Citric acid has many uses in wine production. Citric acid is a weak organic acid, which is often used as a natural preservative or additive to food or drink to add a sour taste to food. It can also be used to neutralize surfaces that have been treated with basic substances. Citric acid elicits antimicrobial activity against some molds and bacteria. It may create a relationship with antioxidants by chelating metal ions to help prevent browning. Citric acid occurs in the metabolism of almost every organism because it is an important intermediate in the tricarboxylic acid cycle (TCA cycle aka citric acid cycle). In the TCA cycle, pyruvate is decarboxylated and reacts with Coenzyme A to yield acetyl-CoA. The enzyme catalyzing this reaction is pyruvate dehydrogenase. Acetyl-CoA condenses with oxalacetate to produce citrate and other intermediate organic acids. In one turn of the cycle, 2 molecules of CO2 are formed and the oxalactetate is regenerated. Also three molecules of NADH and one FADH2 are produced, for a total of 14 ATP for each pyruvate metabolized.
Application in wine microbiology:
TCA cycle has an important role in biosynthesis reactions. The cycle provides intermediates for amino acid and nucleotide biosynthesis. The TCA pathway can regenerate NAD+ with the production of malate and succinate, acids which can be produced by yeast during wine fermentations.
Malolatic fermentation is a normal winemaking practice that involves the conversion of L-malic acid to L-lactic acid by means of a malolactic bacteria. The conversion is a direct carboxylation by a single enzyme. Malolactic bacteria also have to capacity to metabolize citric acid. Citric acid is only present in small amounts in grapes, unless added. The metabolism of citric acid is not seen in commercial winemaking, but its metabolism can have an important effect on the formation of diacetyl.
The most important significance associated with citrate fermentation is the production of diacetyl. The lactic acid bacteria,Oenococcus oeni is used during malolactic fermentation for the deacidification of wine during which the metabolism of diacetyl occurs. The biosynthesis of diacetyl is dependent on the citric acid metabolism. Citric acid is first degraded to acetic acid and pyruvic acid. Most of the pyruvic acid is then metabolized to lactic acid with a portion going to diacetyl, acetoin, and 2,3-butanediol. Co-metabolism of citrate-glucose has been shown to enhance the growth rate and biomass yield of the bacterium O. oeni. The increased growth rate and yield will result in increased ATP synthesis. The metabolism of citric acid usually occurs after malic acid in wine during malolactic fermentation, and is not initiated until more than half of the malic acid has been metabolized.
Increasing citric acid concentrations will generally increase concentration of diacetyl. In winemaking, the citric-sugar co-metabolism can also increase the formation of volatile acid in wine which can affect the wine aroma negatively if present at excessive levels. Citric acid addition will affect the titratable acidity, and since some yeasts are capable of metabolizing citric acid, unexpected changes in the concentration of diacetyl and TA could result.
Citric acid is often added to wines to increase acidity, complement a specific flavor or prevent ferric hazes. It can be added to finished wines to increase acidity and give a “fresh” flavor. The disadvantage of adding citric acid is its microbial instability. Since bacteria use citric acid in their metabolism, it may increase the growth of unwanted microbes. Often to increase acidity of wine, winemakers will add tartaric acid instead.
Excess iron in wine results in the formation of the white precipitate (FePO4-2H2O) with phosphate ions. Ferric phosphate is a colloidal substance hat coagulates under the inﬂuence of calcium, potassium ions, and proteins and precipitates when its solubility product is exceeded. This problem can be treated with the addition of citric acid that forms a citric complex of Fe(III).
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- Lui, S. Q. Malolactic fermentation in wine – beyond deacidification. 2001. Journal of Applied Microbiology 2002, 92, 589–601.
- Sharma, R.K. Citric Acid. CRC Press LLC. 2000.