August 1, 2014

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Measuring Sugar in wine

Learning how to measure sugar in your grapes, juice, and wine is the most fundamental analysis that winemakers learn. It is sugar that will be converted to alcohol by your yeast, so an accurate measurement in the vineyard and in the juice or must at harvest can give you a good estimate of your wine’s potential alcohol. It is rarely the case that your wine will have too little alcohol – early harvest Riesling in Germany often has final alcohol levels between 7 and 10%. Even with alcohol levels this low, the wine’s low pH helps to keep it stable against microbes. Proper sanitation through the vinification process will ensure a clean and crisp wine. If your potential alcohol is too high, on the other hand (> 14%), your fermentation may struggle towards the end, depending on the type of yeast involved.

Refractometer

The first tool often used to measure sugar is a refractometer. I won’t go into too much detail on it’s use as it’s pretty straightforward.  A drop of juice is placed on a quartz surface at one end of the instrument, and you look through the sight glass on the other end. The sugar in the juice will cause light to bend at a certain angle, depending on the quantity. The refractometer measures this angle and contains a scale corresponding the the quantity of dissolved sugar in the mixture. The scale is typically given in °Brix measurement (% sucrose by mass – ie grams sucrose/100 g of solution). It is important to realize that this tool will only give you an accurate measure of your sugar when used in juice. Once your wine starts fermenting, any reading will be inaccurate due to the fact that alcohol has a higher refractive index than water. If there is any alcohol present when using a refractometer, your brix reading will be artificially high. On more than one occasion, a winemaker will discuss their vinification process with me using the term “brix.” Often it’s used when discussing residual sugars in their wines, or perhaps the level of sugar remaining when the wine was pressed. This always causes me to cringe a bit inside, because I know that if they are using Brix to measure remaining sugar in their wine, there is no doubt that the measurement is incorrect.

Once fermentation begins, a hydrometer should be used to measure the specific gravity of your wine. Often hydrometers come with more than one scale on the side. Many times, there is a scale used to measure Brix. Again, °Brix is a measurement of the percentage of sugar by weight in your solution. Because alcohol weighs less than water, measuring your °Brix by specific gravity will give you an incorrect measurement of the actual amount of remaining sugar if there is alcohol in the solution. A hydrometer is not capable of determining the amount of alcohol present in a solution. So, depending on the sugar you started with, the percentage of alcohol can vary by a few degrees with the same quantity of sugar remaining. If alcohol is present when you measure °Brix by specific gravity, the number you get for your brix measurement will be lower than it actually is. If you have a hydrometer with a Brix scale, it should only be used when measuring the sugar quantity in grape juice. You should not be using it to track the fermentation of your wine.

During fermentation, one should use the hydrometer’s specific gravity scale. Tracking your specific gravity will help you determine how quickly the sugar in your wine is being converted into alcohol. All hydrometers are calibrated at 20°C, so you should also measure the temperature of your wine and correct your specific gravity based on the temperature. Your hydrometer should come with a temperature correction chart. Take your reading by looking at the bottom of the meniscus and line it up with the corresponding numbers on the scale. Another common error is measuring a must that contains lots of particles of skins or pulp. This will interfere with your measurement. Carbon dioxide can also push the hydrometer up in your graduated cylinder, so be sure to take your reading quickly if your wine is fermenting.

Once the s.g. falls below 1.0, you know that there is less sugar in the wine than alcohol. It DOES NOT mean that your wine is now “dry,” but it is getting close to dryness. This is another mistake that I’ve come across over and over again. I’ve had people come to me wondering why their wine started re-fermenting in the bottle. They insist that the wine was dry when they bottled it, and when I ask how they measured the residual sugar I’m told that the specific gravity was less than 1.0. Remember that the specific gravity is the result of a mixture of mainly water, alcohol, and sugar.  If your alcohol is very high, you can still have quite a bit of residual sugar left in your wine and still have the s.g. fall below one. In most cases, there is still 2% residual sugar – a sufficient quantity to cause re-fermentation at a later date. Another serious issue is starting malolactic fermentation (MLF) with this much residual sugar. The bacteria responsible for the conversion of malic acid to lactic acid also love to munch on sugar (like most any living creature). The problem is, unlike yeast, bacteria will convert sugar to acetic acid, which increases the wine’s volatile acidity. Thus if you have a wine destined to undergo MLF, you should be certain that it is dry.

Once the specific gravity drops below 1.0, another test is needed to measure residual sugar. Many home winemakers use Clinitest for this purpose. Some commercial winemakers may also use it as a quick way to estimate remaining sugar in the wine. It was once an important tool used to measure residual sugar in the urine of diabetics. It is a fairly simple test: a few drops of wine are placed in a test tube with a tablet that reacts strongly with the liquid. The tablet’s reaction with the sugar causes a color change that is then compared with a standard color strip that indicates the percent of sugar in the solution. The downside of Clinitest is that it can be difficult to measure the color change in red wines.  Also, because an eye-dropper is used to measure your wine sample, you cannot count on your results to be accurate. It is a good idea to run the test several times so you can be confident in your results. Wine with a residual Sugar that is < 0.5% can be considered dry. It is rare for a wine to have zero sugar at the end of fermentation.

If you have access to a spectrophotometer, enzymatic analysis of residual sugar is one of the best and most accurate ways to determine the quantity of sugar left in your wine. When sending a sample to a lab for analysis, this is likely the method that they use. I highly recommend that any winery interested in doing their own lab analysis invest in a spectrophotometer. It is one of the most important pieced of equipment for wine analysis. Click the link above to get a great article from Cornell University on the many uses of a spectrophotometer. A basic model for wine and juice analysis can be purchased for less than $1000, and will open the door to a whole new range of testing capabilities.

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.)

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.