Table sugar has the scientific name sucrose. If we dissolve sucrose in water and add yeast, nothing happens. This simple experiment shows us that not all sugars can ferment. In the case of sucrose, one must add yeast that contains the enzyme invertase which will “invert” sucrose into fermentable sugars so that fermentation can begin.
It is not just that some sugars are not fermentable; it is that they are not all equally fermentable.
Some Theory and Names
Sugars that are based on a 6 carbon atom unit are called hexose. Let it be stated that other sugar lengths also exist. Rye bread, for example, contains a significant amount of pentose sugars (sugars that are based on 5 carbon atoms). This composition partially explains why rye cannot be baked like wheat, but requires a sourdough process.
A common property of sugars is their high solubility in water. All sugars are also heat sensitive. This is important to remember when we prepare highly concentrated sugar solutions. It is also important to know that almost all sugars do not want to crystallize, as honey shows convincingly. Bees would have a serious problem if they had to mine sugar from their own hive.
To finalize the above description, it is important to mention that not all sugars are equal. Not all sugars are equally sweet, and some lack any sweetness at all (starch, for example). Some sugars have a “straight” sweetness, and some a “funny” sweetness. This sweetness will alter the taste of the finished wine.
Fruits are dominated by fructose, glucose, and sucrose, in this order. In most fruits, the fructose to glucose ratio is near 1, with slightly more fructose. Apples and pears have a fructose to glucose ratio of about 2, which is extreme. In grapes, this ratio is 1.1.
Measuring Sugar Solutions
The density of sugar solutions changes significantly as the concentration of sugar increases, and this worsens with the fact that sugar concentrations can become very high. Interestingly, the concentration / density relationship of sugar solutions is linear, which makes calculations simple. A common calculation method is measuring the density of the solution, then measuring the concentration. A concentration increase of 100 g/L increases the density by roughly 0.0368 g/mL. This increase is independent from the chemical nature of the sugar (at least for sucrose, glucose, and fructose, the most important sugars in winemaking).
The density of wine is usually measured with a hydrometer (not to be confused with a hygrometer). In vineyards, a refractometer is used, because it requires only a drop of juice from a single berry. Obviously, this can cause sampling errors, because one cannot assume that each berry has the same concentration of sugar. Therefore, the concentration of sugar for several berries is taken to determine the average concentration.
Since density can easily be measured, it has been used to measure the sugar concentration of grape juice for almost two centuries. These measurement units deserve to be mentioned not only because of their historical significance, but also because they are still occasionally used.
Sugar Density Scales
Some German-speaking countries (Germany, Luxemburg, Switzerland) use the Oechsle scale, abbreviated ° Oechsle or ° Oe. This scale was developed by Christian Ferdinand Oechsle (1774 – 1852), a citizen of the Grand Duchy of Baden (now part of southwest Germany).
The Oeschle scale relies on an areometer, a device that measures the gravity of liquids or the density of solids. Oechsle put out this invention in 1836 with the publication Über den Gebrauch der Most- und Weinwaage (“On the use of must and wine balances”).
The Oechsle scale uses the excess weight in grams that one litre of juice (must) has.
Let us assume that a hypothetical litre must weighs 1,084 g, so that the excess weight is 84 g. Therefore, this must has 84° Oe. Obviously, the density of said must has to be 1.084 g/ml, so the density scale in modern units lines up nicely with the antiquated (but still used) Oechsle scale.
As one might expect with a sacred German tradition, this country that otherwise is strict in its use of SI (metric) units does not just sanction the use of the Oechsle scale, but even enshrines it in some laws, like in the legal relationship between the natural alcohol content of wine and Oechsle degrees, published by the Federal Department of Justice in 1995.
With no apparent connection to Oechsle, in 1843, Carl Joseph Napoleon Balling (1805 – 1868), an Austrian chemist from Bohemia (part of the Austria-Hungarian Empire at the time) developed the saccharimeter, an areometer adapted to measure sugar concentrations. The saccharimeter revolutionized the brewing process by providing an efficient way to measure fermentable sugars, rather than solely relying on the colour caused by malt.
The term saccharimeter was used for the longest time for density measurement, but has taken a new meaning over time and has more recently been used for sugar measurement based on refractive index. The current use of saccharimeter is for measurement by polarization, with the advantage of providing a method that is independent of alcohol content. These three different methods must NOT be confused, and it is regrettable that a scientific term once established has been recycled this way.
Balling’s invention took a life of its own when August Wilhelm Freiherr von Babo (1827 – 1894), the first director of the Niedere Stiftsweinbauschule in Klosterneuburg, Austria developed the Klosterneuburger Mostwaage (KMW) scale based on Balling’s saccharimeter. The KMW scale seems to be in continued use in the countries of the former Austria-Hungarian empire, including the now successor countries of the former Yugoslavia. For example, the Czech Republic and Slovakia continue the use of KMW with the new name normalizovaný moštoměr (NM).
1° KMW = 4.86° Oechsle
Italy, Canada, USA, Australia, and New Zealand prefer the Brix scale, developed by Adolf Ferdinand Wenceslaus Brix (1798–1870). It seems that after the Oechsle scale and the KMW scale (which both preceded the Brie scale), the German-speaking world had no need to adapt another unit of measurement developed by a German. As such, this scale has never received more than casual attention in German-speaking countries (with the exception of the candy industry). English-speaking countries however, embraced the Brix scale; perhaps the name is easier to pronounce than “Oechsle” or “Klosterneuburger Mostwaage”.
Unlike the Baumé scale, the Brix scale has inherent practical value, because 1 °Brix is defined as the density of the solution of 1 g sucrose in 100 g sucrose solution in water. This links the Brix scale and the Oechsle scale with the following approximation:
1 °Brix ≈ 4 °Oe
List of Sugars
Glucose is the most significant sugar base for winemaking. Yeast prefers glucose the most among all sugars, and you should always give fermentation a running start by providing a sufficient amount of glucose, either as such, or by using invert sugar syrup (see below).
Glucose has a sweetness factor of 0.75, relative to sucrose.
You can find out more information about glucose and other sugars in the Chemicals Section.
Simple Sugar Syrup
It can be difficult to pour solid sugar into a wine barrel and dissolve it. Simple syrup is a convenient way to add sugar to wine.
To prepare 1 L of Simple Syrup, 600 g of water is required and 640 g of sugar.
Bring water to a boil, and stir in sugar while continuing to heat. Bring the water to a boil again, then let it cool slightly, and skim off the whitish crust that forms (these are just residual proteins from the manufacturing process). Let the syrup cool completely.
If the simple syrup must comply with Swiss or German pharmacopeia, the syrup must contain 640 g sugar per litre of finished syrup.
CAUTION: This is not the same as using 640 g sugar and 360 g water due to the change in density! The density of this syrup is approximately 1.242 g/mL, which is why the proportion shown above must be followed.
You can make a more Concentrated Simple Syrup with a density of 1.373 g/mL. 1 L of this syrup contains exactly 1 kg sugar, providing an easy measure. This syrup however, is rather viscous, so mixing it into a batch of wine is more difficult than mixing in standard simple syrup.
To prepare this syrup, 373 ml water are required per kg of sugar. Concentrated simple syrup is often used to prepare liqueur, but for liqueur, inverted syrup is usually preferred.
While heating sugar syrup, one must avoid overheating (indicated by the syrup yellowing) because overheating adds an undesirable caramel touch to the syrup.
Owing to its high sugar content, simple syrup has a long shelf life, without the chance of becoming mouldy. As a general rule, the higher the sugar content, the longer the shelf life.
Invert Sugar Syrup
The inversion of sucrose takes place when the temperature of the mixture reaches above 100 °C. The inversion is accomplished by preparing concentrated simple syrup as explained above, but then adding about 3 g citric or tartaric acid to the 1 kg of sugar. The amount of acid is not critical because it just acts as a catalyst. Take special care not to overheat the mixture, because glucose and fructose caramelize easier than sucrose.
NOTE: Invert sugar is always preferred for fermentation, because it already contains the glucose that yeast requires.
Alcohol in this context is used to describe a chemical characteristic, and is not to be confused with alcoholic drinks. Certain sugar alcohols are found in nature, and for this reason, deserve to be mentioned.
Xylitol is a sugar alcohol that is related to the pentose sugar xylose, and is industrially produced from corn cobs after their kernels have been taken out. Xylitol’s sweetness is similar to that of sucrose and very little is resorbed by humans, making it attractive in certain food applications (e.g: low calorie foods, anti-cavity candies), despite its high price. Xylitol can be fatal for animals such as dogs, goats, and rabbits. There seems to be no known toxic effect for humans, other than possible diarrhea when excessive amounts are consumed. Xylitol is naturally found in vegetables (cauliflower) and fruits (prunes, strawberries).
Sorbitol is a sugar alcohol that is related to glucose and fructose, and is industrially produced mainly out of glucose. It can be found naturally in rather high concentrations in rowan berries, and can also be found in pears, prunes, plums, apples, apricot, and peaches. Industrially, sorbitol has various applications, from substituting sugar for diabetics to retaining moisture in toothpaste. Sorbitol is a hexose and contains 6 carbon atoms.
Mannitol is another sugar alcohol found in nature. It is named after the manna ash, a plant which can be found mainly around the Eastern Mediterranean. The dried syrup of the manna ash contains 13% mannitol. Mannitol is an isomer of sorbitol, and also contains 6 carbon atoms.
Micro-organisms can digest most sugar alcohols, but yeast has a clear preference for glucose.