Why Your Sourdough Starter Smells Like Acetone, Vinegar, or Gym Socks (And Whether to Panic)

What it is: An early-stage hunger signal. When the yeast runs out of simple sugars, it starts producing esters and alcohols — the same compound class as acetone. What it means: Your starter is hungry. Not dying, not sick — hungry. What to do: Feed immediately at 1:1:1 with room-temperature water. The smell disappears within one or two feeds. If the smell is very strong and the starter has liquid pooling on top, that's hooch — see the hooch section below. Still safe, still fixable.

What it is: Acetic acid — the same acid in white vinegar — produced by heterofermentative bacteria when the starter is overripe (past peak) or has been fermented at cool temperatures for a long time. Crucially, acetic acid production requires oxygen: the bacteria that make it are aerobic. This is why a stiff, well-aerated starter tends to develop more vinegar character than a wetter, more covered one. What it means: The starter is alive and active — actually too active relative to when you last fed it. It's post-peak and on its way down. What to do: Feed now. If you want a milder, less sour flavour in your bread, feed on a tighter schedule so you catch the starter before it over-sours. A stronger ratio (1:5:5) once also resets the acid balance. If you consistently prefer milder dairy notes over sharp vinegar, reducing oxygen exposure (more water, less air at the top) shifts the balance toward lactic acid bacteria.

What it is: Butyric acid — produced by certain Clostridium bacteria that can temporarily colonise a starter, especially in warm, under-fed conditions. What it means: The balance of bacteria in your starter has shifted. Not dangerous, not permanent — but not pleasant either. What to do: Four to five twice-daily feeds at 1:5:5 with whole wheat or rye flour. The heavy dilution and the bran's yeast populations crowd out the butyric bacteria. The smell usually fades by the third or fourth feed. Keep the starter a bit cooler (under 25°C) until balance is restored.

This is the good smell. Lactic acid — the same compound in yoghurt and mild cheeses — produced by Lactobacillus bacteria in a healthy, well-fed starter. A pleasantly sour, almost fruity, slightly dairy smell means the starter is active and well-balanced. The closer to peak, the more pronounced the tang. Just before peak: maximum tang, domed surface, visibly active. This is the moment to use it.

What it is: Ethanol — a normal fermentation byproduct of yeast. At low levels it's completely normal and part of what makes sourdough smell alive. At high levels it means the starter is well past peak and fermenting past the available food. What it means: Post-peak, needs feeding. What to do: Feed at 1:1:1. If the alcohol smell is very strong and there's liquid on top (hooch), discard the hooch or stir it back in, then feed. The starter is still alive.

Strong, persistent ammonia or a smell like old meat or rotting food — and the starter has visible pink, orange, or green mould. That combination is a contaminated starter. Throw the whole jar out and start a new one. Note that a strong smell alone (without visible mould) is almost never a true contamination — it's just a very hungry, stressed starter that three warm feeds will fix.