countries do not allow the import of wine containing Sorbate
As for the recommended rates of sorbic acid that should be used in wine, there seems to be no clear consensus. The most cited recommendations come from Peynaud (1984), who notes that sorbic acid is half as effective at a pH of 3.5 than it is at a pH of 3.1, but then lists his recommended dosages based on alcohol content. Sorbic acid’s action against yeast is reinforced by alcohol. The following are his recommendations for sorbic acid:
Wine at 10%     150 mg/L
Wine at 11%     125 mg/L
Wine at 12%     100 mg/L
Wine at 13%      75 mg/L
Wine at 14%      50 mg/L
These recommendations by Peynaud assume a pH < 3.5, and adequate SO2 protection. Remember, these numbers are the recommendation for sorbic acid. If you are adding Potassium Sorbate, only 75% is sorbic acid. So you need to divide the sorbic acid amount by 75% to get the equivalent amount in K-sorbate. So 150 mg/L of sorbic acid would mean you should add 200 mg/L of K-sorbate (0.2 g/L).
 
Commercial wineries generally avoid the need to use potassium sorbate because their wines are usually sterile filtered at bottling, so the refermentaion risk is eliminated (sterile filtering means all yeast and bacteria cells are eliminated). Wineries focused on the production of high-quality wines also tend to forgo the use of K-sorbate because they find the aroma of ethyl sorbate to be undesirable. In the end, I can’t make any recommendations for or against it, as it is a preference choice for the winemaker. However, if you are properly sterile filtering your wines, the addition of sorbate is an unnecessary step in the process that comes with risks that should be addressed.
* References:
Auerbach, R. C. 1959. Sorbic acid as a preservative agent in wine. Wines Vines, 40, 26-28.
Ough, C. S.; Ingraham, J. L. 1960. Use of sorbic acid and sulfur dioxide in sweet table wines. Am. J. Enol. Vitic.,11, 117-122
Peynaud, Émile. 1984. Knowing and Making Wine. John Wiley and Sons
Postel, W.; Drawert, F. 1970. Sensory threshold value of sorbic acid in German white wines. Lebensm. Unters. Forsch. 1970,144, 245-252.