Is My Sourdough Starter Healthy?

We often receive messages from customers worried about the growth of mould in their sourdough starters. They are unsure if that smelly contents of the jar are actually good to be used in a recipe.

While mould is expected to occur occasionally as the unwelcome guest there, most of the time there is nothing to worry about and the starter is easy to be recovered, and any future contamination prevented.

Read on to find out more! 

What lives in a sourdough starter?

Each sourdough starter on the planet is made up of a different microbiome. What it means is that it is very difficult to tell exactly which yeast and which bacteria species live in the culture. On top of the obvious – flour – every kitchen and every baker’s hands would contribute different microbial flora that is quite unique when combined. So, no two starters are the same.

However, what we know for sure about microorganisms in sourdough starters is that they include both wild yeast and lactic acid bacteria (LAB). The former is mostly Saccharomyces spp. and the latter is Lactobacillus spp.

These yeasts convert sugars into carbon dioxide bubbles during fermentation and therefore cause the dough to rise. Everyone should be familiar with this basic process, especially having used the instant yeast before.

The bacteria further break down products of the fermentation and produce lactic acid and acetic acid which both provide the sour flavour. They tend to lower the pH and thus act as preservatives, which prevent the growth of contaminating pathogens.

Both yeast and bacteria live in symbiosis in sourdough starters and are perfectly normal and expected. They are the ‘good microbes’ there, making the sourdough bread taste so amazing.

What could contaminate the sourdough starter?

Mould is a type of fungus and a common food contaminant. While not all types of mould are dangerous if consumed (some are in fact purposedly used to make blue cheese), it is certainly nothing pleasant and should be discarded immediately if noticed on your bread or sourdough starter. The spores may trigger allergic reactions and may be harmful, so be wary.

Mould is typically fuzzy in appearance, so any other signs of starter contamination could actually be attributed to bad bacteria, too. But remember, lactic acid bacteria naturally deter mould growth, so it is rather unlikely it will ever develop. Unless… read below.

From our experience, mould or bad bacteria growth will occur on two occasions:

  1. Prolonged transportation in high temperatures, especially during summer.
  2. Poor hygiene/maintenance of the jar where sourdough starter is kept.

In the first instance, please reach out to us if you see any dark spots in the plastic zip-lock bag the starter comes in. While we always strive to ship our starters as fresh as possible, the shipping conditions and lengthy delays are out of our hands. Ideally, starters should travel with ice packs, but for obvious reasons it would not be possible or economically justified. So, we take that risk on our shoulders and will replace any starter that arrived in ‘poor shape’.

We believe it is best to give your new starter a feed within 1-2 weeks since packing to ensure it stays healthy and thriving. Each bag will have a ‘packed on’ date so you know exactly how long it has been since the last feeding. If you don’t have time immediately once you receive the starter, please keep the bag in the fridge for a maximum of 4 weeks.

Of course, sometimes things go south, and you will still notice some dark spots which may indicate contamination – in such case please let us know right away and a new bag with the starter will be dispatched your way immediately! We trust our process based on thousands of starter packs shipped to date.

Now, let’s focus more on the second scenario. You already have given your starter a feed (or a few) and one day you realise there is a foul smell and/or dark spots. Mind you, this would normally happen if you neglected the starter for many days or weeks. Mould takes time to grow and if you maintain your sourdough culture regularly, this should never occur.

You would identify it by the fuzz (spores). There is no fuzz, but it still smells bad? It is probably some bad bacteria. Either way, it is quite easy to prevent starter contamination:

  • After each feeding, wipe the walls of the jar clean with a wet paper towel. Contamination usually occurs on the walls where there are bits of flour and old starter.
  • Feed regularly at least once per week if kept in room temperature, or if stored in a fridge, give your starter a feed every 3-4 weeks 

From our own experience, hygiene is key, so simply keep the jar clean and mould or bad bacteria will never happen! Give your starter some love and feed regularly, and it will thank you by rising wonderful loaves of sourdough that everyone craves for.

A white layer on the surface, is it mould too?

A separate occasion is when you see a white layer on the surface of the sourdough starter. You feed your starter every few days (min. once per week) and you do everything according to the instructions provided in the kit. You were just about to scoop some starter off and prepare the leaven or add directly to ferment the dough but noticed a white layer. What is this and is it harmful?

We believe this is unlikely such white layer forms due to bacterial or mould contamination, especially if you maintain good hygiene and follow a regular feeding routine. This would probably be the strains of lactic acid bacteria growing on the surface. Remember they are meant to be there, so simply give your starter a stir, or better discard half and feed as per usual for some extra activity.

We see this white layer quite frequently on our wheat starter (a little less often on the rye starter), and we are certain they are perfectly normal and indicate a healthy sourdough culture. Usually, the layer forms after a few days since the last feed.

Reach out to us to discuss further if you still have concerns!


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