Mold on Sourdough Starter: How to Tell and What to Do

Short answer: if you see fuzzy, raised growth in green, blue, or black on your starter, it's mold and you must discard. Flat color changes (gray, tan, brown liquid) are usually normal and recoverable.

What real mold looks like

True mold:

• Fuzzy, raised, three-dimensional
• Distinct spots, not even color
• Green, blue-green, black, or pink
• Often has a darker center

If you can see texture and depth to the discoloration, it's mold.

What is NOT mold

Frequently confused with mold but actually normal:

These are not mold and don't require discarding (except pink, which is bacterial).

Why mold happens

Mold spores are everywhere. They land on starters when:

• The lid is left fully open for hours
• The starter sits unfed for 1+ weeks
• The kitchen has high humidity and poor airflow
• A previous moldy item was nearby

A healthy, regularly-fed starter rarely molds because the acidic environment and yeast competition prevent it.

The discard decision tree

See discoloration on starter →

• Is it fuzzy/raised/textured? → MOLD → discard
• Flat color, no fuzz, no off-smell? → likely normal → stir, feed
• Pink or orange streaks? → bacterial → discard
• Brown liquid only? → hooch, normal → pour off, feed
• Sweet/fruity smell + color change? → bacterial → discard

Discarding properly

When you have to discard:

• Throw out the entire starter (don't scrape off mold)
• Compost or trash, not the sink (mold spores persist)
• Wash the jar thoroughly with soap and hot water
• Sterilize with boiling water if you can
• Air dry fully before reuse

Mold spores can stay in a jar even after washing. Hot sterilization is best.

Restarting

After discarding:

Day 1:

• Clean jar
• 50g whole wheat flour
• 50g filtered water
• Mix, cover (loosely)

Day 2 onward:

• Daily refresh, 50% discard, 1:1:1 feed
• After 14 days, healthy starter

The new starter benefits from being kept warm (75–78°F), which helps yeast outcompete potential mold.

Prevention

To prevent mold:

• Don't leave the starter open for hours
• Use a clean jar every 1–2 weeks
• Keep at warm temperature (75°F+)
• Refresh at least weekly
• Don't store near other moldy items

Refrigerator mold

Mold can grow in the fridge if a starter sits for months. The cold slows but doesn't prevent mold.

For long-term storage:

• Refresh the starter once
• Wait 2 hours
• Refrigerate
• Use within 4 weeks ideally
• For longer: dry the starter (see drying article)

A drying alternative

If you bake infrequently:

• Dry the starter into flakes
• Store in a sealed container
• Reactivate in 2–3 days when needed
• No mold risk during storage

This is a better solution for occasional bakers than fridge storage.

Black spots in jar but starter looks fine

If you see black spots on the jar above the starter (not in it):

• It's likely jar staining
• Or a small bit of mold from spillover
• Wipe with a paper towel
• The starter itself may be fine

Inspect the surface and body of the starter. If it's clean, feed and continue.

When to be paranoid

Some bakers throw out starter at the first sign of any color change. This is overcaution but safer than the alternative.

If you're unsure, discard. Starting over takes 14 days. Eating bad bread takes longer to recover from.

Mold during a bake

If you find mold when you're about to mix dough:

• Don't bake
• Discard the starter
• Discard any dough already mixed
• Start over after sterilizing

Don't take chances with mold in food.

Mold-resistant practices

Bakers who never have mold issues:

• Keep starter on the counter, fed daily
• Use warm temperature
• Wash jars often
• Use clean utensils
• Don't double-dip with bread or food utensils

These habits make mold extremely rare.

A confidence-builder

Most "is this mold?" panics turn out to be normal starter changes. Healthy starters develop:

• Brown liquid (hooch)
• Slight color variation
• Surface films

These aren't mold. Stir, feed, continue.

True mold is unmistakable: fuzzy, colorful, and growing. If you've never seen it, you'll know it when you do.