Midwest Grape and Wine Industry Institute

How do breweries prevent “lightstruck” beer?

The goal is simple:
Keep light away from beer.

Common prevention strategies:
Avoid clear or green bottles
Store beer in cool, dark conditions
Educate distributors on proper storage
Minimize UV exposure during packaging and service

Even brown glass only slows the reaction over time; it doesn’t stop it completely.

Once beer becomes lightstruck… There’s no fix.

Prevention and proper handling are the only real solutions.

Want to learn more about MBT and beer quality?
Explore the full factsheet here: https://shop.iastate.edu/extension/family-food-health/food-and-nutrition/food-safety/fs65.html

Beer Faults Series: MBT - Lightstruck | Iowa State University Extension Store Buy Beer Faults Series: MBT - Lightstruck at ISU

Why does light make beer “skunky”?

It all starts with hops.

During brewing, hop alpha acids are converted into iso-alpha acids, the compounds responsible for beer bitterness.

Then light enters the picture…

When exposed to UV light, one iso-alpha acid reacts with naturally occurring riboflavin (vitamin B2).

That reaction creates:
MBT: the same sulfur compound family associated with skunk spray.

The challenge? MBT has an extremely low sensory threshold. That means people can detect it at incredibly tiny concentrations. This is why packaging and storage matter just as much as brewing.

Tomorrow: how breweries prevent beer from becoming lightstruck.

Before using potassium sorbate, know this:
It can prevent problems, but it can also create them if used incorrectly.

Biggest risk: Geranium taint
Some bacteria can convert sorbate into compounds that create a strong geranium-like off aroma

This is why it’s not recommended for wines that have undergone malolactic fermentation.

Best practices:
Add just before bottling (1–2 days)
Use with proper SO₂ levels
Ensure wine is clarified (low yeast count)
Consider pH, alcohol, and intended shelf life

Also worth noting:
Legal limit = 300 mg/L
Sensory threshold ≈ 135 mg/L

The takeaway: Potassium sorbate is effective, but only when used thoughtfully and in the right context.

Want the full breakdown (including dosing calculations)?
Read the full factsheet here: https://shop.iastate.edu/extension/family-food-health/food-and-nutrition/food-safety/fs51.html

🔬 What does potassium sorbate actually do?

It’s often called a preservative, but more specifically:

It stops yeast from growing and reproducing
It helps prevent refermentation in bottled wine

But here’s what it doesn’t do:
It does NOT kill existing yeast (but stops their growth)
It does NOT stop bacteria
It does NOT work on all spoilage organisms

That’s why it’s always used with other practices like SO₂ additions and proper clarification.

Key factor:
Its effectiveness depends on:
pH (more effective at lower pH)
Alcohol content (more effective at higher levels)

It’s a tool, not a fix-all.

Tomorrow: key considerations before using it.

🍷 Sweet wine + time = potential problem

If a wine has residual sugar, there’s always a risk of refermentation in the bottle.

And that means:
Unwanted carbonation
Off-flavors
Even bottle pressure issues

One tool winemakers use? Potassium sorbate.

Helps inhibit yeast growth
Reduces the risk of refermentation

But it’s not a standalone solution; it works best alongside proper SO₂ use, sanitation, and filtration.

Tomorrow: how potassium sorbate actually works (and what it doesn’t do).

How do brewers prevent oxidation?

It’s all about controlling oxygen at every stage.

Key strategies include:
✔️ Minimizing oxygen during transfers & packaging
✔️ Using CO₂ or nitrogen to displace oxygen
✔️ Keeping equipment clean & sealed
✔ Monitoring dissolved oxygen levels

Even small amounts above ~0.5 ppm can impact beer quality over time.

The reality: There’s no true “fix” for oxidation, only prevention.

That’s why handling, packaging, and storage matter just as much as brewing itself.

Want to learn more about beer oxidation and how it’s managed?
Check out the full factsheet here: https://shop.iastate.edu/extension/family-food-health/food-and-nutrition/food-safety/fs69.html

Beer Faults Series: Oxidation | Iowa State University Extension Store Buy Beer Faults Series: Oxidation at ISU

What does oxidation taste like in beer?

Once you know it, you can’t untaste it.

Common oxidation flavors include:
Cardboard / paper
Bruised apple
Cheesy (from oxidized hops)

What’s happening behind the scenes? Oxygen reacts with compounds from malt and hops, forming new chemicals that change flavor and aroma.

One key culprit:
(E)-2-nonenal, responsible for that classic “wet cardboard” flavor (detectable at incredibly low levels).

Oxidation can also:
Dull hop aroma (up to 50% loss over time)
Darken color
Alter bitterness

Most beers are meant to be enjoyed fresh; this is why.

Tomorrow: how brewers prevent oxidation.

🍺 Why does beer taste “stale”?

It usually comes down to one thing: oxygen.

Oxidation happens when oxygen reacts with compounds in beer, leading to:

Loss of fresh aroma
Muted flavors
“Stale” or cardboard-like notes

Here’s the catch:

Oxygen isn’t always bad.

Yeast actually needs oxygen early on for healthy fermentation, but after that, it becomes the enemy.

Timing is everything in brewing.

Tomorrow: what oxidation actually tastes like (and why it happens).

🧪 How do we measure SO₂ in wine?

There’s no single method, just the right one for the job.

Common approaches include:
Aeration/Oxidation (industry standard)
Titration methods (fast, practical)
Colorimetric analysis (efficient for multiple samples)
Gas detection tubes (measure “true” free SO₂)

Each method has trade-offs, and results can vary depending on the wine.

The key: consistency and understanding your system.

📚 Want to learn more?
Dive deeper into SO₂ analysis methods here: https://shop.iastate.edu/extension/family-food-health/food-and-nutrition/food-safety/fs62.html

Sulfur Dioxide (SO2) Analysis Methods for Hybrid Wines | Iowa State University Extension Store Buy Sulfur Dioxide (SO2) Analysis Methods for Hybrid Wines at ISU

🍇 Not all wines behave the same.

Cold-hardy grapes (like those grown in Iowa) have different chemistry than traditional wine grapes.

⬇️ Lower tannins (natural antioxidants)
⬆️ Higher pigments (anthocyanins)

This means SO₂ behaves differently, and even measuring it can be more complex.

Tomorrow: how winemakers actually measure SO₂.