November 21, 2014

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High Total Acidity AND high pH?! How to handle it…

One of the reasons that grapes have been used to make wine for thousands of years is that they are one of the few fruits in the world that contain large concentrations of tartaric acid. The strength of acids is measured by their ability to shed protons – or more specifically, hydrogen ions (H+). Without going too deep into a chemistry lecture (which I’m sure will lose most of you in a few sentences), when you measure the pH of your wine, you are measuring the concentration of these ions – that’s what the big ‘H’ in pH stands for. The tricky thing to remember is that while pH is a measurement of H+, the formula for its calculation causes the pH to be inversely proportional to the H+ concentration. Thus, as the H+ concentration increases, your pH decreases.

So what is the big deal about pH? Because tartaric acid is relatively strong, it works to keep a wine’s pH near 3.0, which in turn keeps the wine stable against microbes. This is one of the reasons why wine made from grapes has flourished around the world: it doesn’t spoil easily, and acts as an antiseptic. The combination of ethanol and the acidic environment are extremely inhospitable to most microbes. In an indigenous yeast fermentation, after the wine hits 5-6% alcohol, one yeast will dominate the fermentation: Saccharomyces cerevisiae or S. bayanus. After the sugar is depleted, there isn’t much left in the wine to act as a food source for microbes that are capable of surviving in those harsh conditions. Lactic Acid bacteria, if present, will begin to consume the malic acid (transforming it to lactic acid), while Acetobacter species are capable of turning ethanol into acetic acid (vinegar). However, Acetobacter needs oxygen in order to do this, so as long as you keep your containers full, you don’t need to worry much about them.

This year, like in 2010, we saw problems with high pH in many of our wines, but we saw it especially in Marquette. The most likely explanation is that Marquette grown under certain conditions has an excess of potassium, which can drive up the pH. Malic acid concentration likely also plays a role in increasing the pH, since it is a weaker acid that in turn is converted to an even weaker acid (lactic acid) in red wine vinification. In any case, the high pH is worrisome and steps need to be taken to ensure that the wine remains stable.

Sulfur Dioxide Addition. While it is still possible to limit microbes with sulfur addition when the pH creeps up to 3.8, you need to use substantially more SO2 as your pH increases. Most of the sulfur you add to wine becomes bound to sugars and other compounds in your wine. The rest of the sulfur exists as “free” or unbound SO2. At a pH of 3.4, you should aim for 35 mg/L of free sulfur in your wine in order to be sure that it’s protecting your wine against microbial spoilage. However, at a pH of 3.8, you’d need nearly 90 mg/L of free sulfur to get the same protection. Considering that the legal limit for TOTAL sulfur in your wine cannot exceed 400 ppm, one can see how maintaining a high free SO2 rate can quickly make it possible to exceed that limit. Though it’s possible to keep your wine clean with a high pH, it isn’t easy. One should consider a pH greater than 3.8 the breaking point where acidification becomes necessary.

Wine Sensory. The pH has a huge effect on the color of red wine, as it affects the colored pigments. If you start to keep track of your wine color and corresponding pH, it becomes almost possible to predict your wine’s pH based on color alone. A high pH wine will lose the vibrant red tones, and become more of an eggplant purple color. Low pH wines will have a bright pink rim and vibrant red hue. Differences occur between grape cultivar, of course, but generally if you observe the rim of color at the edge of the wine when you tilt your glass, if it’s purple then the pH is high. High pH wines also have a tendency to be described as “flabby” or “flat,” however it is difficult to say whether or not that holds true when the wine has a corresponding high total acidity, like we often see in Marquette. In Riesling, wines with equal sugar/acid ratios can taste sweeter at a higher pH.

Cold Stabilization. Wines with a pH greater than 3.65 should not be cold stabilized. When wines are cold-stabilized, the goal is to precipitate potassium bitartrate crystals so that they don’t fall out of solution in the bottle. Above pH 3.65, this salt acts like an acid. So, by removing an acid from the solution, it causes your pH to increase. However, if the wine’s pH is LESS THAN 3.65, cold stabilization will help to LOWER your pH. Below this point, potassium bitartrate acts as a base, so removing from solution causes the solution to become more acidic. Pretty cool, huh?

What we were faced with this year. The Marquette grapes that were harvested this year arrived at the winery with a pH of 3.6, but also had a total acidity of almost 1.0%! Knowing that the pH would increase during skin maceration (potassium is extracted from the skins), and again during malolactic fermentation, I acidified the must at harvest with tartaric acid at a rate of 0.2%. This brought the pH below 3.5. During Malolactic fermentation, we saw the pH creep up again to 4.0, so we were forced to once more acidify the wine to make it stable.

So here’s where a decision needed to be made: how much tartaric acid should we add? The total acidity was around 0.65%, which is pretty good for a red wine. Adding too much tartaric acid would make the wine tart and unpalatable. If I was working in a commercial winery, these are the options I’d see:

1) Acidify with Tartaric Acid. Aim to get the pH to 3.8, and hope that the tartaric acid additions didn’t make the wine too tart, then avoid cold-stabilization. A rule of thumb to use when acidifying:  1.0 g/L of tartaric acid will generally lower the pH by 0.1 (this is a guideline, of course… to be accurate, always perform bench trials before making a large addition).

2) Acidify with Tartaric Acid. Aim to get the pH below 3.65 and KNOW the wine was going to be very tart, but then cold-stabilize. With this option, the cold-stabilization will further lower the pH another 0.1 to 0.2 points (depending on the potassium bitartrate concentration). Then, working at a pH of 3.4-3.5, we will have room to remove the tartaric acid using chemical deacidification methods. Chemical deacidification comes with the worry of losing some of the aromatics, so bench trials should be performed to determine the amount of additive works best for the individual wine.

3) Blend the wine with a lower pH wine (of course do bench trials to see if you like the blend). This of course is still an option if you choose option 1 or 2, especially if you find the wine is still too tart. Blending is one of the the real arts in winemaking.

4) Use an anion exchanger. However, while an ion exchanger is available on the commercial scale for wineries, the cost of the equipment isn’t practical unless your last name is Mondavi.

We went with option #2. Since we are an experimental winery, blending is not an option. If I went with the first option, the amount of tartaric acid needed to get the wine under a pH of 3.8 made the wine too tart.  The wines were acidified with 4 g/L of tartaric acid, which brought the pH down below 3.6 (and the TA above 1.0%), and they are now chilling  at 28°F. I’m hoping that cold stabilization removes 1-2 g/L of total acidity, and we can use potassium bicarbonate to remove an additional 1-2 g/L.  In the end, I’m hoping that nearly all of the added tartaric acid that was added to the wine can be removed, and we’ll be left with a wine that has a healthy pH between 3.6-3.8, with a palatable TA around 0.6%.

 Results Post Cold-Stabilization

To recap what we did to this high pH/high TA juice:

The Marquette fruit arrived at the winery and was separated into 6 different lots for trials.

The Total acidity at crush ranged from 8.5 – 9.1 g/L (0.8-0.9%), and the pH was around 3.6

At Crush, we added 2 g/L of Tartaric acid to bring down the pH during maceration and fermentation on the skins (I anticipated an increase in pH during fermentation).

Post malo-lactic fermentation, the pH had risen to 3.9-4.0 and the total acidity was averaging 6.5 g/L

We added 4 g/L of tartaric acid to bring the pH below 3.6, and cold-stabilized.

Final wine pH post cold-stabilization (avg of 6 lots) = 3.44

Final Total Acidity (avg of 6 lots) = 7.7 g/L

In the end, we had added a total of 6 g/L of tartaric acid. We see that most of that addition dropped out during fermentation and cold-stabilization. Our final wine has a low enough pH that we can do some tweaking to the acidity via carbonate additions if we find that necessary.

Sulfur Dioxide as an Antimicrobial

Sulfur Dioxide (SO2) has many benefits in winemaking. It acts not only as an antioxidant and antioxidasic (inhibits oxidative activity by enzymes), but also as an antiseptic. It is extremely important to accurately measure the sulfur levels in your wine, as it’s effectiveness against microbes changes in function with your free sulfur concentration and the wine’s composition. Excessive SO2 not only is a health concern, but it will also inhibit the bouquet of your wine. It first neutralizes some aromatic compounds, but as your concentration increases a noticeable burning sensation will be felt on both the nose and the palate. Here, I will focus on how to determine what quantity of SO2 to use in your wine to make sure that it is stable against microbes while remaining undetectable.
First, some chemistry. When you add SO2 to wine (most wineries either add it as a compressed liquid or in the salt form: Potassium Metabisulfite), not all of it will be actively working to protect the wine. Most of it gets bound to other elements in the wine: proteins, aldehydes, anthocyanins, and sugars (we’ll discuss this a bit later). It’s in it’s unbound, or “free” form where it goes to work.
Free Sulfur exists as two forms: molecular SO2 and bisulfite. As the pH decreases (the wine becomes more acidic), a greater percentage of the free sulfur exists as molecular SO2. This is the part that inhibits microbes (yeast and bacteria). In other words, SO2 is more effective at lower pH, so you need to add less. As the chart below shows, at a pH of 3.0, 6% of the free sulfur in your wine is molecular SO2, as the wine pH increases, less than 1% of your free sulfur is actually working to protect your wine against microbes!
The guideline to follow when adding sulfur is that you need at least 0.8 mg/L of molecular SO2 to inhibit the development of bacteria and yeast in wine. So, at a pH of 3.0, your free sulfur should measure at least 14 mg/L, while at a pH of 3.9 your free SO2 should be 109 mg/L. The above chart is extremely important for winemakers. I would suggest copying it and posting it to the wall in your lab or winery. Now, knowing that the legal limit of total SO2 (free and bound) in US wine is 350 ppm, one can see how it may be difficult to adequately protect a high pH wine from spoilage microbes during aging. Another important number to be aware of is the sensory threshold of SO2 (in other words, at what level the majority of people will be able to smell it). Literature states this value as 2.0 ppm molecular SO2. Many winemakers will add more than 0.8 molecular SO2 in order to be on the safe side, especially when a wine contains residual sugar. If you are planning on doing this, make sure that you are not passing this threshold value – especially as you approach bottling. Immediately after fermentation this is less important, as much of your first SO2 addition will end up bound. You can also expect your free sulfur levels to drop over time, so an initial big dose after fermentation usually isn’t as worrisome as a large dose later on. Also keep in mind  that dry red wines that have completed malolactic fermentation need less protection against microbes than wines that still contain sugar and malic acid. Even though 350 ppm is the maximum dose allowed, think of this value as the maximum for sweet wines. Dry red wines are often safe with less than 100 ppm of total sulfur.
Remember that most of the sulfur you add to your wine will NOT end up as free sulfur. Most of it ends up bound to other substrates in the wine, notably acetaldehyde, anthocyanins, and sugars. This is another reason why a larger initial dose of sulfur is recommended, especially in sweet wines. Studies also show that a larger initial dose will end up with you using less sulfur over time. Some of the bound sulfur is stable – meaning that the binding reaction is irreversible. Much of the bound sulfur is unstable, which means that it can revert to free sulfur again.
Because each individual wine will go through various equilibrium reactions between the bound and free forms, it is important that you measure your free and total sulfur levels a few days after you make an addition in order to make any readjustments. The various interconvertible states of sulfur dioxide are difficult to predict. Therefore, you should  continue to measure and readjust levels on a regular basis. Only when all the free carbonyl* compounds in your wine have combined with SO2 will your free sulfur content show a direct linear relationship with what is added. I would recommend checking the sulfur on a monthly basis in wines containing residual sugar, and on a quarterly basis in dry wines.
Finally, while SO2 is one of the best antimicrobial agents available to add to wine, it is important to understand that it works by inhibiting the metabolism of yeast and bacteria. It does not control certain spoilage yeasts, and does not directly kill yeast and bacteria. It is therefore not effective as a means to stop fermentation or spoilage by itself. Spoilage yeasts such as Brettanomyces spp. and Zygosaccharomyces bailii have been shown to have high tolerance to SO2. The best protection is prevention. Once spoilage bacteria and yeast start to develop in the wine, SO2 doesn’t work well at eliminating them. Thus, good sanitation and maintaining proper levels of SO2 throughout the winemaking process are important in keeping wines free from spoilage. Also be aware that even wine with no added sulfur contains small amounts of SO2, as yeast produce it naturally as a byproduct of fermentation.
One more final point: when adding sulfur in the form of Potassium Metabisulfite (PMS), don’t forget that it contains 57% sulfur by weight. Therefore, 100 mg of PMS contains 57 mg of sulfur. 
So, the calculation for sulfur addition with PMS would be as follows:
[Desired free SO2 concentration (ppm) – Actual Free SO2 concentration (ppm)] x 1.75 x [volume of wine in L] ÷ 1000 = grams PMS to add
If you want to know how to convert this equation to grams PMS/gallon of wine, I’ll refer you here to learn more about using metric and English units together.
* a carbonyl compound contains a carbonyl functional group – a Carbon atom double-bonded to an oxygen atom: C=O. Examples in wine include acetaldehyde, sugars, esters, anthocyanins…
For further reading