Chemistry Friday; Measuring sugar content in Degrees Brix

October 7, 2011



One of the most important parameters that is analyzed for in the winery lab during harvest is sugar content; we use it to determine ripeness and to monitor the progress of fermentation, both of which are critical to the winemaking process. At Amapola Creek (as in just about every other winery in California) we express sugar content in terms of degrees Brix.

Imagine you have one hundred milliliters of water (around half a cup). When there is no sugar in the water, we say that it is at 0 degrees Brix.

Now imagine that you have one gram of sugar (a very small spoonful), and you dissolve it in enough water that the final sugar solution has a volume of one hundred milliliters. This would be 1 degree Brix.

If there are two grams of sugar in one hundred milliliters of solution, it would be 2 degrees Brix, and so on. One gram of water (at 20 degrees C) is equivalent to one milliliter, so the Brix scale is a way of expressing what percentage of the solution (by weight) is sugar. A solution that is 10 degrees Brix would be made of ten grams of sugar and ninety milliliters of water, a solution that is 50 degrees Brix would be made of fifty grams of sugar and fifty milliliters of water, etc., etc.

Most grapes when they are ready to harvest are somewhere in the neighborhood of 25 degrees Brix (roughly 25% sugar by weight, this is only approximate because of course there are other compounds, like natural acids, that are dissolved in the juice as well). This can vary a few degrees Brix in either direction depending on the vineyard, weather, and style of wine to be made, but as a rule of thumb 25 degrees Brix is a good approximation.

When measuring the sugar content of juice that is not fermenting yet, such as vineyard samples or samples from grapes that have just been crushed at the winery, we use a device called a refractometer.

The denser a liquid is, the more it will bend the light that passes through it. A refractometer works by measuring how much light passing through the liquid is bent, and then inferring the density (and by extension the sugar content) from that measurement. Many refracs use a simple setup of lenses and a prism to make the measurement, but we happen to have this nice digital one.
Once the juice has started to ferment, however, the refractometer will no longer work, because bubbles of CO2 interfere with the path of the light as it passes through the juice. To measure the Brix of fermenting juice, we use a device called a hydrometer.

Hydrometers are neat. They are designed so that you float them in a container of juice, and then read the number off the scale printed on the stem at the point that it emerges from the surface. The one in the picture above is floating in water, so if you were to look at the point where the stem emerges from the surface, it would read '0'. If there were sugar in the water, the water would be denser and therefore displace more of the mass of the hydrometer, pushing more of it up out of the liquid and changing the point at which the stem and and the liquid surface meet, giving a higher reading.

 Hydrometers are quick and easy to use, but you do have to be careful of certain sources of error. If a sample is fermenting very hard, the CO2 bubles can lift the hydrometer up and give an artificially high reading. Similarly, if the sample has a lot of solids in it, it can become viscous enough that hydrometer readings can be artificially high.

One more interesting thing to note about degrees Brix is the impact of alcohol on measurement. Alcohol is less dense than water. So, in wine, where the sugar is gone but there is plenty of alcohol, the Brix readings will actually be negative. A negative Brix is when we usually decide to schedule pressing, because it implies that the sugar is largely used up.
That’s it for now, see you Monday!
To visit the Amapola Creek Winery main website, please click here.

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