Sorbic acid

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Sorbic acid is a short-chain fatty acid which, together with its salt, potassium sorbate, exhibits antimicrobial properties.[1] Owing to its solubility, potassium sorbate is often used instead of sorbic acid. It is added as a yeast inhibitor in sweet wines (greater than 2.0 g/L reducing sugar). This compound does not kill yeasts but, if properly employed, inhibits their growth. Sorbate has little practical inhibitory effect on the growth of acetic acid bacteria, lactic acid bacteria, or oxidative (film forming) yeast and the spoilage yeast Dekkera. The effectiveness of sorbic acid (potassium sorbate) in controlling fermentative yeast in wine is dependent upon the following: wine pH, free sulfur dioxide content, percent alcohol by volume, sorbic acid concentration. The lower the pH of the wine, the greater is the percentage of sorbic acid that is undissociated. It is the undissociated acid which controls yeast inhibition. An increase in pH (the result of a malolactic fermentation, for example) decreases the percentage of the undissociated compound and, therefore, reduces the effectiveness of yeast inhibition. Potassium sorbate should only be added to sweet wines, just prior to bottling. Oxidation of wines containing potassium sorbate results in a flat, butter-like off odor. Wines bottled with this compound should contain sulfur dioxide to avoid oxidation. The growth of lactic acid bacteria in the presence of sorbic acid produces a potent odorous compound (2-ethoxyhexa-3,5-diene) that is responsible for the so-called "geranium tone." High free sulfur dioxide levels (approximately 30 mg/L, depending upon the pH) must be achieved at bottling to prevent lactic acid bacterial growth in the presence of sorbic acid, and to aid in limiting wine oxidation. Potassium sorbate is generally employed at levels of 150-300 mg/L sorbic acid. Sweet wines low in alcohol and free sulfur dioxide, and wines high in pH, require more of this compound for stabilization. The average detectable threshold is 135 mg/L sorbic acid, with the minimum detection level approximately 50 mg/L. It should be noted that the threshold is much lower for wines that have undergone oxidation in the presence of sorbic acid and to aid in limiting wine oxidation. The use of sorbate in wines designed for long-term aging is contraindicated. As stated, owing to its solubility, potassium sorbate is often used instead of sorbic acid. To determine the amount of potassium sorbate to employ to obtain a given level of sorbic acid, the following relationship can be used: Weight of potassium sorbate required = 1.34 x addition level of sorbic acid (mg/L)

Sorbic acid and its potassium salt have been extensively tested and used as a preservative in many foods. In the United States, BATF permits the use of sorbic acid and potassium sorbate to preserve wine. The maximum concentration of sorbic acid allowed in finished wine is 300 mg/L.[2]

Sorbic acid (2,4-hexadienoic acid) is a straight chain unsaturated fatty acid with a molecular weight of 112.13. The potassium salt of sorbic acid is commercially available as a powder or granules. Its molecular weight is 150.22 and it is very soluble in water.[2]

The antimicrobial action of sorbic acid is primarily against yeasts and molds. Its action against bacteria appears to be selective. At concentrations used in wine it does not seem to prevent spoilage from either acetic or lactic acid bacteria. Must and wine related yeasts inhibited by sorbic acid include species of genera Brettanomyces, Candida, Hansenula, Pichia, Saccharomyces, Torulaspora, and Zygosaccharomyces.[2] The inhibitory effect of sorbic acid on yeast strains is not uniform. Certain species are more tolerant than others. Sorbic acid also inhibits mold growth. Some of the important species that are suppressed by sorbic acid belong to the genera Alternaria, Botrytis, Cladosporiwn, Fusariwn, Mucor, Penicilliwn, Rhizopus, Trichoderma. Mold can be a problem in wine cellars. To control mold in the wine cellar, sorbic acid could be included in the antimicrobial compounds used for sanitizing. Several microorganisms can metabolize sorbic acid particularly when it is present in small concentrations. For this reason, it is not a suitable preservative in foods with high microbial counts. To derive the maximum benefit from the antimicrobial action of sorbic acid, it is important to clean the wine well and keep the microbial count low in the bottled wine. It should be emphasized that sorbic acid inhibits yeast and mold, but not acetic and lactic acid bacteria. In fact, lactic acid bacteria can metabolize sorbic acid and produce off flavored compounds. It should be emphasized that a wine must be clarified to reduce the yeast population below 100/ml for sorbic acid to be effective.

The inhibitory influence of sorbic acid is greatest when it is in undissociated form. The pKa of sorbic acid is 4.75. The antimicrobial action increases as the pH value decreases below 4.75. In other words, the proportion of undissociated form of sorbic acid increases (above 50%) as the pH drops below 4.75, this can lead to increased antimicrobial action.[2]

The antimicrobial action of sorbic acid is due to its inhibitory influence on various enzymes in the microbial cell. The enzymes inhibited by sorbic acid include the following:[2]

  1. Enzymes involved in carbohydrate metabolism such as enolase and lactate dehydrogenase.
  2. Enzymes of citric acid cycles such as malate dehydrogenase, isocitrate dehydrogenase, ketoglutarate dehydrogenase, succinate dehydrogenase, and fumerase.
  3. Several enzymes containing SH group, and other enzymes such as catalase and peroxidase.

The taste threshold for experienced tasters has been reported to be about 130 ppm. Addition of sorbic acid often results in the formation of ethyl sorbate, which is said to impart an unpleasant odor when present in a significant level. As mentioned earlier, lactic acid bacteria can decompose sorbic acid and produce 2-ethoxyhexa-3,5-diene, and other compounds which give a geranium like off odor.[2] Other related intermediate compounds also contribute to the overall off odors in wines containing sorbic acid and spoiled by lactic acid bacteria. To prevent bacterial spoilage in sweet wines it is important to add a sufficient amount of sufur dioxide in addition to sorbic acid.

Besides pH, the ethanol content of a wine also influences the antimicrobial action of sorbic acid. For this reason, with a relatively high amount of alcohol in the wine, lower levels of sorbic acid would be needed. Peynaud (1980) recommended the following doses of sorbic acid in clarified wine based on alcohol content.[2]

Recommended Sorbic Acid Levels[2]
% Alcohol in wine Sorbic acid mg/L
10 150
11 125
12 100
13 75
14 50


The key points in sorbic acid use are summarized below.[2]

  1. Potassium sorbate (most soluble form of sorbic acid) should be used. However, this can cause bitartrate precipition problems.
  2. The solubility of potassium sorbate is influenced by temperature, therefore, it should not be added to a cold wine.
  3. Wine should be mixed well after sorbate addition.
  4. Sorbate should be used in conjunction with sulfur dioxide.
  5. Certain yeast and bacteria are not inhibited by sorbic acid.
  6. Properly clarified wine (low yeast count), low pH, and relatively high alcohol would help in reducing the amount of sorbic acid needed for effectively controlling yeast.
  7. Sorbic acid addition should never be considered as a substitute for poor sanitation.

Potential sources[edit]

References[edit]

  1. ↑ Zoecklein BW. Controlling microbial growth in wine. Wine/Enology Grape Chemistry Group at Va Tech.
  2. ↑ a b c d e f g h i Dharmadhikari M. Sorbic acid. Vineyard & Vintage View. 1992;7(6):1–5.
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