Under certain conditions, moulds can form on a sourdough starter, even in the fridge. Why? What should I do if this happens?
Sourdoughs are set up by nature to be safe and healthy by default, providing simple good practice is followed. However, in the first day or two of a starter’s life, when the lactic acid bacteria (LAB) are still ‘finding their feet’ and have not produced sufficient selective anti-bacterial and anti-fungal compounds to sterilise the mixture, it is possible for moulds to get a foothold, usually as a result of some cross-contamination (a waft or splash of something else in the kitchen) or something lurking in the flour you are using.
In an older starter that is being kept in a tub (perhaps with an incomplete seal), moulds sometime creep in where there is a lot of ‘headspace’ above the starter itself, i.e. it isn’t occupying much of the tub. This space can allow enough oxygen for any adventitious moulds that may settle on the sides of the tub or even on the surface of the sourdough to multiply, unimpeded by the acidity of the main bulk of starter that would almost certainly inhibit any such growth.
Black moulds are unusual and are likely to be from ‘dirty’ flour or over-relaxed kitchen hygiene. White moulds are more common. Indeed, yeasts and moulds are biologically pretty similar and it is sometimes hard to distinguish between a white mould and the natural efflorescence of yeasts on the surface of an active sourdough starter.
With black moulds, caution is certainly advisable, especially knowing what can easily grow on (for instance) cooked rice left in a warm kitchen. So if there is any degree of contamination, it is best to throw the starter away, clean the tub well and begin again. If there is just the odd spot, remove it carefully, stir up the sourdough (with a clean spoon) and give it a ‘mini-refreshment’ by adding some fresh flour and water. If the black spots reappear, it would be best to start again – perhaps with a different source of flour.
With white moulds, either simply stir the moulds into the body of the mix and, if necessary, transfer to a smaller tub with less space between starter and lid, or skim off the majority of the mould before proceeding as just described.
In either case, a couple of refreshments with fresh flour should allow the good microbes to re-establish themselves in your sourdough eco-system. And one final thing: firmer sourdoughs (i.e. those with a lower water content) don’t ferment as quickly but they don’t decompose so quickly either and are somewhat less prone to getting layers of mould on their surface.
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