August 14, 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.

Balancing Sugar and Acid to make a more food-friendly Minnesota Wine

One of the biggest challenges we face in Minnesota is trying to make well-balanced wines from grapes that often have less-than-ideal chemistry at harvest. Deacidification is nearly always obligatory (either by chemical or biological methods), and stopping fermentation early to leave residual sugar (or back-sweetening wines) is often done in order to balance the high acids. Creating balanced wines from Minnesota fruit is not easy. The trend at the moment seems to be making wines on the sweet side. I would argue that it is easier to make a sweet wine than a balanced wine. Another advantage to creating sweet wines is that they are fairly easy to sell. This is something that is difficult for people who’ve been in the wine industry a long time to readily admit. The vast majority of people who drink wine in the US are new wine drinkers, who prefer sweet, fruity wines. That’s ok… I’ll admit that boxed white zinfandel is how I first became acquainted with wine. It’s a style that’s more approachable than the dry, acidic, or tannic wines. However, it’s the dry, acidic, and tannic wines that make the best food wines, and this is a style that I would like to see more of in the state of Minnesota. I’d also like to throw out this thought: just as many people grow-out of drinking Light Beer and Kool-Aid, many wine drinkers start to move toward drier and more bitter-tasting (tannic) wines over time. So, many of those “new” wine drinkers who prefer sweet wines now, may prefer a drier style down the road.
Why make food-friendly wines? While I understand that the majority of Americans don’t sit down to dinner with a glass of wine, I still think it is important that winemakers in the US strive to make wines that can be enjoyed at the dinner table. Here’s why: whether it’s sitting down to delivery pizza or a meal prepared with some love and effort, for most of us, eating is a time to relax. It’s the one time of day where we can be alone with our thoughts, or be joined by friends and family. When we have a good dining experience it tends to be a memorable experience – even if it’s simply pizza with friends. In fact, I’d argue that the reason pizza is a popular food comes down to the fact that it is usually something that is ordered for parties or events. Plenty of other foods are as easy and simple (and can even be delivered), yet pizza is often the first food you think to order when a group of friends get together. Pizza is associated with fun. Now, imagine if wine had the same association for the general American public? For many people, wine is associated with fancy dinners or special occasions (anniversaries, weddings, holidays…). What if wine were a part of everyday occasions?
According to the most recent Wine Market Council survey, 20% of the US adult population consumes 91% of the wine in our country. Of that “core” group of wine drinkers, only 9% consume wine on a daily basis. Considering that about 3.4 trillion bottles of table wine (678 million gallons) were consumed in the US in 2010, imagine how just a 1-2% increase in daily consumption would affect total wine sales in the US?  Or, imagine if some of the marginal wine drinkers (14% of the US adult population), started drinking wine on a regular basis?

For many people, the idea of pairing food with wine is daunting. They think they need special training, or that they aren’t that sophisticated. The fact is, most people already have had experience with good food and beverage pairings their whole lives. It doesn’t take a sophisticated palate to experience the pleasure of warm cookies with a glass of cold milk, or perhaps the satisfaction of salty pretzels and beer. We can imagine that something tart and acidic like lemonade would taste awful with cookies, or that something syrupy sweet just wouldn’t be right alongside a juicy steak. If wine is thought of as more a condiment or seasoning, then it makes wine and food pairing less daunting. Imagine squeezing a lemon over fried fish. Now think of drinking a nice dry, acidic white wine with that same piece of fish. See, easy!

When the titratable acidity in your wine is high enough where fermenting it dry would make it taste more like biting into a lemon than drinking Chablis, it makes sense to leave some sugar in the wine to make it less tart. It has been known for quite some time that a high-acid variety like Riesling can retain some residual sugar and still taste dry. The International Riesling Foundation (IRF) has done great work determining what ratio of sugar to acid is needed in order to have a wine that tastes dry, medium-dry, medium-sweet, etc. This could be a good guideline to follow with some of our high-acid varieties. So, if you have a wine with a pH of 3.1-3.2, and a TA of 10 g/L, according to their guidelines (see chart below), the wine would taste dry as long has you had less than 10.0 g/L of residual sugar (1.0%). However, as this chart was made with riesling in mind, I can’t say whether it will work perfectly with our varieties. A riesling with a TA > 10 g/L would be considered high in acid, while we are lucky to see La Crescent or Frontenac lower than 12 g/L. **

That’s not to say that sweet wines don’t pair well will food. It’s just that it can be more difficult to find a wine with the right kind of sweetness to balance-out the meal. For example, if you’re serving something savory (like pork), that has a sweet element (baked apples), a wine that has the same level of sweetness as the apples could be a good compliment (say, an off-dry gewurztraminer?). If, however, you pair something that is much sweeter, the balance is thrown off. You have the wine competing with the food rather than complimenting it. Slightly sweet wines can also help tame the heat in spicy foods (like Thai or Indian dishes), but go too sweet and the wine will take center stage. However, when serving a sweet wine with dessert, you want the wine to be as sweet or even sweeter than the dessert. Sweet wines can also be a good contrast to salty foods like blue cheese (however they don’t always work well with hard cheeses). The best way to learn what works with a particular wine is to try a food and wine together.

There are plenty of people who enjoy drinking Minnesota wines, yet even with our wonderful local food movement, many restaurants serving locally-grown food have yet to serve that wine made with locally-grown grapes. The Minnesota Landscape Arboretum will be hosting a wine and food pairing event in September this year, with local chefs choosing their favorite food-friendly wines to pair with a dish of their own creation. Wineries are asked to submit their best food-friendly local wines to the Minnesota Landscape Arboretum Learning Center by June 6th to be vetted by local chefs. I encourage local wineries to participate, as I believe this is the next logical step for Minnesota wine – to be seen at the Minnesota dinner table.

**Remember that this is a taste profile, and a wine that retains more than a few g/L of sugar is technically not considered “dry.” This is an important distinction to make, as a wine that is not technically dry still risks refermentation. Red wines that you intend to put through malolactic fermentation should be technically dry. Lactic acid bacteria will metabolize remaining sugar into acetic acid – resulting in a wine with a vinegar taste.)

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

Potassium Sorbate as a Wine Preservative

Potassium Sorbate (K-sorbate) is a relatively recent wine additive (it only first started to be used about 50 years ago), used primarily as a preservative to help prevent re-fermentation of sweet or semi-sweet wines. It is widely used in many types of foods ranging from cheese and yogurt to dried fruit and meat. It is even used in cosmetics to help give them a stable shelf life. It is generally considered as safe, having about the same toxicity as table salt.
To be honest, I hadn’t heard much of wineries using K-sorbate before returning to Minnesota, and had never used it in off-dry or sweet wines in either Alsace or Australia, and now I get questions about it on a regular basis. So, I’ve been doing a bit of research on it lately and figured I’d share with you what I have learned.
When added to water, K-sorbate breaks down into sorbic acid (sorbate) and ionic potassium (K). It is the sorbic acid that is active as an anti-microbial. It doesn’t kill yeast cells, but only prevents them from growing and being active. It has no effect on lactic acid and acetic acid bacteria at the amounts added in wine. Therefore, it should only be added to wine that is already stable via its pH and free sulfur. Another important point to remember is that just as sulfur dioxide (SO2) is more active at lower pH, so is sorbate.
Here are some more important points to know:
  • Like SO2, sorbic acid is detectable in wines when it is added above certain levels, though the reported values vary from 135 to 400 mg/L. * 
  • Over time, sorbic acid will be reduced to form ethyl sorbate, which has been described as having pineapple and celery aromas. You cannot prevent this from happening in wine, as this reduction occurs naturally with the presence of ethanol. While these aromas aren’t inherently objectionable, they will mask other fruity aromas in your wine. Some people may consider it a flaw. The concentration of ethyl sorbate will continue to rise over time, and is dependent on your initial sorbic acid concentration. Therefore, sorbate is generally added to wines which aren’t destined to be aged.
  • It will not inhibit bacterial activity. If you add it to wine before bulk storage or bottling, you need to be absolutely certain that the wine is stable. If there is any lactic acid bacteria present in your wine, it will still be there after sorbate is added. Potassium Sorbate must ALWAYS be used in conjunction with proper SO2 addition.
  • If lactic acid bacteria is present in the wine, it will metabolize sorbic acid and produce a chemical that has a strong odor of Geranium leaves and is considered a major wine flaw. 
  • The amount of sugar in the wine has no effect on the amount of sorbate needed. The only concerns are pH, alcohol, and the initial population of yeast cells (which should be less than 100/mL – make sure the wine is very clear before adding K-sorbate).
  • When adding K-Sorbate to wine, remember that it contains about 75% sorbic acid by weight (100 mg of K-sorbate contains 75 mg of sorbic acid).
  • The BATF limits sorbic acid addition to wines to 300 mg/L (the European Union regulations limit its addition to 200 mg/L).
  • Sorbate SHOULD NOT be added to dry red or white wines. There is no risk of refermentation when there is no sugar present. You are only adding the risk of off-odors from ethyl sorbate as well as risking the production of geranium taint.
  • Sorbic Acid is not very soluble in water. Precautions need to be made when adding it to wine to ensure that it is properly dissolved in the wine.
  • Sorbate is not allowed as an additive in production of organic wine
  • Certain countries do not allow the import of wine containing Sorbate
As for the recommended rates of sorbic acid that should be used in wine, there seems to be no clear consensus. The most cited recommendations come from Peynaud (1984), who notes that sorbic acid is half as effective at a pH of 3.5 than it is at a pH of 3.1, but then lists his recommended dosages based on alcohol content. Sorbic acid’s action against yeast is reinforced by alcohol. The following are his recommendations for sorbic acid:
Wine at 10%     150 mg/L
Wine at 11%     125 mg/L
Wine at 12%     100 mg/L
Wine at 13%      75 mg/L
Wine at 14%      50 mg/L
These recommendations by Peynaud assume a pH < 3.5, and adequate SO2 protection. Remember, these numbers are the recommendation for sorbic acid. If you are adding Potassium Sorbate, only 75% is sorbic acid. So you need to divide the sorbic acid amount by 75% to get the equivalent amount in K-sorbate. So 150 mg/L of sorbic acid would mean you should add 200 mg/L of K-sorbate (0.2 g/L).
 
Commercial wineries generally avoid the need to use potassium sorbate because their wines are usually sterile filtered at bottling, so the refermentaion risk is eliminated (sterile filtering means all yeast and bacteria cells are eliminated). Wineries focused on the production of high-quality wines also tend to forgo the use of K-sorbate because they find the aroma of ethyl sorbate to be undesirable. In the end, I can’t make any recommendations for or against it, as it is a preference choice for the winemaker. However, if you are properly sterile filtering your wines, the addition of sorbate is an unnecessary step in the process that comes with risks that should be addressed.
* References:
Auerbach, R. C. 1959. Sorbic acid as a preservative agent in wine. Wines Vines, 40, 26-28.
Ough, C. S.; Ingraham, J. L. 1960. Use of sorbic acid and sulfur dioxide in sweet table wines. Am. J. Enol. Vitic.,11, 117-122
Peynaud, Émile. 1984. Knowing and Making Wine. John Wiley and Sons
Postel, W.; Drawert, F. 1970. Sensory threshold value of sorbic acid in German white wines. Lebensm. Unters. Forsch. 1970,144, 245-252.

Bench Trials – Why you should get comfortable using the Metric System

We’re getting to the point in the year where wineries are thinking about some ways in which they may fine-tune their wines before putting them in the bottle. Perhaps you will be blending different batches together, or doing some fining to soften some of the wines. Maybe you realized that your deacidification wasn’t quite enough or perhaps that wine that you initially wanted to leave about 10 g/L of sugar in ended up fermenting to dryness, no matter how hard you tried to kill off the remaining yeast. Now that it is stable, you want to add back some sweetness. However, in all these cases the final result irreversible so it is important to get an idea of what the final outcome may be before you do it to an entire tank or barrel of wine.

Here are some important tools you will need:

  • a scale that will accurately measure to 0.1 g
  • a 50 or 100 mL graduated cylinder
  • a volumetric flask of 50 or 100 mL
  • a micropipette (ideal) with tips, or a pipette that will accurately measure 0.5 – 1.0 mL
  • a friend to help you taste your trials

Now let’s see why the metric system makes bench trials so much easier.

Let’s say that your wine has too much Hydrogen sulfide (H2S), that reduction aroma produced during fermentation that smells a bit like rotten egg. Only small amounts of copper sulfate (CuSO4) are usually sufficient to remove the offending odor. There is a legal limit of how much copper you may add, and it is very small – only 6 ppm (mg/L). Because excess copper additions can lead to other problems such as haze formation, and copper casse, as well as the fact that any remaining copper in the wine can’t exceed 0.5 ppm by law, you want to use the smallest amount possible to remove the offending odor (here you will find a standard procedure for copper addition bench trials along with others).

To carry out the bench trial, measure out 100 mL of the stinky wine into 4 clean glasses. To make sure the glasses don’t smell like soap, it is a good idea to rinse the glasses with wine before starting. One glass is your control, the next three we will add copper sulfate at the following rates: 5 ppm, 7 ppm, 9 ppm.*

In order to make our addition easier, we will make up a 1% w/v solution of copper sulfate (CuSO4) by measuring 1.0 g into our 100 mL volumetric flask, and filling to the volume graduation mark with distilled water (see photo).

Now the calculation is easy (in the metric system):

Since our solution has 1000 mg CuSO4 in 100 mL of water, each mL of our stock solution contains 10 mg of CuSO4.

1000 mg / 100 mL = 10 mg/L

We want to add CuSO4 to our wine at a rate of 5.0 mg/L (5.0 mg/1000mL). That means that each mL of wine will have 0.005 mg of Copper Sulfate. So, our 100 mL sample of wine will have 0.5 mg of CuSO4.

5 mg / 1000 mL = 0.005 mg/L      0.005 mg/L x 100 mL = 0.5 mg

Now, knowing that 1.0 mL of our stock solution has 10 mg of CuSO4, we can calculate how much of the solution we should add to our wine sample by dividing 0.5mg by 10. We therefore need to add 0.05 mL (50 µL) of the solution to our 100 mL wine sample.

5 mg / (1 mL / 10 mg) = 0.05 mL = 50 µL

This is where a micropipette comes in handy. It allows you to add very small and precise amounts, and you aren’t causing error by diluting your sample. Of course, if you don’t have one, you would have to simply dilute your stock solution by a factor of 10 (1% to 0.1%).

All of those calculations were done in my head. Try doing that working in the Imperial System! It even is easier when it comes time to actually use these trials to make the final addition in your winery. Let’s say you did your 3 trials of Copper Sulfate additon, and you and your tasting buddy found the 5 ppm addition to be the best (ie: the least amount required to be effective in removing the offensive odor). Now, if you know the volume of your tank in liters, the amount of copper sulfate to add to the tank is also a calculation you can do in your head (no need to convert back to the imperial system). If you had 1500 Liters of wine, and want to add 5 mg/L;

5 mg/L x 1500 L = 7500 mg = 7.5 g

Simple.  Try using the metric system in your winery. I’ll bet you never go back! And, if you insist on using some sort of combination of both systems in your winery (like bottling and commerce in the metric, and winemaking in Imperial), I refer you here for further guidance.

*For simplicity, we won’t calculate how that translates to the actual amount of copper that you are adding. Copper Sulfate is most often purchased as a hydrated salt – Copper Sulfate Pentahydrate (CuSO4·5H2O). It looks like a blue crystalline powder. When adding it to wine, you have to remember that only about 25% of the compound is actually copper. So, if you add 5 ppm of (CuSO4·5H2O), you are actually adding 1.25 ppm of actual copper. I also am not endorsing these trial values as the value you should use for your trials. You may find that only 2 or 3 ppm of copper sulfate is sufficient to help rid the wine of H2S. The numbers I’m using are for example only.