Free Sulfur (F-SO2)
Free SO2 is a measurement of SO2 that is unbound to other molecules. Its molecular form (SO2) is an antimicrobial preservative that’s speciation in wine is highly pH, alcohol, and temperature dependent. The majority of Free SO2 in wine is bisulfite (HSO3), a powerful antioxidant in winemaking.
The traditional calculation used to determine the speciation of sulfur in wine is the Henderson-Hasselbalch equation, but recent studies have shown that free anthocyanins in red wines complex with free SO2 converting most of it into its inactive bound form. Furthermore, the Henderson-Hasselbalch equation doesn't account for variables like temperature or ABV.
While our lab uses the reference methodology for free sulfur determination in wine, aeration-oxidation (A-O), it’s worth noting that A-O doesn’t distinguish between anthocyanin-bound SO2 and truly free SO2. So, the following updated formula from Jenkins et al., 2020 can be used for molecular SO2 determination in white wines and accounts for pH, temperature, and ABV (click here). As for red wines, it is worth noting that Free SO2 may not always play the role we think it does in winemaking.
To learn more about Free SO2 and its importance in winemaking, become a Bound member.
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FAQs
Frequently Asked Questions
Supplies
- We supply 50 mL centrifuge tubes and labels upon request.
Sample
- Sample 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 sample type. ”Juice” applies to grapes and juice samples before fermentation. ”Must” applies to samples undergoing primary fermentation. ”Wine” applies to samples after fermentation.
- Select “Standard” for our corresponding juice, must, and wine panels, or select “Phenolics” for our phenolics panel. SO2 analysis can be included with the standard panel by selecting “with SO2”. 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 scheduling a pickup, delivering directly to our lab, or 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. Established clients can also use our quick scheduling function by clicking the “LAB CLIENTS” tab on the bottom right hand corner of the screen. Be sure to select the correct date and provide all necessary details including instructions for pickup or requests for sampling supplies. 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.
- Our invoices are sent via email and are due upon receipt.
- Your payment is securely processed through Bound’s website using one of the most trusted e-commerce platforms in the world.
- You can also opt-in to our invoice autopay feature by clicking “SIGN UP FOR AUTOPAY” on your account page.
To ensure their stability, we ask our clients either freeze or boil their juice / fermenting wine samples before shipment. Please note that boiling or freezing your samples may influence your results and we cannot guarantee their validity. 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".
Boiled
We generally do not recommend boiling your samples as it can significantly alter the chemistry of your sample. If necessary, samples can be boiled in their 50 mL polypropylene centrifuge tubes with a loosely fitted cap on top. Polypropylene tubes can be boiled up to 10 minutes, but be sure not to over boil as exposure to high temperatures can change your results. Label each sample as "BOILED".
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