D-Glucose and D-Fructose (GluFru)

D-Glucose and D-Fructose (GluFru)D Glucose and D Fructose (Glu Fru) represent the vast majority of naturally occurring sugars in grapes. They are fermentable and can be oxidized or reduced to form other derivatives in winemaking that are fundamental to wines matrix. Together, Glu Fru can be used to estimate a more accurate potential alcohol of an unfermented must or characterize dryness at the end of fermentation. A symptom of many stuck fermentations is residual D Fructose due to a

D-Glucose and D-Fructose (Glu-Fru) represent the vast majority of naturally occurring sugars in grapes. They are fermentable and can be oxidized or reduced to form other derivatives in winemaking that are fundamental to wine’s matrix. Together, Glu-Fru can be used to estimate a more accurate potential alcohol of an unfermented must or characterize dryness at the end of fermentation. A symptom of many stuck fermentations is residual D-Fructose due to a slight preference in D-Glucose uptake during glycolysis, which is why many yeasts used for restarting stuck fermentations are “fructophilic”. Whether or not a winemaker decides to incorporate residual sugar into the finished wine is an important stylistic decision because sweetness changes flavor and mouthfeel characteristics such as sourness, bitterness, astringency, and body (Lawless and Heymann, 2010). 

Note: A common conversion is Potential Alcohol = GluFru / 16.83, although this may vary by yeast strain and winemaking practices. For example, a cold closed top white wine fermentation may have a lower conversion rate than a hot open top red wine fermentation. Either way, estimating potential alcohol with GluFru is more reliable than using °Brix as Glucose and Fructose represent the actual fermentable sugars instead of total dissolved solids. 

Another valuable aspect of measuring GluFru is to track the period of active sugar accumulation in grapes as a metric of ripeness. The accumulation of sugars coincide with the accumulation of anthocyanins because all anthocyanins contain glucose in their chemical structure. Sugar accumulation signals anthocyanidin accumulation and enzymes synthesize them together (Das et al., 2012; Ma et al., 2021; Walker et al., 2021). This metric, known as Berry Sugar Loading, is a way to track sugar accumulation irrespective of dehydration. It also serves as a valuable metric of ripeness for both red and white varietals.

Simply put, Berry Sugar Loading is the average dry weight of sugar per berry, whereas the traditional measurement of °Brix is a ratio of dissolved solids to berry water. This is why °Brix increases with berry dehydration. A good example to distinguish the two measurements is during a heat wave. Under extreme stress, vines use grapes as sinks and metabolize their sugars. While the °Brix increase due to dehydration, berry sugar loading decreases due to vine stress. Winegrowers can gain much more information about the metabolism, resiliency, and recovery of their vineyard by tracking berry sugar loading as a metric of vine metabolism than simply tracking °Brix.

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