Is Your Sourdough Starter Overflowing? Unmistakable Causes

Sourdough starters operate on the simple principle of fermentation. The combination of lactic acid bacteria and wild yeast that constitutes a sourdough starter breaks down the sugars in the flour. As they metabolize these sugars, they excrete carbon dioxide gas and alcohol. 

The elastic nature of the starter, thanks to gluten proteins, traps this carbon dioxide, leading to the bubbling and rising we associate with active fermentation. This natural expansion process is a testament to a thriving microbial community in your starter. However, under certain conditions, this expansion can be so vigorous that the starter might outgrow its container, leading to an overflow.

Overflows can be messy and can also mean the loss of a significant portion of your precious starter. Here are refined strategies to ensure that your starter stays within its bounds:

To prevent a sourdough starter from overflowing, use a larger jar at least three times the volume of the starter post-feeding, ensuring ample space for growth. Increase feeding ratio to 1:2:2 if it’s too active, and maintain a moderately cool temperature. Stir occasionally to release gases and refrigerate if leaving unattended for extended periods.

1. Reasons for the Overflow

a) Container Size

One of the primary culprits behind a starter overflow is the size of the container. A confined space leaves little room for expansion. Considering that a healthy starter can grow exponentially, there’s a risk that it might outgrow a small container, especially if the starter amount and the feed are substantial in proportion to the container’s size.

b) Vigorous Starter

The vitality of a starter isn’t just dependent on the feeding routine but also on the particular strains of microorganisms it harbors. Some microbial combinations are intrinsically more energetic, breaking down sugars more efficiently and producing gas at an accelerated rate, leading to faster and more robust growth.

c) Warm Environment

Microbial activity, particularly yeast fermentation, is temperature-dependent. At warmer temperatures, typically between 78°F (25°C) to 82°F (28°C), yeasts are invigorated and tend to metabolize sugars more rapidly. So, if your starter’s environment leans towards the warmer end, you can expect a faster rate of gas production and, consequently, a quicker rise.

d) High Hydration

The consistency of your starter plays a role in its behavior during fermentation. A starter with high hydration (meaning more water content relative to flour) offers less resistance to the expanding gases. The fluid nature of such a starter facilitates faster and more substantial growth compared to a denser, low-hydration starter.

2. Preventing the Overflow

a) Use a Larger Container

The key to preventing overflows starts with selecting the right container size. An active sourdough starter, in favorable conditions, can easily double or even triple in size. It’s not just about the volume of the starter but also the headroom needed for the gases. 

Always lean on the side of caution. Choose a container that can accommodate at least three times the volume of the starter post-feeding, ensuring ample space for growth and preventing any messy spill-overs.

b) Adjust Feedings ratio 

The feeding ratio, which represents the proportion of starter to fresh flour and water, has a direct impact on the starter’s growth rate. A highly active starter might benefit from a diluted feed, such as a 1:2:2 (starter:flour:water) or even a 1:3:3 ratio. This provides the microbes with a more substantial food source, potentially slowing down their activity and making the fermentation process more gradual.

c) Reducing the Temperature

The temperature at which you keep your starter is a crucial determinant of its activity. Microbial metabolism is accelerated in warmer conditions, leading to rapid gas production and swift rises. While warmth can rejuvenate a slow starter, it can supercharge an already active one. To moderate the fermentation pace, place the starter in a cooler area of your home.

Refrigeration drastically slows down the fermentation process. By placing an active starter in the fridge, you’re essentially hitting the ‘pause’ button on its growth. This method is especially useful if you foresee being unable to attend to your starter for a day or more. Just remember to let it return to room temperature and feed it once you’re ready to use it again.

d) Stirring

An often-overlooked method to regulate an overactive starter is simply to stir it. This gentle agitation releases trapped carbon dioxide gas, momentarily collapsing the starter’s structure. By doing so, you not only create space in the container but also momentarily slow down the starter’s growth by redistributing the food source for the microbes.


An overflowing sourdough starter is a testament to its health and vitality, but it can be a messy problem to contend with. By understanding the reasons behind the overflow and implementing preventive measures, you can maintain a vibrant sourdough culture without the unexpected spills.


Hey there! I'm Sam, your go-to pal for all things sourdough. I've been baking and kneading for 10 fun-filled years, and I can't wait to share the joy of turning simple ingredients into heavenly sourdough bread with you. Grab your apron and let's dive into this amazing world of sourdough bread together on this blog.

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