By: Thomas J. Payette, Winemaking Consultant
In a recent symposium the speaker stood at the podium. He started his talk saying “If stranded on a deserted island and the island happened to have grapes planted on it, and he were only allowed just three things he thought he could make some excellent wine.” These three items were:
• A wine glass
• An excellent pH meter
• An accurate and precise tool to measure SO2
The above statement gave each attendee at that conference an idea of how important the measurement of pH is to making wine. Understanding the relationship of pH to each wine is very important for a winemaker to better understand how to handle a wine and to predict where a certain wine’s development may be heading. Using the experiences drawn from past winemaking developments, one will soon understand how pH is a major factor to consider when making wine.
What is pH?
pH is a numerical measurement of the acidity or alkalinity of a chemical solution. The measurement is of the hydrogen ion activity and it is logarithmic. This understanding is all the reader needs at this time. Going much beyond this understanding may only bore the readers and discourage them from moving forward in their reading. For more in depth knowledge of pH – feel free to research it.
What is a normal pH Range for a wine?
Most wines range between a pH of 2.90 and 4.00 with the large majority of wines ranging between 3.20 and 3.80. Red wines will often be more near 3.40-3.80 and whites wines will normally be in the range of 3.20 – 3.58. Some wines will stretch these limits but most will conform. Winemakers must intervene, as needed or desired, to bring the wine back into the proper pH range.
What Influences the pH of a Wine?
The grape condition: how those grapes are grown, potassium levels in the grapes, maturity level and skin contact after harvest (crushing) will be some of the largest influences on the wine pH.
Grape condition: Wine grapes that have been allowed to hang will often show an increase in pH moving toward a pH of 4.00 and sometimes higher. Most winemakers get increasingly nervous as pHs near 3.70 on red wines unless the tannin and anthocyanin structure are intense enough that the wine has some protection. This protection is often from the antiseptic 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 and other factors will affect the pH of the wine. These relationships need to be studied further to be conclusive, but many factors may be controlled in the vineyard to help achieve the pH desired at harvest. Much of this is related to canopy management. Fruit that is grown in dense canopies with greater shading are more likely to come in at a higher pH than their exact same counterpart with hedging and some light exposure on the fruit zone.
Potassium Content: The potassium content of the grapes may have an influence on the pH of the wine and more importantly the ability of the winemaker to adjust the pH. When grapes are produced 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 for fruit maturation needed has shown to increase the level of potassium in the fruit. This potassium may result in higher pH’s in the mature fruit but more importantly offer a buffering capacity in the wine. This buffering capacity will be a challenge to a winemaker dealing with a wine that has a relatively high titratable acidity and a high ph. The winemaker may do a tartaric acid addition trial to a batch of wine then find more acid is needed than predicted to achieve the pH desired. This is a result of the buffering capacity resisting movement of the pH with each incremental addition of acid. This is one of the many reasons winemakers should find themselves in the vineyard as much as possible working with the vineyard manager to show the vineyard crew how a wine may get out of balance and not have chemical stability with the higher pH.
Maturity and Malic acid: The maturation process of most wine grapes is another factor that tends to affect the ph. As wine grapes mature, more malic acid, especially in warmer climates, will respire leaving more tartaric as the principal acid. Winemakers watching this rise in pH on the fruit are urged not to worry too much. This decline in malic acid does result in a higher pH of the fruit at harvest but it will often result in less of a shift of the pH upward after malo-lactic fermentation. The reason is there is less malic in the wine to be consumed by the malo-lactic 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 wine making as well. The reader should keep in mind this malic shifting process happens in warmer climates mostly and less so in cooler climates.
Skin contact: Skin contact is another less mentioned factor when speaking of influencing the wine pH. Juice in contact with the skins after crushing will start to show a rise in the pH soon after crushing. This process will be more rapid and 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 off a white wine grape or red wine grapes are crushed, an alcoholic fermentation is usually started soon after with yeast. Alcohol has a higher pH than sugar so it stands to reason that as a wine ferments the pH will start to rise. Often, one will see a shift upward of about 0.15 to 0.22 in pH so a juice starting at a pH of 3.2 could result in a wine of about 3.40. This tends to stabilize while under a pH 3.65 as the tartrates precipitating out of solution will help counteract the pH shift upward from the alcoholic fermentation. Restating – the pH will rise upward during alcoholic fermentation and then drop while cold stabilizing the wine often bringing our pH back to near the original value at harvest in the juice in this example.
pH Shifts During Malo-Lactic Fermentation
After alcoholic fermentation, depending on the wine being made, the winemaker may want to induce a malo-lactic fermentation. This is often to reduce the acidity of a wine and to soften the wine by reducing the malic acid content to levels of unfermentable malic acid left over. The malic that is converted will transform to lactic and provides a pH shift upward in most cases. Many wines will shift upward by about 0.20 ph units. This is directly linked to how much malic was present prior the malo-lactic fermentation and how much was consumable by the Malo-lactic bacteria. Quantitatively measuring the malic acid prior to inducing the malo-lactic fermentation will give the winemaker a better rough idea as to how much pH shift to expect once completed.
3.62 Bifurcation
As noted in a previous addition of The Grapevine Magazine in the “Cold Stabilization article” a very interesting phenomenon does exist in grape wine when the pH of the wine is near roughly 3.62. This is more directly related to how the pH will react during cold stabilization as potassium bitartrate crystals form and precipitate out of solution. When a grape wine is below 3.62 and potassium bitartrates fall out during crystallization and precipitation the wine pH will go down. Conversely – when a wine pH is above 3.62 the pH will shift upward while the tartrates fall out. Keep in mind this may happen in portion during the alcoholic fermentation as the alcohol level goes up.
pH and Sulfur Dioxide
The use of Sulfur Dioxide is closely related to the pH value obtained on a certain wine. Wines with lower pH values will need less free sulfur dioxide added to them in order to do the job needed. Conversely wines at a higher pH may need more SO2 to protect them properly. Another interesting factor with sulfur dioxide does exist on a very small level. When sulfur dioxide reacts with oxygen it will form H2SO4 or sulfuric acid. This can have a very small effect on the pH of a wine, especially longer aging reds, in the wine production cellar.
pH and Sorbic Acid
Sweeter wines will often have potassium sorbate (sorbic acid) added to them to prevent any refermentation issues with that wine. These sorbates may be used in conjunction or the absence of sterile filtration. When using sorbates one may lower the amount used if the pH is lower. An example may be: a clean “yeast free” wine at pH 3.10 may only need 150 ppm sorbic acid to avoid a refermentation issue. This is closely linked to the SO2 value measured at bottling and post bottling. Oxygen pick up must be moderate to little in order to preserve the bottled wine. Sulfur Dioxide and Sorbic acid work together to prevent yeast fermentation.
Aging Ramifications
Wines at lower pH values will generally age slower than wines of higher pH values. This is true of wines treated the same. A red wine at a higher pH value may soften and age faster. Some reds may develop a brickish color on the edges. Try to avoid this and treat wines with a higher pH in a delicate fashion. Use protective oxygen avoidance measure to help slow the rate of oxidation. Conversely wines with lower pH may age slower yet may be handled best as well in the absence of oxygen.
pH and Micro-organisms
The higher the pH of a certain wine, the greater the chance that wine may succumb to a potential unwanted wine microbe. At higher pH’s a whole host of other bacteria may grow in the wine ranging from lactobacillus, dekkera, acetobacter, brettanomyces and Low Acid Bacteria just to name a few. So, as our wine pH values increase, we open ourselves up almost exponentially to other potential bacteria issues. Many of these can greatly change the quality of a wine in a negative fashion. When monitoring your wine pH’s in the cellar – be aware of which wines are higher and keep a special eye on those wines.
Mouthfeel
To build confidence with your palate, learn to relate how the mouthfeel is at certain pH’s. One cannot taste pH yet one can taste acidity and make general assumptions about the pH. A nice balanced wine may actually have a higher pH lending toward a broader mouthfeel. This is true with Chardonnay as well as Viognier. Many reds may do well with higher pH’s if the body, tannins and structure support to give backbone to the palate.
Measuring pH
The reader is encouraged to obtain a very nice, precise and accurate pH meter with automatic temperature compensation for his or her lab. Calibrate this unit often and most likely daily when used. This is a critical measurement in winemaking and obtaining the proper measurement is imperative to make sound winemaking decisions. Be sure the sample is free of any Carbon Dioxide during measurement because that may interfere. The meter should measure to the 100th for example 3.46. Invest in a great meter and use it often.
Summary
Keep an eye on your pH’s while growing your grapes and making wine. This tool is of greater importance than most winemakers may think. Managing the grape’s pH in the vineyards can be a first step and taking action in the cellar should be
the next protocol. Get a firm understanding of how the pH’s react to each process in your cellar mentioned above to make the best wine possible.
The next issue will be on How to Raise the pH of your wine. Till then – be well.
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.
