BakeHQ

Sourdough starter has mould

Raised, fuzzy or hairy growth on the surface or up the sides of the jar, often green, black, pink or orange, and visibly different in texture from the culture itself.

Usually established at: fermenting

Narrow it down

Answer what you can. Anything you skip simply leaves its causes in play — there is no requirement to answer everything, and no answer is treated as more authoritative than what you actually saw.

What does the growth actually look like?
Where on the jar is it?
Has the jar been covered?
How recently had it been fed when this appeared?

All 3 possible causes are shown below. Answer a question above to start ruling them out.

Every possible cause

The complete model, whatever you answered above. Each cause states the mechanism — what physically happens — because a cause without a mechanism is folklore with a confident tone.

Mould was introduced on the jar, lid or utensil

Common

A healthy culture resists mould largely through acidity and competition — the established population occupies the substrate and keeps the pH low enough to be hostile. Spores introduced in quantity on a dirty jar, an unwashed spoon or a lid with dried residue can establish faster than that defence suppresses them, particularly on residue up the side of the jar where the material is drier, less acidic and out of the culture proper.

What to change

  • Discard it and start again

    Discard the whole culture, including any portion that looks unaffected, and wash the jar thoroughly before starting again. Do not scrape off the visible growth and feed what remains: mould in a soft, wet medium extends beyond what you can see, and some moulds produce toxins that persist after the visible growth is removed.

    Applies where there is fuzzy or coloured growth — green, black, pink, orange, or anything raised and hairy. It does NOT apply to a dark liquid layer, a strong solvent smell or a sharply sour taste, all of which are normal signs of hunger.

    Trade-off: You lose the culture and the time invested in it. BakeHQ takes the cautious position here deliberately: a starter takes about a week to replace, and that is a small price against an uncertain risk.

  • Keep the jar and utensils clean, and cover without sealing

    Wash the jar at intervals rather than topping up indefinitely, wipe residue from the sides and rim where it dries out and loses acidity, use a clean utensil each time, and keep the jar covered but not airtight — a culture produces gas, and a sealed jar builds pressure.

    Preventive. It applies to any maintained culture and is most important where mould has occurred before.

    Trade-off: None beyond the effort.

It was left open to the air for long periods

Common

Mould spores are ubiquitous in kitchen air, and an open jar collects them continuously. A cover does not need to be airtight — a culture produces gas and a sealed jar is its own problem — but an uncovered one is a landing surface, and one that dries at the top into exactly the less-acidic crust where spores establish most easily.

What to change

  • Discard it and start again

    Discard the whole culture, including any portion that looks unaffected, and wash the jar thoroughly before starting again. Do not scrape off the visible growth and feed what remains: mould in a soft, wet medium extends beyond what you can see, and some moulds produce toxins that persist after the visible growth is removed.

    Applies where there is fuzzy or coloured growth — green, black, pink, orange, or anything raised and hairy. It does NOT apply to a dark liquid layer, a strong solvent smell or a sharply sour taste, all of which are normal signs of hunger.

    Trade-off: You lose the culture and the time invested in it. BakeHQ takes the cautious position here deliberately: a starter takes about a week to replace, and that is a small price against an uncertain risk.

  • Keep the jar and utensils clean, and cover without sealing

    Wash the jar at intervals rather than topping up indefinitely, wipe residue from the sides and rim where it dries out and loses acidity, use a clean utensil each time, and keep the jar covered but not airtight — a culture produces gas, and a sealed jar builds pressure.

    Preventive. It applies to any maintained culture and is most important where mould has occurred before.

    Trade-off: None beyond the effort.

The culture was too weak to defend itself

Common

The protection a mature culture enjoys is ecological rather than magical: an established, actively fermenting population keeps the pH low and the substrate occupied. A culture that has been neglected to the point of exhaustion is neither, and it becomes as colonisable as any other damp flour paste. This is why mould appears after neglect far more often than in a culture being fed regularly.

What to change

  • Discard it and start again

    Discard the whole culture, including any portion that looks unaffected, and wash the jar thoroughly before starting again. Do not scrape off the visible growth and feed what remains: mould in a soft, wet medium extends beyond what you can see, and some moulds produce toxins that persist after the visible growth is removed.

    Applies where there is fuzzy or coloured growth — green, black, pink, orange, or anything raised and hairy. It does NOT apply to a dark liquid layer, a strong solvent smell or a sharply sour taste, all of which are normal signs of hunger.

    Trade-off: You lose the culture and the time invested in it. BakeHQ takes the cautious position here deliberately: a starter takes about a week to replace, and that is a small price against an uncertain risk.

  • Feed it often enough that it can defend itself

    Keep the culture on a regular refreshment schedule, or refrigerate it deliberately between bakes rather than leaving it at room temperature to exhaust itself. The protection against mould is the active population and the low pH it maintains, and both need feeding.

    Preventive, and the reason mould appears after neglect far more often than during regular use.

    Trade-off: Regular feeding produces regular discard.

Preventing it next time

  • Wash the jar at intervals rather than topping it up indefinitely, and wipe residue from the sides and rim.
  • Use a clean utensil at every feed.
  • Cover the jar without sealing it — a culture produces gas, so an airtight lid is its own problem.
  • Keep it on a schedule, or refrigerate it deliberately between bakes rather than leaving it to exhaust itself.

Telling mould from a normal, hungry starter

This is the distinction that matters most on this page, and the two are not similar once you know what you are looking at.

A dark liquid layer on the surface — often grey or brown, sometimes almost black — is normal. It is what a hungry culture produces, it is flat and liquid, and it stirs back in or pours off. A sharp solvent or nail-varnish smell accompanies it and is also normal.

Mould is a growth rather than a liquid. It is raised, it has texture — fuzzy, hairy or velvety — and it is usually coloured: green, black, pink or orange. It sits on the surface or on dried residue rather than being part of the culture.

If what you have is the first, feed it. If it is the second, discard it.

Why BakeHQ says discard rather than scrape

Advice to scrape off visible mould and continue circulates widely, and BakeHQ does not follow it. A starter is a soft, wet medium, and growth in such a medium is not confined to the part that is visible. Some moulds produce toxins that remain after the growth is removed, and a domestic baker cannot identify which mould they have.

The judgement is a cost comparison rather than a claim of certainty: a starter takes roughly a week to replace, and the alternative is an unquantified risk taken repeatedly. Where the safer answer costs that little, BakeHQ takes it and says why. This is also why these pages carry no commercial recommendations — a page that advises throwing something away should have no interest in what you do next.

Related

Possible causes

  • Mould was introduced on the jar, lid or utensil

    Spores arriving in quantity on a jar, lid or spoon can establish faster than acidity suppresses them.

  • It was left open to the air for long periods

    An open jar is a continuous landing surface, and the dried crust at the top is where spores establish most easily.

  • The culture was too weak to defend itself

    The protection a starter enjoys is ecological. A neglected culture is neither acidic nor active enough to defend itself.

If none of this matches what you saw, the model has a gap. Tell us — that is how this section gets better.

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