How to Strengthen and Deacidify a Weak Sourdough Starter

By Tom Cucuzza, The Sourdough Journey © February 2024

Many sourdough bakers struggle with “sluggish” starters from time to time.  A strong starter will occasionally weaken and lose its rising power.  What causes this to happen and how do you correct it?

Unless you are discarding and feeding your starter every day (or multiple times per day), it is inevitable that your starter will become acidic and will lose some if its rising strength over time. 

Learning how to identify an acidic starter, and learning the method to periodically strengthening and deacidify your starter is routine maintenance for a sourdough starter and an essential skill for sourdough bakers.

Is Your Starter Strong or Weak?

Measuring the strength of a sourdough starter is more of an art than science for home bakers.  Here are the classic indications of strong and weak sourdough starters:

Understanding Starter Strength

A starter’s “strength” is generally measured by its ability to rapidly create carbon dioxide to raise a batch of dough.  When bakers talk about a “strong” starter, they are generally referring to a starter with a large, productive yeast population.  It is easy to anthropomorphize your starter and assume that individual yeast cells may be stronger than other cells. But there is very little difference in one cell versus another.  It is better to think of a large population of cells, versus a small population. 

When your starter is sluggish or weak, it loses its ability to quickly or fully raise a loaf.   Is this simply because the yeast population is too small?  It is not quite that simple. 

The Impact of Acidity on Starter Strength

Your starter contains two microbes, yeast and lactic acid bacteria (LAB). These two microbes coexist in a symbiotic relationship. The yeast does most of the “rising” work in a batch of dough by creating carbon dioxide (CO2) and the lactic acid bacteria creates the lactic and acetic acids that produce the signature sour flavor. 

In a strong, healthy starter the yeast and LAB enjoy a productive working relationship.  When you feed your starter with water and flour:

• The yeast produces carbon dioxide (and to a lesser extent, alcohol)
• The lactic acid bacteria produces lactic acid and acetic acid

Both types of microbes are swimming in the byproduct of their symbiotic partner.  The LAB is unimpacted by the carbon dioxide (and consumes the alcohol), but the yeast is massively impacted by the acidity produced by the LAB. 

Yeast thrives in an environment with low acidity, and it struggles tremendously in an environment with high acidity.  As your starter becomes acidic, it “chokes off” the yeast, and limits its ability to produce carbon dioxide.   Acid also acts as a form of “yeast birth control,” and it limits the yeast’s ability to produce offspring.  When your yeast is swimming in an acidic environment it is in a very unhappy and unproductive place. 

Yeast + Acid = “Weak” Yeast

Understanding the interplay of the yeast and lactic acid bacteria is an essential skill for sourdough bakers. 

The strength of a sourdough starter is a function of two variables – the yeast population and the acidity level.

Consequently, a weak starter can be caused by:

1)     A small yeast population, and/or

2)     A high acid load.

The vast majority of starter “strength” problems are caused by high acidity, not a small yeast population!

What Causes an Acidic Starter

Your sourdough starter can become acidic by:

• Letting your starter fall completely flat before discarding and refeeding
• Using low feeding ratios (e.g., 1:1:1 or lower) which carry over a high acid load
• Keeping your starter too warm (above 85F/29C)
• Leaving your starter for a long time in the refrigerator without feeding

If you keep your starter at room temperature it is very common for your starter to become acidic over time (i.e., 1 to 4 weeks) unless you are baking daily and feeding your starter multiple times per day.  Bakers who keep their starter in the refrigerator have less risk of acidity build up, but it can still happen over a longer time period (e.g., one to two months).

Deacidifying your starter is routine maintenance for your sourdough starter.  It is not a sign of negligence or doing something wrong. It is the natural order of things.  A sourdough starter will always want to become more acidic.  Lactic acid bacteria will always outrun the yeast, given enough time. 

How Do I Deacidify My Starter?

The two methods of deacidifying your starter are:

1)     High Feeding Ratio, and

2)     Peak-to-Peak Feedings

Method 1: High Feeding Ratio

Sometimes you can remediate a weak, acidic starter with a single high feeding ratio.

The most common “high feeding ratios” are 1:5:5 and 1:10:10.  These ratios help knock down the acidity of your starter with one large feeding.  For example, if you keep approximately 150g starter you would use the following ratios (with some rounding for simplicity):

1:5:5 ratio = 15g carryover starter, 75g flour, and 75g water.

1:10:10 ratio = 7g starter, 75g flour, and 75g water

Note, a high feeding ratio will take longer to peak than your normal feeding ratio.  Be patient!

There are two important rules when using high feeding ratios:

1. Never discard and refeed before your starter peaks, and
2. Always discard and refeed before your starter falls flat

If your starter still smells acidic after one feeding, you can repeat a second high ratio feeding.  If this does not work, proceed to Method 2: The Peak-to-Peak Method.

Method 2: The Peak-to-Peak Method

To deacidify your starter, you will use the “Peak-to-Peak” feeding method for 3 to 5 feedings over a few days.  With the Peak-to-Peak method, you ignore the clock, watch your starter’s activity, then discard and feed your sourdough starter as it is peaking.  The interval between feeding and peaking should shorten as the starter strengthens so it is impossible to establish a fixed feeding time (e.g., every 12 hours) with this method.

I also recommend using very small quantities of starter in this method, because most of it will be discarded.

• Feeding #1 – Take a small amount of your existing starter (10g) and feed it 2x flour (20g) and 2x water (20g).  This is a 1:2:2 feeding.
• Wait for Peak #1 – Watch and wait for your starter to peak.  Your starter will peak more quickly if it is warm, versus cool.  There is no benefit to warming up the starter to try to get it to peak faster.  You can warm or cool your starter to fit the feeding times into your schedule, if necessary, but warming or cooling your starter has no material impact on its strength. For the first few feedings, I will sometimes put my starter on a warming mat at 80F/27C, just to speed up the first few feedings so I can hopefully do three rounds on the first day.  Never let your starter temperature get above 85F/29C.
• Peak #1 – When your starter peaks in height or bubble activity, discard 80% (retain 10g), and feed 1:2:2 again (20g flour and 20g water).  It is important to wait for the peak, then discard and refeed at peak, or slightly after.
• Repeat this process three to five times over a few days (or longer if your starter is sluggish and/or your kitchen is cold). 

With each successive discard and refeeding, keep track of the time it takes for your starter to peak.  As your starter strengthens and deacidifies, you should see the time to peak becoming shorter. 

FAQ: What if your last feeding of the day is in the evening and your starter will peak overnight? 

This is the trickiest part of the method.  Without getting up in the middle of the night (which I have done), you have two options:

1. Put your starter in the refrigerator and resume the process the next morning,
2. Use a higher feeding ratio, and/or
3. Reduce the temperature of your starter

If I’m feeding my starter in the evening and I don’t want it to peak until morning, I will increase the feeding ratio to 1:5:5 or 1:10:10.  This will stretch out the time to peak by about 30-50%.  And, I reduce my starter temperature overnight.  By reducing your starter temperature by 15 degrees Fahrenheit or 8 degrees Celsius, you can double the time it takes to peak.  An easy way to do this is to put your starter in your oven with an ice pack or a bowl of ice.  This will slow it down substantially overnight so it should peak the next morning.  As I demonstrate in the video, you can also put your starter jar directly in a bowl of ice and it will delay the start of the rise time by about 4 hours (see video for details on this method).  

When have I done enough Peak-to-Peak feedings?

You should look for a few characteristics:

Time to Peak

The time between feeding and peaking should shorten (assuming the temperature and feeding ratio is unchanged).  If you do two back-to-back feedings and the time to peak is the same (e.g., 4 hours), then your starter has reached a plateau and you can stop.

Consistency and Smell

As your starter becomes less acidic, you should see it become lighter, more frothy, stringy and it should smell more yeasty than acidic.  Use your nose each time the starter peaks.  You will smell it when the acidic smell abates and the starter smells ripe and yeasty.

The Science of Peak-to-Peak Feeding

There are many starter feeding methods used by bakers and it is difficult to assess the difference between them all (many work).  The Peak-to-Peak method results in a measurable reduction in acidity.  

Here is an example of three feedings using the Peak-to-Peak method.  

In this example, the acidic starter had a very low pH of 3.5.   

·        At the first peak, the pH had risen to 3.62 (lower acidity)

·        At the second peak, the pH had risen to 4.01 (lower acidity)

·        At the third peak, the pH had risen to 4.11 (lower acidity).

In three Peak-to-Peak feedings, the acidity at peak reduced significantly from a very acidity pH of 3.5, to a very low acidity pH of 4.11.    

FAQ: Why don’t you just do a massively high feeding in the first feeding (e.g., 1:20:20 feeding ratio)?

This seems like a logical method, but I have tested it and it is not as effective as the Peak-to-Peak method. When you use a very small feeding ratio, you are still propagating the initial LAB to Yeast population throughout that feeding.  So if you use a very small amount of a very acidic starter, you are already starting with an unbalanced population, and replicating that microbial balance. 

Also, when deacidifying your starter, time is the enemy.  The longer it takes for your for your starter to peak, the more acidic it becomes.  Although a very high feeding ratio seems like a sensible approach, it elongates the time to peak, which gives the LAB more time to outrun the yeast.  This is one of the most fascinating (and confounding) findings in my starter experiments.  There is a constant “background acidity” that develops in even with the highest feeding ratios, simply with the passage of time.  It is likely that there is an optimal feeding ratio, but I have not yet discovered it in my experiments.

FAQ: Can I use a higher feeding ratio with the Peak-to-Peak method?

Yes, you can go up to a 1:5:5 feeding ratio with that method.  I prefer a lower feeding ratio simply to compress the overall time to get through multiple cycles of the feeding.  For example, with a 1:2:2 feeding, I can do 3-5 feedings in 2 days or less.  With a higher feeding ratio, it will stretch out the overall timing.

FAQ: How often do I need to use this method?

Sourdough starters naturally want to become more acidity over time. I generally accept that and assume that I need to deacidify my starter about once a month, based on my baking schedule and feeding methods.  Everyone’s starter will behave differently depending on how you feed it.  Watch for the telltale signs of a weak starter and use your judgement.  Sometimes, a few peak-to-peak feeds can quickly snap your starter back into shape. 

FAQ: How do I keep my starter from becoming acidic in the future?

It all depends on your baking schedule and starter feeding schedule and method.  For example, you will rarely hear commercial bakeries talk about deacidifying their starters because they are baking every day and sometimes feeding their starters more than once per day.  They also may keep their starters refrigerated at night.  Daily feedings (or multiple times per day) and refrigeration will keep a starter very strong and with very low acidity.

I use a combination of feeding ratios and refrigeration to keep my starter from becoming acidic. 

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