PH is the measure of the acidity or alkalinity of a solution. The lower the pH, the higher is the release of Hydrogen ions concentration by the free acids and the greater is the acidity of the solution. The main sources of acidity in wine will be the tartaric, malic, and lactic acids which release 50%, 35%, and 20% of their ions respectively. Tartaric acid has the lowest pH level and lactic acid the highest.
A balanced pH is of great importance for the final wine as it will affect color, taste, balance, and ageing ability. Both the viticulturist and the winemaker can have a direct influence in the pH of the resulting wine. Not only during wine making itself, but also during must treatment and during the growing season in the vineyard.
IMPORTANCE OF PH
The final level of pH has several effects on the balance and the taste of the final wine. As a measure of acidity, it will influence the final balance of the wine in all color. For white and/or sweet wines, the acidity will reduce the sweet taste of the wine. It will be necessary to balance the alcohol and the residual sugars of the final wine. In red and/or rosé wines, acidity will reinforce tannins and increase their astringency. The total contents of salts in wine will interact with the acids and change the degree of acidity of the acids, making them taste more or less sour depending on their levels.
Low pH will have an important stabilizing effect on color and be a key factor in the prolongation of the life of the wine. For red wines, it will have a direct effect on anthocyanins and make the red color brighter and more durable over time. It will also act to reduce the browning of the wine phenolics and thus preserve colors during the ageing process. Low pH releases a high concentration of ions in the wine and this will have the effect of keeping a higher proportion of free sulphur dioxide in non-ionized form thus increasing its effectiveness. Non-ionized SO2, its molecular form, has protective properties and will counter against the effect of oxygen, thus increasing the life of the wine. Finally, low pH has an antibacterial effect and helps to protect the wine against microbial spoilage.
In the vineyard, the viticulturist will have many options available to help altering pH levels. The most important factor influencing the final acidity of the wine will be the weather and the timing of harvest will be instrumental to the final pH level of the resulting wine. The riper the grapes, the lower will be the acidity of the must. Therefore, picking grapes at slightly unripe levels can help keep the pH low.
All other factors being equal, different grape varieties will have different levels of natural acidity at harvest. A high vigor vineyard and one highly fertilized with nitrogen will produce higher levels of malic acids which is higher in pH than tartaric acids. High levels of potassium in the soils will have a neutralizing effect on pH and increase the final pH level. By keeping weeds or cultivating cover crops in the vineyard can help assimilate the excess nitrogen from the soil. Low vigor rootstocks will reduce the negative effects of potassium as will the reduction of shading by careful canopy management.
In the winery, the winemaker will also have many options to alter the final pH levels of the must. As we saw, harvest date will greatly influence the final levels of acidity in the must. However, it will be possible for the winemaker to acidify with addition of tartaric or malic acid in case the must is found to have a pH beyond desired levels. This is a common practice in hot regions such as Barossa Valley in Australia or in Maipo Valley in Chile. However, if the pH of must is found to be too low, is will be possible to deacidify by the addition of calcium carbonate or potassium bicarbonate. These methods are often used in cool climate after bad vintages especially in the Loire and Chablis in France. However, these methods remove mainly the tartaric acids and leave the sharp tasting malic acid.
During fermentation, it will be possible to alter pH in several ways. For red wine, the maceration with the grape skins will increase the levels of salts by increasing potassium in the resulting wine. This will have the dual effect to increase pH and make the wine taste sourer. However, during fermentation, the yeast will synthesize various organic acids with lower pH such as malic acids. This will have the effect of increasing the proportion of tartaric acids in the resulting wines and therefore lowering its pH.
In the final blend, it will also be possible to alter pH at the desired level. By blending grapes of different acidity, it will be possible to obtain the proper balance. The best example is the use of Sauvignon Blanc in the blend with Semillon in Sauterne. The Sauvignon provides freshness and acidity to balance the sweetness of Semillon, especially when affected by noble rot. Blend can b made from grapes of the same variety picked at slightly different time during the harvest period. Some grapes might be picked more of less ripe to provide the final blend with more or less acidity as desired.
Finally, before bottling, it will be possible for the winemaker to make final adjustments in the final acidity of the wine either by acidification or by deacidification as above.
As we saw, pH level is important to the balance and taste of the final wine. It can also affects key decisions during wine making. It is therefore important for the viticulturist and the winemaker to take into consideration during vine growing as well as wine making. Many options are available both in the vineyards and the cellar. It is even more important in countries with marginal climate whether they be too cold or too warm to attain natural levels of balanced acidity.