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Wine Reducing sugars

During alcoholic fermentation, yeast consume sugars found in the grape juice or must and convert it to ethyl alcohol and carbon dioxide. In the case of certain styles of wine such as semi-sweet or dessert wines, some sugar is allowed to remain post-fermentation. This residual sugar can serve to provide a sweeter character to the final blend or play a role in microbial stability.

The primary fermentable sugars found in grapes are glucose and fructose. These two simple sugars are also known as reducing sugars because they contain functional groups capable of being oxidized under certain conditions. A winemaker interested in confirming the residual sugar content of a product post-fermentation, or a finished wine product, can use a redox titration to facilitate the oxidation and analysis of these sugars.

For this titration, an alkali solution of copper complex, known as Fehling’s reagent, is combined with a sample of wine. Catalyzing the reaction with heat, the reducing sugars present reduce the copper from Cu(II) to Cu(I). Potassium iodide is added to reduce any excess Cu(II), resulting in iodine as a product. The iodine, present in an amount equal to the residual Cu(II), is then titrated with sodium thiosulfate to determine the original amount of residual sugar present in the wine sample. The results are reported as g/L of reducing sugar.

Hanna Instruments offers titration systems along with a photometer for measuring reducing sugars in wine.


Titration Systems

A variety of titrations systems are available including a version that is programmed with only wine methods. The reducing sugar method is pre-programmed along with free and total sulfur dioxide, acidity and yeast assimilable nitrogen. Titration systems are available with 1 or 2 analog input boards that allow multiple sensors to be used. These systems can also be configured to have one or two dosing pumps. Having multiple dosing pumps allow for the use multiple titrants.

For high throughput some of the titration systems can be connected to an auto sampler for automated analysis of multiple samples.



Portable Meters

A portable photometer is available for the determination of the concentration of reducing sugars in wine. In this method a sample of wine is added to a vial that contains a pre-dosed amount of reagent. The sample is then digested in a test tube heater. After digestion and cooling the meter is used to measure the color change that occurs and then displays the concentration.





A variety of ORP electrodes are available that include application specific versions made specifically for grape juice and must. These versions feature a ground glass junction with a plastic sleeve. These features are part of Hanna CPS (clogging prevention system) technology to prevent clogging of the reference junction by solids found in grape juice and must.



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