Smells, layers, and slow rises: what they mean

When your starter is misbehaving, the first instinct is to throw it out and start over. Don't. Almost every problem you'll encounter is fixable, and the symptoms are usually easy to read once you know what they mean. This article catalogues the most common ones and what to do about each.

One blanket rule before we start: the only reason to throw a starter out is mould. Pink, orange, fuzzy growth on the surface, or any colour besides off-white or grey, means the culture has been overrun by something you don't want. Throw it out and build fresh. Everything else on this list is recoverable.

Smells: what each one means

Mild yogurt or beer: healthy

A balanced, mature starter at peak smells like mild yogurt, mild beer, sometimes faintly bready. This is the target. If your starter smells like this and rises predictably, nothing is wrong.

Sharp vinegar or acetone: too acidic / past peak

A starter that smells aggressively of vinegar or nail polish remover (acetone) is past peak and acidic. The bacterial side of the culture has outpaced the yeast side. The starter is still alive, just out of balance.

Fix it: increase the feeding ratio (1:5:5 or higher), feed twice a day at warm room temperature for 3–5 days. The yeast population will catch back up to the bacteria. The smell will normalise within a few feedings.

No smell, just raw flour: too young or too cold

A starter that smells like wet flour and nothing else is either still establishing (first 7 days) or living somewhere too cold for the microbes to be active.

Fix it: move to warmer location (22–26 °C / 72–79 °F), continue feeding daily, give it 3–5 more days.

Cheese, gym socks, ammonia, garbage: bacterial overgrowth

These smells are unusual but they happen. They mean an undesirable bacteria has taken hold, usually because the starter has been sitting too long without feeding, often at warm temperature.

Fix it: take a small amount (20 g), feed at 1:10:10 with fresh flour and water at warm room temperature. Repeat twice a day for 5–7 days. The high-ratio feedings will dilute the unwanted bacteria below sustainable levels and let the lactobacillus and yeast re-establish. If after a week the bad smell hasn't resolved, build fresh.

Sweet, beer-like, mildly fruity: healthy variation

Some starters develop a sweet, almost fruity character, particularly young ones, particularly with whole-grain flours. This is fine. It usually shifts toward the standard yogurt/beer profile as the culture matures, but a sweet starter isn't broken.

Layers and surface conditions

Greyish liquid on top: hooch

Hooch is alcohol, a by-product of yeast fermentation when the starter has been sitting hungry past peak. It's a normal stress signal, not a problem in itself. The starter is telling you it's been waiting longer than usual for food.

Fix it: pour the hooch off (or stir it in if you want a tangier flavor), feed normally. If hooch is showing up consistently, your feeding interval is too long or your ratio is too low.

White, papery film on the surface: kahm yeast

A thin, dull-white film on the surface that doesn't have the bubbly texture of healthy starter is often kahm yeast: a wild yeast variant that grows on top of fermented foods. It's not dangerous and it doesn't produce harmful toxins, but it's a sign the surface conditions favour it.

Fix it: skim off the film, take a small amount of the starter underneath as your seed, feed at high ratio at warm room temperature, and reduce surface exposure (smaller jar, looser fill; don't fill the jar more than 1/3rd to leave less surface area).

Pink, orange, or fuzzy growth: mould

Bin it. This is the only situation where a starter is genuinely lost. Mould produces toxins that can persist even if you scrape off the visible growth. Don't try to rescue it. Build fresh.

Mould growth almost always indicates contamination from outside (a dirty utensil, food residue in the jar, improper covering). When you start over, sterilise the new jar and use a clean spoon every time.

Rise problems

Slow rise (takes more than 12–14 hours at room temperature)

Most common cause: kitchen too cold. Move to warmer location and time the next 2–3 feedings.

Second cause: feeding ratio too high for the culture's current strength. If you're feeding 1:10:10 and the starter takes 18 hours to peak, drop to 1:5:5 for a few feedings to rebuild population, then go back up.

Third cause: bacterial overload (the smell will be sharp). Treat as described in the smell section above.

Stops rising entirely

A starter that has been peaking normally and suddenly stops rising is usually responding to a temperature change (a cold weekend, draft from a window) or a flour change. Identify what changed in the last 1–2 feedings and reverse it. The starter will catch back up within 2–3 feedings.

Rises and falls very fast (within 2–3 hours)

Sometimes a sign of a very warm kitchen. Sometimes a sign of bacterial dominance. Look at the smell: if it's mild, just increase the feeding ratio. If it's sharp, treat for bacterial overgrowth as above.

Problems specific to young starters (first 4 weeks)

Young starters are more volatile than mature ones. A few oddities are normal during weeks 2–4 that wouldn't be okay later:

• Smells that shift from feeding to feeding: fruity one day, vinegary the next. Normal during weeks 2–3.
• Inconsistent peak times. One feeding peaks in 6 hours, the next takes 10. Should stabilise by week 4.
• Brief return of the day-3 dramatic over-rise. Sometimes happens around week 2. Wait it out.

If the issue persists past week 4, treat it as a real problem and use the diagnostics above. Before week 4, give it more time.

When in doubt: the recovery feed

If you can't tell what's wrong but the starter is clearly not behaving normally, the safest fix is what serious bakers call a recovery feed:

1. 1.Take only 20 g of the misbehaving starter as a seed.
2. 2.Feed at 1:10:10 with 200 g flour and 200 g water.
3. 3.Stir thoroughly, cover loosely, leave at warm room temperature.
4. 4.Repeat twice daily for 5 days regardless of what you're seeing.

The high ratio dilutes whatever's wrong. The warm temperature accelerates the recovery. The repeated feedings rebuild population. Most non-mould problems clear up within 5 days of consistent recovery feeding.

The mindset

Sourdough starters are forgiving. They'll do strange things from time to time and almost always recover with patience. The wrong response is to panic and start over; the right response is to keep feeding, watch what happens, and wait. The only fatal problem is mould; everything else is just a starter telling you something.