Top Reasons Why Your Sourdough Starter Might Grow Mold

(Scroll down for information on Pink and Orange bad bacteria)
As passionate sourdough enthusiasts, we know how disheartening it can be to see mold growing on your carefully nurtured starter. Mold can appear during the rehydration process or after your sourdough starter has been rehydrated, turning what should be a joyous baking journey into a frustrating experience. Understanding the common causes can help you prevent this issue and keep your sourdough starter healthy and thriving. Here are the top reasons why your sourdough starter might grow mold:

1. Unsanitary Conditions
One of the most common reasons for mold growth is contamination from unclean containers or utensils. Always ensure that your jars, bowls, and mixing tools are thoroughly cleaned and, if possible, sterilized before using them to store or mix your sourdough starter.  I recommend only using bowls, jars and mixing utensils directly from the dishwasher.

2. Improper Storage Temperature
Sourdough starters thrive in a specific temperature range, typically between 70-75°F (20-24°C). If the temperature is too high, it can promote the growth of harmful bacteria and mold. Conversely, too low temperatures can slow down the fermentation process and allow mold to take hold.  This is a large reason I never suggest putting your Summit Sourdough Starter into the oven with the light on.  Not only is it too hot, which can cook and kill your culture, it also  encourages the growth of bad bacteria and mold.

3. Exposure to Contaminants
Mold spores are everywhere, and exposure to contaminants can easily lead to mold growth. Make sure your starter is covered but not sealed airtight. A breathable cover like a loose fitting lid, or a reusable cloth jar cover allows air to circulate while keeping out contaminants.

4. Insufficient Feeding Schedule
Sourdough starters need regular feedings to stay healthy. Neglecting to feed your starter regularly can lead to an imbalance in the microbial environment, allowing mold to grow. Stick to a consistent feeding schedule to keep your starter robust and active. I recommend feeding your Summit Sourdough Starter once every 24 hours by weight in grams.  A proper feeding of a 1:2:2 (Starter : water : Flour) will ensure  your sourdough starter is well nourished and healthy.

5. Using Contaminated Flour or Water
The quality of your ingredients matters. Using flour or water that contains mold spores can introduce unwanted microorganisms to your starter. Opt for high-quality, fresh flour and filtered or bottled water to minimize the risk of contamination.  Using a spoon that was dipped into a pot to mix, and then dunked into a bag of flour to scoop has easily left your flour contaminated with food particles that will grow mold in your sourdough starter jar.   

6. High Humidity Levels
High humidity can create a conducive environment for mold growth. If you live in a humid climate, be extra vigilant about your starter’s environment. Consider using a dehumidifier in your kitchen or keeping the starter in a less humid part of your home.

7. Inadequate Airflow
While it's important to cover your starter, it still needs to breathe. Using a tight lid can create an anaerobic environment that encourages mold growth. Use a breathable cover, like a reusable jar cover or a loose-fitting lid, to ensure proper airflow.

8. Using a Metal or Plastic Container
Sourdough starters are acidic and can react with metal containers, which might not only affect the starter's health but also contribute to mold growth.  Plastic containers which can be surface scratched can have bacteria in the scratched layer, which in turn will grow mold too.  I suggest only using glass containers to store your starter safely.  I use the Weck Tulip Jars.

9. Cross-Contamination
Cross-contamination can occur if you use utensils or containers that have been in contact with other food items or substances that carry mold spores.   Never, ever allow the utensil that you'll mix your sourdough starter with touch your counter.  You may inadvertently pick up a crumb which once it is introduced to your starter will encourage your starter to grow mold.   Always use dedicated tools for your sourdough starter to prevent cross-contamination and only use tools that have been freshly sanitized from the dishwasher.

10. Neglect During Travel or Absence
If you leave your starter unattended for an extended period, especially in a warm environment, it can become susceptible to mold. If you need to leave your starter for a while, consider refrigerating it to slow down fermentation and mold growth. Remember to feed it before you put it into the fridge, and immediately upon your return to revive it.

Pink and Orange Discoloration: Indicators of Bad Bacteria
In addition to mold, you might notice pink or orange discoloration in your sourdough starter. Unlike mold, which can appear as fuzzy patches in various colors like green, black, or white, pink and orange hues are signs of harmful bacteria. Here's what you need to know:

Why It Happens

1. Bacterial Contamination: Pink and orange discoloration indicates the presence of harmful bacteria, such as Serratia marcescens, which thrive in conditions where the starter is not properly maintained.
2. Warm and Humid Conditions: Bacteria flourish in warm, humid environments, which can be exacerbated by improper storage conditions.
3. Inadequate Acidity: A healthy sourdough starter is acidic enough to inhibit harmful bacterial growth. If your starter's acidity is too low, it can become a breeding ground for unwanted bacteria.

How to Prevent It

1. Maintain Proper Hygiene: Always use clean utensils and containers. Sanitize your hands and surfaces before handling the starter.
2. Consistent Feeding Schedule: Regular feedings help maintain the right balance of beneficial bacteria and yeast, keeping harmful bacteria at bay.
3. Optimal Storage: Store your starter in a cool, dry place with adequate airflow. Avoid environments that are too warm or humid.
4. Monitor Acidity Levels: Ensure your starter remains sufficiently acidic. If you notice a change in smell or color, it may be time to refresh your starter or start anew.

Mold and bacterial growth in your sourdough starter can be a sign of underlying issues related to cleanliness, environment, or maintenance practices. Starters displaying mold and / or bacterial growth should be thrown away!  By paying attention to these top reasons, you can take proactive steps to prevent mold and keep your sourdough starter healthy and thriving. Remember, a little care goes a long way in maintaining a vibrant and active starter, ensuring you can enjoy delicious sourdough bread for years to come.

Happy baking! If you have any questions or need further assistance with your sourdough journey, feel free to reach out or explore our other resources on the website.