(Glucose + Fructose) / 17 Using glucose + fructose analysis to predict potential alcohol provides a more accurate measurement of the levels of fermentable sugar compared to using ºBrix. Note that in ripe fruit, glucose + fructose numbers often appear higher than the corresponding ºBrix results. The reason for this is that ºBrix is measured as a percentage by weight, meaning ºBrix values are greatly influenced by the density of the juice, while glucose + fructose is measured as weight by volume and is independent of juice density. An official conversion rate formula used in Europe is: Potential Alcohol (% vol) = glucose + fructose (g/L) / 16.83. In practice, rounding the 16.83 conversion factor to 17 is common.
The Reality While client feedback suggests that glucose + fructose values improve the quality of their predictions, it is important to remember that yeast populations and fermentation conditions
may vary, and any prediction of potential alcohol is only an approximation. Alcohol conversion ratios are always subject to variability, so it is possible your actual alcohol may be lower or higher than the estimate. Many clients have found that the conversion rates observed for their own yeasts and fermentation conditions generally remain relatively constant, and use their internally observed historical conversion rates to calculate potential alcohol content based on their glucose + fructose values. With white wines, predictions are usually fairly accurate. When making red wine, however, using a truly representative juice sample can be a challenge and can affect potential alcohol predictions. A juice sample taken soon after a tank is filled may not take into account un-popped berries, often unripe (containing less sugar and more acids) and raisins (sometimes an overlooked source of large amounts of sugar, acid and potassium). We suggest sampling after an initial 10°Brix drop, and analyzing the fermenting sample for glucose + fructose and alcohol simultaneously for a more accurate potential alcohol estimate.
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