Understanding Wine Acidity, Sugar, and Sulfites

Posted on May 13, 2024 in Chemistry

1. Acidity in Wine

Total Acidity

Total acidity is the sum of all free acid functions present in must or wine. It is determined by titration and expressed in g/L of tartaric acid (in France, in g/L of sulfuric acid). Must have a total acidity greater than or equal to 4.5 g/L tartaric acid.

Volatile Acidity

Volatile acidity is the sum of acids belonging to the acetic series present in wine in both free and salified states. It is expressed in g/L of acetic acid (in France, in g/L of sulfuric acid) and cannot exceed 1 g/L.

Fixed Acidity

Fixed acidity is the difference between total acidity and volatile acidity, expressed in g/L of tartaric acid.

Real Acidity (pH)

Total acidity only considers the amount of free acids, regardless of their strength. Real acidity, expressed by pH, measures the amount of dissociated hydrogen ions present in wine or grape juice. These H+ ions are responsible for the sourness perceived by taste. The pH of musts and wines is related to the amount and strength of acids and typically ranges from 2.8 to 3.8.

2. Sugar-like Substances in Wine

A) Sugars

Some sugars are not fermented in wine, particularly in varieties like Uvay, Petit Rouge, and Blancos y. These sugars include:

  • Glucose: Found in grapes at approximately 15%.
  • Fructose: Found in grapes at approximately 25%. In fully fermented wines, a small proportion of fructose and some glucose always remain.
  • Mites: Present on grapes, they disappear during fermentation.
  • Arabinose and Xylose: Found in grapes and do not disappear during fermentation.

B) Polyvinyl Alcohols

Inositol, Mannitol, Arabitol, Erythritol, and Sorbitol are polyvinyl alcohols found in grapes.

C) Alcohol

Alcohols appear during alcoholic fermentation. The main alcohols in wine are:

  • Ethanol: Found in proportions of 72-120 g/L, it is responsible for the odor of wine and, after water, is its most important component.
  • Glycerol: Present at 1-5 g/L, it contributes to the sweetness of wine. Glycerol is formed at the beginning of fermentation and during the decay of grapes (especially those attacked by the fungus Botrytis cinerea), and is therefore present in grape must.
  • Butylenglycol: Found at 0.3-1.5 g/L, it has a sweet taste that can be almost bitter.

3. Advantages and Disadvantages of Sulfites

Disadvantages of Sulfites

  1. Slows or prevents malolactic fermentation.
  2. Can cause odors of hydrogen sulfide, reminiscent of rotten eggs or onions.
  3. Can be toxic to consumers, potentially causing allergies, headaches, and fatigue (rare). This problem can be avoided by using appropriate doses.

Benefits of Sulfites

1. Antiseptic and Selective Action

Sulfur dioxide (SO2) has an antiseptic action, preventing the growth of bacteria that cause malolactic fermentation and wine diseases (e.g., chopped wine). It also prevents the growth of yeasts that can lead to alcoholic refermentation in barrels or bottles, causing other wine diseases (e.g., flower of wine).

SO2 has a selective effect because not all organisms have the same sensitivity to it. Bacteria are more sensitive than yeasts, so adding SO2 can prevent malolactic fermentation while allowing alcoholic fermentation to proceed. Some yeasts are more sensitive than others, particularly those with a lower alcohol conversion rate and those that produce endogenous SO2. This selectivity is used to delay the start of fermentation, allowing desirable yeasts to become more active and outcompete undesirable yeasts and bacteria. This helps protect wines from the action of certain microorganisms.

2. Solvent Action

SO2 causes the death of cells in the grape skin, promoting the extraction of substances, mainly anthocyanins (with which it combines, causing fading), tannins, and aromas.

3. Antioxidasic and Antioxidant Action

SO2 protects the must from the action of oxygen (O2) in two ways:

  • Antioxidasic action: SO2 prevents enzymatic oxidation by blocking the action of must enzymes like polyphenoloxidase. These enzymes act on phenols in wine and must, altering their organoleptic characteristics, including color and taste. Examples include tyrosinase, an enzyme found in grapes, and laccase, an enzyme produced by the fungus Botrytis cinerea.
  • Antioxidant action: SO2 prevents chemical oxidation by combining with O2 to form sulfuric acid (H2SO4), thereby protecting other substances from oxidation. The formation of H2SO4 also lowers the pH.

4. Taste Improvement

SO2 reacts with acetaldehyde, a compound that gives wine a rotten apple flavor and contributes to hangovers, improving the taste of wine and preserving its flavor and freshness.

4. Chemical and Biological Deacidification

Deacidification is performed to achieve an optimal pH in wine. It can be carried out by biological means, such as alcoholic fermentation (which decreases acidity through the action of yeasts), malolactic fermentation, or winemaking techniques like carbonic maceration.

Sometimes, chemical deacidification is necessary. Products used for this purpose include potassium tartrate, potassium bicarbonate, and calcium carbonate. The first two are preferred because potassium is the most abundant cation in wine. Calcium carbonate has the drawback of enriching the wine with calcium, which can promote the precipitation of neutral calcium tartrate. This precipitation occurs later than with potassium tartrate, and the thickness of the crystals is slower to develop, but its solubility has little influence on alcohol and temperature.

The mechanism of action of potassium bicarbonate and calcium carbonate is similar. Both salts react with tartaric acid, causing it to precipitate as potassium acid tartrate or neutral potassium tartrate. The maximum deacidification limit is 1 g/L of total acidity expressed as tartaric acid. The doses required to lower total acidity by 1 g/L are 0.66 g of calcium carbonate or potassium bicarbonate, and 1.5 g of neutral potassium tartrate.

Deacidification can be performed at various stages: in the must before fermentation begins, in stabilized wine, and during wine development. The timing should consider the effects of alcoholic fermentation on acidity, as well as the potential for cold stabilization of the wine.