⭐ “Karl on Freezing Sourdough Cultures”

Most home bakers are told never to freeze a sourdough starter — as if cold temperatures will kill it, weaken it, or somehow “ruin the microbes.”
But the man who has preserved more sourdough cultures than anyone alive tells a very different story.

Karl De Smedt — the world’s only Sourdough Librarian — oversees more than one hundred living cultures in Belgium’s Puratos Sourdough Library. These aren’t grocery-store starters; they’re centuries-old, culturally significant, one-of-a-kind microbial ecosystems. And Karl freezes them on purpose.

Why?

Because when it’s done correctly, freezing is not harmful.

It’s preservation.

Karl’s approach to freezing is calm, practical, and backed by microbial science. Some yeasts slow down, some lactobacilli get groggy, the balance shifts for a feeding or two — but the culture wakes back up, re-stabilizes, and carries on as stronger than ever.

If you’ve ever wondered whether freezing your starter is safe, smart, or even recommended… the Sourdough Librarian has your answer.

Let’s make some dough — and learn how to protect the microbes that make it possible.

Karl views freezing as a safe, practical, long-term storage method, but only when done correctly.

He routinely freezes rare and geographically unique starters collected for the Puratos Sourdough Library.

Here is his stance:

✔ 1. Freezing is Safe for Long-Term Preservation

Karl often freezes backup samples of rare or irreplaceable starters.

Why?

Because freezing stops microbial activity without killing the culture, allowing it to stay genetically and microbially intact for months or years.

This is his go-to method when:

• a starter is culturally/historically important

• the library needs redundancy

• a culture is being shipped or transported long-distance

✔ 2. Freezing DOES Change the Microbial Ratio Temporarily

This is the nuance many people miss.

Karl notes:

• yeasts tolerate freezing better than some lactobacilli

• some LAB species temporarily lose strength after thawing

• acidity and flavor may shift for the first few refreshments

• but the culture stabilizes again after 1–3 feedings

So the key is:

“Freezing is not harmful, but the starter must be refreshed after thawing to return to balance.”

This is exactly how the Sourdough Library handles frozen backups.

✔ 3. Karl Uses a Simple Freeze Protocol

His standard guidance:

• store starter at 100% hydration (equal flour + water)

• freeze in an airtight, small container

• thaw slowly in the refrigerator overnight

• refresh once or twice before use

✔ 4. He Strongly Prefers Freezing Over Dehydrating for LAB Preservation

Karl has said in several interviews that:

• freeze-dried or sun-dried starters lose some lactobacillus diversity

• freezing preserves the entire microbial community more faithfully

What that means for your culture:

Dried culture = convenience + reliability
Frozen culture = maximal microbial preservation
Fresh mother dough = highest continuity

✔ 5. Freezing Is NOT a Daily-Use Maintenance Method

Karl does NOT recommend freezing as part of a normal feeding cycle.

It’s for:

• long-term storage

• emergencies

• preserving unique cultures

Normal bakers should use:

• the Mother Dough method 

• refrigeration (10–14 days between bakes)
  or

• regular bi-monthly feeding 

Until next time, let’s give thanks for our daily bread…

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