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.
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.