The Importance of pH in Winemaking


In a recent symposium, a speaker stood at the podium. He began to speak, telling the audience that if stranded on a deserted island and grapes were planted there, and he was allowed only three items, he could make some excellent wine. The three items were a wine glass, a high-quality pH meter, and an accurate and precise tool to measure sulfur dioxide. In this statement, the speaker gave each attendee an idea of the importance of pH in making wine.

Understanding the relationship of pH to wine is necessary for every winemaker to understand not only how to handle wine well but to predict where an individual wine’s development may be heading. Using the experiences drawn from past winemaking developments, winemakers can understand how pH is a major factor to consider when making wine.

pH in Wine

Simply put, pH is a numerical measurement of the acidity or alkalinity of a chemical solution. The typical pH range for most wines is between 2.90 and 4.00, with the vast majority of wines ranging between 3.20 and 3.80. Red wines will often have a higher pH, between 3.40-3.80, and whites wines will normally be in the range of 3.20 – 3.50. Some wines will test these limits, but most will conform. Winemakers must intervene, as needed or desired, to bring the wine back into the proper pH range.

Influences the pH of Wine

A number of factors influence the pH of wine, including grape condition, how the grapes are grown, potassium levels in the grapes, maturity level and malic acid content, and skin contact during crushing.

Grape Condition

Wine grapes that have been allowed to hang on the vine will often show a pH of 4.00 and sometimes higher. Most winemakers get increasingly nervous as pHs near 3.70 on red wines unless the tannins and anthocyanin structure are intense enough that the wine has some protection. This protection is often from the antiseptic properties and oxygen scavenging properties of the tannins and deep color compounds.

How the Grapes are Grown

Many factors – from the soil, weather, amount of water at the root zone – will affect the pH of the wine. These relationships need to be studied further to be conclusive, but these factors, in particular, can be controlled in the vineyard to help achieve a winemaker’s desired pH at harvest.

Much of this is related to canopy management. Fruit grown in dense canopies with greater shading is more likely to have a higher pH than their counterpart with hedging and some light exposure on the fruit zone.

Potassium Content

The potassium content of grapes has a high influence on the pH of the wine and the ability of the winemaker to make adjustments to the pH.

When grapes grow on a vine, research has shown only a certain amount of leaf surface area is needed to bring the grapes to ripeness. Canopy density and leaf surface area beyond the amount required for fruit maturation has shown to increase the level of potassium in the fruit. A high concentration of potassium may result in a higher pH in mature fruit but, more importantly, offer a buffering capacity in the wine. The buffering capacity will be a challenge to a winemaker when the wine has a relatively high titratable acidity and a high pH. Even after adding tartaric acid to a batch of wine, a winemaker may find more acid is needed than predicted to achieve the pH desired. This result is due to the buffering capacity resisting movement of the pH with each incremental addition of acid. To help combat this issue, winemakers and vineyard managers should work together to show the vineyard crew how to protect grapes from becoming out of balance.

Maturity and Malic Acid

The maturation process of most wine grapes is another factor that tends to affect pH. As wine grapes mature, more malic acid, especially in warmer climates, will respire, leaving tartaric as the principal acid. Winemakers watching this rise in pH are urged not to worry too much. At this time, there is less malic in the wine to be consumed by the malolactic bacteria and therefore less of a shift while converting malic acid to lactic. This principal applies mostly to red grapes, but it can have applications to white winemaking as well. The malic shifting process happens primarily in warmer climates and not as often in cooler climates.

Skin Contact

One of the less-mentioned factors influencing the pH of wine is skin contact. Juice in contact with the skins will start to show a rise in the pH soon after crushing. This process will be more rapid and is directly related to the temperature of the must. For this and many reasons, most winemakers prefer to press just after crushing white wine grapes, and some prefer whole cluster pressing.

pH Shifts During Alcoholic Fermentation

When the juice is pressed from a white wine grape or red wine grapes are crushed, the next step is fermentation. Alcohol has a higher pH than sugar, so it stands to reason that as wine ferments the pH will start to rise. Often, one will see a shift upward of about 0.15 to 0.22 in pH. For example, a juice starting with a pH of 3.2 could result in wine with a pH of 3.40. The pH tends to stabilize, however, as tartrates precipitate out of the juice during cold stabilization when the wine pH is near but below 3.65.

As the tartrates crystallize and fall out, they will help counteract the pH shift upward from the alcoholic fermentation, often lowering pH back near the original value at harvest. Conversely – when a wine pH is above 3.65 the pH will shift upward while the tartrates crystals fall out.

pH Shifts During Malolactic Fermentation

After alcoholic fermentation, depending on the wine being made, the winemaker may want to induce a malolactic fermentation (MLF). MLF reduces the malic acid content by converting it to lactic acid, reducing the overall acidity of the wine and softening it. During this process, the pH of many wines will shift upward by approximately 0.20. This result is directly linked to how much malic was present before MLF and how much was consumed by the malolactic bacteria. Measuring the malic acid before inducing MLF will give the winemaker a rough estimate of what to expect when the pH shifts.

pH and Sulfur Dioxide

Sulfur Dioxide is an important part of winemaking because it protects wine from oxidization and microbial contamination during aging and storing. The amount of SO2 used is closely related to the pH of the wine, as wines with lower pH will need less free SO2 to protect them. Charts are available online or through a local extension office.

pH and Sorbic Acid

Sweeter wines will often have potassium sorbate (sorbic acid) added to them to prevent re-fermentation within the bottle. These sorbates are used in conjunction with, or in the absence of, sterile filtration. In the case that a wine has a lower pH, fewer sorbates are necessary. For example, if a clean, “yeast free” wine has a pH of 3.10, it may only need 150 ppm sorbic acid to avoid re-fermentation.

Aging Ramifications

Wines with a lower pH will age slower than wines with higher pH values. Because of this, wines with a higher pH should be treated delicately and monitored closely for brick-colored edges. It is also recommended that winemakers use protective oxygen avoidance measures to help slow the rate of oxidation.

pH and Micro-Organisms

The higher the pH of an individual wine, the greater the chance to lose that wine to an unwanted wine microbe. At higher pH levels, a whole host of bacteria may grow in the wine. From Lactobacillus, dekkera, acetobacter, Brettanomyces to Low Acid Bacteria, winemakers open themselves up almost exponentially to other potential bacteria issues. Many of these can substantially change the quality of wine negatively. When monitoring the pH of your wines as they age – be aware of which wines are higher and keep a close eye on them.

Mouthfeel

To build confidence with your palate, learn to relate how the mouthfeel is at certain pHs. While pH has no taste, one can taste acidity and make general assumptions about the pH. A nice balanced wine may have a higher pH lending toward a broader mouthfeel, as with Chardonnay and Viognier. Many red wines may do well with a higher pH if the body, tannins, and structure support to give backbone to the palate.

Measuring pH

Winemakers are encouraged to obtain a high-end, precise and accurate pH meter with automatic temperature compensation for their lab. Calibrate the meter often during the off-season and daily during crushing and fermentation, when it will often be used. Critical to the winemaking process, obtaining the proper measurement of pH is imperative to make sound winemaking decisions. Be sure the sample is free of any carbon dioxide (CO2) as this may interfere with the reading.

The importance of pH in winemaking is greater than most winemakers may think. By consistently monitoring the pH of their grapes and juice throughout this process, winemakers will gain a solid understanding of what to expect of their final product, allowing them to make the best wine possible and improve their bottom line.

References:

Amerine, M.A., Berg, H.W., Cruess, W.V. 1972.
The Technology of Wine Making

Dharmadhikari, M.R., Wilker, K.L. 2001.
Micro Vinification.

Zoecklein, B.W., Fugelsang, K.C., Gump, B.H., and Nury, F.S. 1999. Wine Analysis and Production

Verbal conversations with
Jacques Boissenot, Jacques Recht,
Chris Johnson and Joachim Hollerith

Vinquiry website for Sulfur Dioxide chart