Midwest Grape and Wine Industry Institute

How are total soluble solids (TSS) measured?

The two most common tools used to measure TSS are refractometers and hydrometers.

Refractometers measure how light bends as it passes through grape juice, while hydrometers measure the density of the juice. Both methods can be used to estimate °Brix and monitor sugar accumulation during ripening.

Because temperature can influence refractometer and hydrometer readings, measurements should be taken near the instrument's calibration temperature (typically 20°C/68°F) or corrected using appropriate temperature compensation to improve accuracy.

👉 Want to learn more? Check out our latest Lunch and Learn webinar, where Erin covers essential pre-harvest laboratory analyses. https://www.youtube.com/watch?v=g8y4Lu3Z7vw

Lunch and Learn Series: Lab Analysis Review and Best Practices On this month's Lunch and Learn, Erin discusses lab practices perta...

Grape Maturity Series: Total Soluble Solids (TSS)

What happens to sugar during grape ripening?

Ripening begins at veraison, when berries soften, change color, and begin
accumulating sugar. As ripening progresses, glucose and fructose levels increase
inside the berry.

Sugar concentration increases
Berries soften
Color develops
Titratable acidity (TA) decreases

Monitoring TSS over time helps growers and winemakers track ripening progress
throughout the season.

Tomorrow: How do growers and winemakers measure °Brix in the vineyard?

Featured ISU Extension Fact Sheet: Estimating Grape Maturity by Total Soluble Solids

°Brix is one of the most common measurements used to assess fruit maturity. It measures total soluble solids (TSS) in grape juice, which consist of up to 99% sugars.

As grapes ripen, sugars accumulate in the berries, causing °Brix levels to increase. Because of this, °Brix serves as a reliable indicator of berry ripeness and is often used to estimate a wine's potential alcohol content.

While TSS is an important maturity parameter, it is only one piece of the harvest decision puzzle. Acidity, pH, flavor development, and overall fruit condition also play critical roles in determining the optimal harvest date.

Curious about how growers and winemakers use °Brix to track grape ripeness? Learn more about estimating grape maturity using total soluble solids (TSS) at the link below.

Tomorrow: Our weekly berry maturity report goes live! Join our mailing list to receive these reports directly in your inbox. 🍇🍷

https://shop.iastate.edu/extension/family-food-health/food-and-nutrition/food-safety/fs49c.html

Estimating Grape Maturity by Total Soluble Solids - Grape Maturity Series | Iowa State University Extension Store Buy Estimating Grape Maturity by Total Soluble Solids - Grape Maturity Series at ISU

Get ready for the 2026 grape harvest! In partnership with Randall Vos Commercial Fruit Production - ISU Extension and Outreach we will be issuing weekly Wine Grape Berry Maturity reports to our growers and winemakers. These reports provide updates (weight per berry, Brix, pH, and TA) of Midwest cultivars to help you determine ideal harvest timing for your region. Randall shares these weekly on his website, but sign up for email updates from us to keep them coming to your inbox!

Check out the first week's report at the link below. 🍇🍷

Midwest Wine Grape Berry Composition Report: 2026 Week 1 | Iowa State University Extension and Outreach Commercial Horticulture Introduction August 12, 2026, Randall VosWelcome to the first edition of the 2026 Midwest Wine Grape Berry Composition Report. Beginning next week, this report will be released each Tuesday throughout the sampling season.Several growers have indicated that harvest of early-ripening cultivars, includ...

QUICK TIP: Start Tracking Berry Maturity Early

Don’t wait until harvest is around the corner to begin sampling.

Tracking fruit maturity over several weeks can help you identify trends in:
Brix
pH
Titratable Acidity (TA)

Watching how these values change over time often provides more insight than a single measurement.

Harvest timing is a process, not a single test.

When do you typically begin maturity sampling?

MYTH: A wine with no faults is automatically a high-quality wine.

FACT: Being fault-free is the starting point, not the finish line.

A wine can be free of oxidation, volatile acidity, and microbial spoilage and still lack:
Balance
Aroma complexity
Varietal expression
Overall quality

Fault-free wines are technically sound, but great wines go a step further by delivering a positive sensory experience.

Quality isn’t just the absence of problems; it’s the presence of desirable characteristics.

What qualities do you value most in a wine?

WHO ARE WE?

Meet one of our valued Extension partners, Randall Vos, Iowa State University Extension and Outreach’s Statewide Field Specialist of Horticulture – Commercial Fruit Crops.

Over the years, Randall has made countless contributions to the Midwest Grape and Wine Industry Institute through his collaboration, expertise, and dedication to Iowa’s grape and wine industry. His knowledge of cold-hardy grape production continues to strengthen the educational resources and support available to growers and winemakers across the Midwest.

Learn more about Randall and his viticulture resources: https://www.extension.iastate.edu/commercialhort/viticulture

QUICK TIP: Know Your YAN Before Fermentation

Yeast needs more than sugar to perform their best.

Testing Yeast Assimilable Nitrogen (YAN) before fermentation can help you:
Develop an effective nutrient strategy
Support healthy fermentations
Reduce the risk of sluggish or stuck fermentations
Improve fermentation performance

Don’t wait until fermentation problems appear to ask whether nutrients were sufficient.

Good fermentations start with good preparation.

Do you routinely test YAN before harvest?

MYTH: All yeast strains perform the same.

FACT: Different yeast strains can produce dramatically different results, even when fermenting the exact same juice.

Yeast does much more than convert sugar into alcohol.

Different strains can influence:
Fruit and floral aromas
Fermentation speed and temperature tolerance
Mouthfeel and texture
Nutrient requirements
Alcohol tolerance

That’s why winemakers carefully select yeast strains based on the grape variety, wine style, and desired sensory profile.

For example, a yeast selected to enhance tropical fruit aromas may produce a very different wine than one chosen for mouthfeel or fermentation reliability.

Yeast is an important tool a winemaker has for shaping a wine's final character.

Have you ever fermented the same juice with two different yeast strains and compared the results?