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 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.
To learn more about Glu-Fru and its importance in winemaking, become a Bound advising client.
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FAQs
Frequently Asked Questions
Supplies
- Centrifuge Tubes (SKU: G1005-50-2)
- Blank Label Sheets
- Label Template (Download)
- We supply 50 mL centrifuge tubes and labels for local Santa Barbara clients upon request.
Best Practices
- Collect a sample of your wine in a way that is most representative of the entire lot (i.e. practice flushing your sample valve, collecting after movements like pumpovers, stirring your barrel, etc.).
Label
- Label each sample appropriately with your Client ID, Sample Date, and Sample ID. Samples for Phenolics analysis also require a Crush Date, Varietal, and Appellation. The analysis cannot be performed without the applicable information for each sample.
- Mark the panel.
- Individual parameters can be added at the bottom of the label. A full list can be found on our ANALYSES page.
Clients are able to submit samples by contacting us directly and scheduling a pickup, delivering directly to our lab, or sending via the mail.
To arrange for pickup, clients must be located near Lompoc, Buellton, Santa Ynez, Goleta, or Santa Barbara and notify us by 11 am for same-day retrieval. Please contact us for more information.
- We provide same-day results by 7 pm with a guaranteed turnaround within 24 hours of sample receipt.
- Samples that are not analyzed same-daly are refrigerated overnight and analyzed first thing the next morning.
- Results are delivered via email in PDF format and uploaded to your account.
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To ensure their stability, we ask our clients freeze their juice samples before shipment. Please contact Bound with any additional questions about sample preparation before shipment. We recommend expedited same-day to one-day shipping with a tracking number included.
Frozen
Samples can be placed in a freezer 24-hours in advance and shipped with an ice pack. Do not over-fill the polypropylene tubes or use glass containers as the frozen liquid will expand and could pose a safety concern. Freezing samples is a better alternative to boiling when analyzing compounds like ethanol, volatile acidity, free sulfur, and phenolics. Label each sample as "FROZEN".