About º Brix, Glucose + Fructose, and Potential Alcohol
The amount of fermentable sugar (glucose + fructose) in juice and the average conversion rate of sugar into alcohol can be used to help predict the potential alcohol level in the corresponding fully fermented wine.
Why Choose Glucose + Fructose instead of º Brix for Alcohol Predictions?
°Brix provides a quick estimate of sugar content, but it is actually a measure of all the dissolved matter in must, not just the fermentable sugars. Using °Brix, which is not a true measure of fermentable sugar, adds an additional layer of uncertainty to alcohol predictions.
The glucose + fructose analysis provides more accurate sugar levels than °Brix for a given sample regardless of fruit conditions. Differences or discrepancies between °Brix and actual fermentable sugar content are more pronounced in immature fruit and in fruit affected by fungal growth.
How do Glucose + Fructose Compare with º Brix?
It might seem surprising that glucose + fructose numbers often appear higher than the corresponding °Brix. The reason for this apparent anomaly is that °Brix is measured as a percentage by weight measurement so °Brix values are influenced by the density of the juice. Glucose + fructose is measured as weight by volume and is independent of juice density.
This helps explain why °Brix (% by weight) appear to increase more slowly than glucose + fructose (weight by volume) during grape ripening and why the equivalent glucose + fructose results usually are higher than °Brix for sugar values greater than 21-23 °Brix.
Conversion Ratios for Potential Alcohol
|ESTIMATED CONVERSION RANGES
||ETHANOL (% Vol)
||10.0 - 10.9
||10.6 - 11.5
||11.1 - 12.1
||11.7 - 12.7
||12.2 - 13.3
||12.8 - 13.9
||13.3 - 14.5
||13.9 - 15.2
||14.4 - 15.8
The conversion rate used by the European Union is:
Potential Alcohol (% vol) = glucose + fructose (g/L) ÷ 16.83.
Keep in mind that since actual conversion rates vary with yeast properties and fermentation conditions, the calculation of potential alcohol is only an approximation. Alcohol conversion ratios are always subject to variability during fermentation so your alcohol may be lower or higher than your potential alcohol estimate.
This calculation provides a good starting point for estimating the potential alcohol content of a wine. Clients have found that the conversion rates observed for their own yeasts and fermentation conditions generally remain relatively constant and use their internally observed rates to calculate potential alcohol content based on their glucose + fructose values.
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