Sorbic acid in sweet wine making

In the realm of oenology, the utilization of various additives is key to achieving the desired characteristics and stability in wines. Among these additives, sorbic acid emerges as a significant component in the production of sweet wines. It complements, rather than replaces, the use of sulfur dioxide (SO2), a common preservative in winemaking.

Sorbic acid, chemically known as C6H8O2, is a natural compound used as a preservative. Initially derived from fruits of the Sorbus genus, it is now predominantly synthesized. In the food industry, it is labeled as E-200 and poses no toxicity or health risks. This compound appears as a white crystalline powder with a mildly acidic flavor. Its solubility is relatively low in water (1.6 grams per liter at 20°C) but higher in 96% alcohol (112 grams per liter at 20°C).

In winemaking, sorbic acid is not used in its acidic form but as potassium or sodium sorbate, due to their higher solubility. However, this transformation results in a significant release of cations – approximately 70 grams per liter of potassium and 200 grams per liter of sorbic acid for every 270 grams per liter of potassium sorbate added.

This compound is metabolized like other fatty acids in humans, converting into water and carbon dioxide, making it non-toxic. The maximum legal dose in European wines, as per Regulation EC No 479/2008, is 200 mg/liter in the form of potassium sorbate.

Sorbic acid is primarily employed in musts or wines for its antimicrobial properties, particularly its fungicidal capabilities. It inhibits the development of mycelium, the germination of spores, and yeast multiplication. According to J. Ribéreau-Gayon, the fungicidal dose required to halt or suppress a high level of alcoholic fermentation is around 5 grams per liter, while the fungistatic dose, which prevents fermentation in a must, varies between 0.3 and 0.5 grams per liter.

Its application is almost exclusively in sweet wines, where doses can be lower than those mentioned due to the effects of alcohol content and pH levels. The effectiveness against yeasts is significantly higher at lower pH values, whereas at pH levels above 3.5, the maximum legal dose of 200 mg/liter may be insufficient. The inhibitory action of sorbic acid is attributed to its non-dissociated fraction, which decreases as the pH level drops, necessitating double the dose when the pH shifts from 3.1 to 3.5.

The alcohol content of wine also significantly influences the activity of sorbic acid, allowing for reduced dosages as alcohol levels increase. Besides its fungicidal effect, sorbic acid also possesses bactericidal properties, although these are much weaker. High doses between 0.5 to 1.0 gram per liter are necessary to inhibit lactic or acetic bacteria. However, at legal doses, sorbic acid can encourage bacterial growth by inhibiting yeasts, thereby eliminating their competition with bacteria.

In wines with added sorbic acid and low levels of sulfur dioxide, there is a risk of lactic bacteria development. This can degrade sorbic acid, producing a substance with a strong geranium-like odor, known as the "geraniol" defect. This compound, difficult to eliminate even with authorized deodorizing treatments, can be attenuated with carbon dioxide or nitrogen gas sweeps and eliminated with an intense oxidation treatment using permanganate.

Sorbic acid should be added to wines before bottling as potassium sorbate, dissolved in water and added slowly and under agitation to prevent its crystallization as sorbic acid.

Its use is indicated for the biological stabilization of sweet wines, to prevent yeast development on their sugars. It is not useful for dry wines. Aqueous solutions of sorbic acid can deteriorate over time, developing a yellow color and an unpleasant, pungent odor. Hence, it is advisable to prepare them just before use in concentrated solutions of 270 grams of potassium sorbate per liter of water. For sorbic acid, a mixture of 200 grams of the acid with 100 grams of potassium hydroxide per liter of water is recommended.

Up to the legal maximum dose of 200 mg/liter, no modifications in the sensory characteristics of treated wines have been detected. However, higher doses over time can lead to astringent sensations, bitterness, and a strange pungent aroma. This is because, as an unsaturated fatty acid, it can oxidize, creating an aldehyde function, potentially leading to these defective characteristics. In calcium-rich wines, calcium sorbate can precipitate, forming fine crystalline needles or amorphous masses of small granules.

Finally, the presence of sorbic acid can lead to an overestimation in the measurement of a wine's volatile acidity, with an excess result of 1.6 to 1.7 meq or 0.09 to 0.10 grams per liter more when the wine contains 200 mg/liter of sorbic acid.