|The Gravity of Sugar...how the tool that measures sugar content in wine works||
Champagne gets its bubbles from sugar and yeast added to a tightly sealed bottle of champagne resulting in a second fermentation. The carbon dioxide from the fermentation is trapped in the bottle and is released as bubbles when a bottle of champagne is opened. In the early production of champagne, wine makers faced the difficult challenge of estimating the amount of sugar to add. If the wine maker added too little sugar the wine did not have the commercially valuable prise de mousse or bubbly character we know today. However, if too much sugar was added the internal pressure could build high enough to rupture the sealed bottle causing injury to the workers and expensive loss of wine. This guesswork made the production of champagne extremely risky until the technological advance of the sucro-oenomètre.
In 1836, a chemist named M. François from Châlons-sur-Marne published his “Nouvelles observations sur le fermentation de vin en bouteilles, suivies d’un procédé pour reconnaître la quantité de sucre contenu dans le vin immédiatement avant le tirage”. ("New observations on bottle fermentation followed by a new process to measure the sugar level in wine before tirage") In this work Francois introduced the sucro-oenomètre. The sucro-oenomètre facilitates the accurate determination of the quantity of sugar to add to the wine to achieve the prise de mousse (about 6 atmospheres* of pressure). *An atmosphere is a measure of pressure. One atmosphere is the pressure we experience walking around on earth at sea level.
One might suspect that a good wine maker could simply taste the wine to determine the sweetness and adjust the sugar levels on this basis. However, the taste sensation of sweetness depends not only on the sugar level but the acidity as well. An acidic wine may have more sugar than a less acidic wine but taste less sweet. The sucro-oenomètre finally took the guesswork out of bubble regulation and propelled the champagne industry forward as a viable economic venture. T. Stevenson noted that the use of sucro-oenomètre known as the “reduction François” was rapidly and readily accepted in many wine-growing regions throughout Europe and especially in Germany. Ironically the winemakers of champagne reluctantly accepted the the use of sucro-oenomètre and it took until the 1860s before its use was commonplace in Champagne.
The sucro-oenomètre is an inexpensive piece of test equipment—a sealed glass tube with a weighted end and calibrated markings. Simple and elegant in design and usually made of blown glass, the sucro-oenomètre consists of a bulbous, weighted bottom with a long thin stem. (See figure) Etched on the stem of the instrument is a calibrated scale ranging from 980grams to 1100grams. The scale represents the specific gravity of the solution being measured by the sucro-oenomètre. Specific gravity means the weight of one liter of the solution. Water has a specific gravity of 1000g because one liter of water weighs 1000grams.
To measure the specific gravity of wine the winemaker places the sucro-oenomètre in a sample of the wine, letting the device sink until the bobbing stops. The reading is taken where the surface of wine meets the etched scale on the sucro-oenomètre. Water with sugar dissolved in it is denser than pure water. Freshly pressed grape juice can be 15-23% sugar (weight/volume). The specific gravity or grape juice before fermentation will be greater than 1000grams. Based on this measurement the winemaker will calculate the amount of sugar need to add to the wine to produce the desired alcohol content or carbon dioxide for the bubbles. M. Francois not only described the sucro-oenomètre but also prescription for the sugar needed for the prise de mousse.
The purpose of this article is to explain how the sucro-oenomètre works—how does floating a glass bobber in wine tell how much sugar is in the solution? To understand this one must think about how things float and what does buoyancy tell us about the liquid an object is floating in.
Why do things float? When an object is immersed in liquid such as wine, the weight of the object pushes it down due to gravity. (Gravity is the invisible attractive force that all objects exert on one another.) As gravity pulls an object down in wine, wine is pushed out of the way. But the wine pushes back with a force equal to the weight of the wine displaced (pushed out of the way). The object, such as the sucro-oenomètre, sinks until its weight is equaled by the up thrust of the wine, then it floats. Thus, the etched scale on the neck of the sucro-oenomètre affords a precise determination of how deep it sinks in the wine or, in other words, the weight of the wine pushed out of the way.
Notice the key actions of wine being pushed out of the way by the sucro-oenomètre and the wine pushing back. Denser wine pushes back harder than less dense wine. A good example of this phenomenon that you may have experienced while swimming is the difference between salt water in the ocean and fresh water in a lake or pool. The denser salt water makes it easier to float than fresh water. It is actually hard to sink in very salty water like the Great Salt Lake. The same holds for wine. Denser wine with more sugar in it will float the scuro-oenomètre higher, more of the neck will stick out of the wine than more watery wine. In essence, the sucro-oenomètre weighs the solution it is in. The winemaker could also very carefully measure one liter of wine and weigh it on an analytical balance and determine the same thing. However, sucro-oenomètre does the same thing without the need for careful measuring of volumes or an expensive analytical scale.
Ultimately, the sucro-oenomètre works on the general principal of buoyancy and does not explicitly tell the winemaker there is sugar in the wine. The winemaker must assume from the source of the juice and its taste that sugar is making the grape juice or wine denser than water. This is why the sucro-oenomètre is but one function for this type of device. The sucro-oenomètre has many more functions and is also known more generically as a hydrometer. The same principles hold and this device can be calibrated differently and be used to measure alcohol content of wine or the activity of battery acid. This simple but elegant device accurately determines the density of solutions with important applications across the world of chemistry and as an invaluable tool for the champagne industry.