Grape processing: managing acidity and oxygen
In the fourth part of our articles on pre-fermentation steps, Westgarth Wines wine specialist Maurizio Broggi delves into managing acidity and oxygen.
Depending on the chemical composition of the grapes and the winemaker’s stylistic goal, the grape must may go through a number of treatments before fermentation. Some of these treatments may be necessary to compensate for nature’s deficiencies such as insufficient sugar levels or excessive acidity, while others, such as clarification in white wines, may be part of standard winemaking procedures to achieve a specific wine style.
Acidity and pH greatly contribute to the structure and balance of a wine. If the acidity is too low, this could make the wine flabby and lacking freshness, whereas too much acidity results in unbalanced, sharp wines. Furthermore, adjusting to an appropriate pH level is essential in winemaking as a lower pH contributes to protecting against spoilage microorganisms and makes sulfur dioxide more effective.
Depending on climate and specific vintage conditions, acidity in wine may be increased (acidification) or decreased (deacidification). In many countries, acid adjustment is regulated by law, which means there are limits to how much a wine can be acidified or deacidified. In Europe, it is forbidden to acidify grape must that has been also enriched. Depending on the style and grape variety, acid adjustment aims to achieve a total acidity (titratable acidity) of 5 to 8 g/l and a pH between 3.1 to 3.4 for whites and 3.3 to 3.6 for reds.
In warm to hot conditions, acidity in grapes is broken down more rapidly during the growing season, as such grapes may be harvested with an insufficient level of acidity. That is why acidification is usually allowed in warm to hot regions such as southern Europe, California, or Australia. In moderate climate regions, acidification may be allowed and more commonly practiced only in particularly hot vintages, such as in 2003 in Europe.
Tartaric acid is the preferred acid for additions as it is considered the most ideal in the way it affects the taste of wine. The process of acid adjustment usually takes place before fermentation. Sometimes, acid levels may also be fine-tuned and tweaked at the final blending stage before bottling.
In very cool climates, such as in Germany or Canada, or particularly cool vintages, grapes may be harvested with extreme levels of acidity and very low pH. In such cases, deacidification may be necessary to balance the wine by reducing the overall acidity and increasing the pH. Deacidification is commonly achieved by chemical methods. Lowering acids is, however, a less straightforward process compared to acidification. Instead of being physically removed, acids must be neutralized by a chemical reaction. Potassium bicarbonate (HKCO3) or calcium carbonate (CaCO3) can be used to neutralize acidity in grape must by lowering the levels of tartaric acid. Potassium bicarbonate is usually the preferred addition because it is faster, and the wine is more tartrate stable than with the addition of calcium carbonate. Double salt deacidification is another chemical method that is typically used only for a portion of the grape must. It works by lowering both levels of tartaric and malic acids by completely neutralizing the acidity. The neutralized portion of the must is then added back to the untreated must.
Deacidification can also be obtained naturally post-fermentation through malolactic conversion. Malolactic conversion converts the sharp malic acid into the softer lactic acid. While it is almost always practiced in red wines, it is not always ideal or desired in white wines because of the flavor change associated with malolactic conversion (e.g. butter).
Depending on the composition and quality of the grapes, other additions to grape must may be necessary before fermentation begins. Yeasts need nutrients to reproduce and grow, particularly nitrogen. Nitrogen-deficient grape musts may result in slow or stuck fermentation which may lead to foul-smelling hydrogen sulfides. The lack of nitrogen in grape musts can be easily addressed by adding diammonium phosphate (DAP). Sometimes, the addition of vitamins may also be necessary. Vitamin B1 (thiamine) is critical for healthy growth of yeasts. Vitamin C (ascorbic acid), added in combination with sulfur dioxide, is used as an additional protection against oxidation.
Oxygen management in grape must
Oxygen in winemaking plays both a positive and a negative role. It is necessary to promote healthy fermentation so that yeasts can reproduce and grow successfully, however, significant exposure to oxygen at the wrong time can be detrimental to wine quality as it leads to oxidation, especially in white and rosé wines.
How oxygen is managed during winemaking, including during the grape must stage, determines the final style of the wine. Wines made excluding as much oxygen exposure as possible are known as being made in a reductive style, which is commonly practiced in white and rosé winemaking. Reductive conditions are achieved by adding sulfur dioxide to protect against oxidation, and by flushing tanks and pipes with inert gasses, such as nitrogen or argon, to remove any oxygen present. Cool temperatures during processing and storage of the must are also fundamental as reactions with oxygen are accelerated by warmer temperatures.
Hyperoxidation is a technique that some winemakers adopt in white musts whereby grape must is deliberately exposed to oxygen to promote the oxidation of the most easily oxidizable phenolic compounds. This technique makes the wine more resistant to oxidation, less bitter and ultimately provides a more stable color and longer shelf life. Some grape varieties such as Chardonnay and Viognier seem to benefit from this technique. It should be noted, however, that this practice removes some of the most delicate aromatic compounds and some winemakers consider it to be detrimental for certain aromatic grape varieties such as Sauvignon Blanc.
Hyperoxidation is achieved using the same equipment used for flotation. An amount of oxygen or air is blown through the must from the bottom of a tank. The addition of sulfur dioxide at the must stage is avoided to allow this process to work. After exposure to oxygen, the grape must initially turns dark brown and the most fragile oxidized phenolic compounds will precipitate during fermentation. Eventually, part of the oxidation is reversed during fermentation, turning the wine’s color bright again.
Fundamentally, must additions and treatments are critical decisions a winemaker has to make to improve grape must composition, and therein lies one of the many balancing acts of winemaking. But these are also highly dependent on the desired stylistic preferences and, ultimately, quality level.
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