Sugar in wine?

Recently while playing on twitter, someone asked a question that got us thinking (we know, rare occurrence), and rather than just shoot from the assumptive hip, we went about looking for an actual factual answer. Taking some time to talk to a couple local winemakers the answer became clear as mud (technical term).

The question is 'how does a consumer know how much residual sugar is in the wine they are drinking?' Well the simple answer is, read the label.

Righty o there smart arses you say. So what is residual sugar and how is it measured?

First off, residual sugar concentration is expressed in grams per liter (g/L) or as a percentage of weight to volume. For example, a wine with 0.2% residual sugar contains two grams of sugar in a liter of win (bottle is 3/4 liter or 750ml). Dry wines are typically in the 0.2–0.3 percent range, off-dry wines in the 1.0–5.0 percent range, and sweet dessert wines in the 5.0–15 percent range. (got to love the metric system) Sounds simple right? Well as a measurement it is, but where things get murky is in determining actual real world percentage.

There are a number of ways residual sugar can be measured. But wait. Returning to our initial question, what is residual sugar? We’re now faced with the question of which types of sugar are going to be counted in this residual amount. Wait, what, this was going to a simple story on measuring something as simple as sugar when in reality we need to determine what is being measured, simple put it in your coffee sugar, or, glucose, fructose, sucrose, pentoses or are we just talking about sugars that yeast can ferment? Blimey Charlie how easy was it to go down that rabbit hole? Of course next up it depends on how residual sugar is to be measured. The most common way to measure sugar in wine only measures reducing sugars like glucose and fructose, but not sucrose. In most cases that means that all of the sugar in the wine is being measured, but there is a notable exception: chaptalization. When winemakers add sugar to must, they often add it in the form of sucrose, or more simply, stick it in your coffee table sugar, because sucrose is cheap. If the yeast dose not digest all of that sucrose (after breaking it down to its component parts, glucose and fructose), the wine may contain more sugar than the standard measurement would suggest. But we digress. So there is another big, huh? 

Next up there is the term Brix. This is a term that is used for expressing sugar content in grape juice but generally not wine and the first tool often used to measure sugar is a refractometer. 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. Simple, and often used to help determine when grapes are to be picked but not for measuring sugar in wine. Now how are we going to measure residual sugar in fermenting wine in a commercial setting. One of two methods seems to be primarily used. For a reading that is plus or minus a percentage or two there is the hydrometer. 

Using the hydrometer’s specific gravity scale, tracking specific gravity will determine how quickly the sugar in wine is being converted into alcohol. All hydrometers are calibrated at 20°C, so measure the temperature of the wine and correct the specific gravity based on the temperature. The hydrometer should come with a temperature correction chart. Another risk to accuracy is measuring a must that contains lots of particles of skins or pulp. This will interfere with the measurement. Carbon dioxide can also push the hydrometer up in the graduated cylinder, so take the reading quickly if the wine is fermenting. Also as the wine gains alcohol this changes the specific gravity and affects the reading and so must be adjusted for. Again though a not very accurate plus or minus a point or two. To avoid the variable of alcohol there is a method that has you boil off the alcohol and replace with water. Time consuming and a pain, but a lot more accurate.

Lastly it seems that the solution the the accuracy issue is to use a spectrophotometer. The spectrometer, enzymatic analysis of residual sugar is one of the best and most accurate ways to determine the quantity of sugar left in your wine and the method used by most if not all accredited labs.

Last but certainly not least is the fact that printing labels a scheduling bottling takes time and often the labels are ordered prior to a good final accurate test (assuming one is done) is done so an 'accurate approximation' is put on the label.

As it turns out this simple subject is not as simple as we first thought and as we discovered the answers are pretty much all answered with the precursor, 'it depends but generally speaking this is sometimes the general rule of thumb sometimes' kinds of non specific answers. So truth in labeling now comes into play. Struth this was supposed to be a simple answer to a simple question. Maybe the following will help!
Here are some specific rules from the ATF Ruling 78-4 for your reading pleasure and some followup to it from the Code of Federal Regulations.

After a few hours of chatting, and a few more to research and check some facts and terms, we have come to the conclusion of 'if we like it, who cares!' as all this stuff is giving us a brain ache! We are off to get a glass or three to clear our heads!

Happy Wine Adventures,

Cheers,

WineWalkabout

Kiwi & Koala