Effects of Sourdough Starter Age

My love for sourdough began in August 2021 when my aunt passed a portion of her sourdough starter along to my mom. Soon after, I asked my mom to pass some of the starter to me as well. At first I simply thought that it was fun to bake bread from only a handful of ingredients; however, I quickly discovered the complexity of those precious air bubbles that cause the bread to come alive. That is, because sourdough starters in their own right, are full of living microorganisms, the two major ones being lactic acid bacteria and yeasts.

These two types of microbes intertwine beautifully to create the perfect storm for bread. Yeasts break down the sugars in flour and turn them into the carbon dioxide gas bubbles that we see. All the while, lactic acid bacteria are busy turning the sugars into lactic acid that both grants sourdough its defining “sour” flavor and protects it from other harmful bacteria. Another bonus of lactic acid bacteria is that they break down gluten over time (2). This breakdown or “pre-digestion” creates a less inflammatory bread and a more desirable option for those with gluten sensitivities. (4)

While experts agree on the presence of yeasts and lactic acid bacteria in sourdough starters, there is a large variety of species within those two microbial umbrellas that make starters unique. Each starter has its own personal microbiome born from the flour that is used, the surrounding environments, the way it’s fed, and any other variable differences that have yet to be pinpointed. How and why each starter has the particular species and balance of species that they do remains a mystery. Further, some of the other microbes themselves are a mystery (1). 

Even though I’ve been using and feeding the same starter for almost two years, the microbiome of the starter has shifted an infinite number of times in that time. Unlike a person’s fingerprint, the genetic fingerprint of a starter is ever changing as the microbiome adjusts to new flours, new environmental factors, etc. 

Since my starter was passed down to me, I was curious as to what differences, if any, I would notice between it and a starter that I made from scratch. My theory was that a mature starter would generate a stronger sourdough flavor in bread than a new one. To test this out, I ran a little experiment.

Note: For the purpose of this blog and avoiding confusion, I will call the passed down starter the “mature” starter and the other my “new” starter.

Question: Does the microbiome of a “new” versus “mature” starter impart differences in the same bread recipe? 

Goal: Pinpoint any differences in how the two sourdough starters perform by baking two loaves of bread with the only changed variable being a mature versus new starter.

The Process:

Step 1- Make a new starter.

As you can imagine based on the basic science of a starter, the time required to develop a starter with enough microbial activity to bake with differs for the very reasons that a microbiome is unique. I followed the Sourdough Starter Recipe by Bless this Mess, who happens to also be the creator of one of my favorite, easy crusty sourdough loaf recipes. The recipe states that the starter could be active and ripe somewhere in the range of 5 to 14 days, but for me it took 8 days. Here is how my starter progressed:

Day 1, ~2:00pm: Added 50 grams each of water and flour to a jar. I used King Arthur’s All-Purpose flour and an old t-shirt cutting secured by the jar’s metal ring. It is important to not completely cut off the starter from fresh air.
Day 2, ~2:00pm: Added 50 more grams each of water and flour to the same jar. At this point, I didn’t see any bubbles, and the mixture was thick. It smelled almost of Play-Doh.
Day 3, ~9:00am: Air bubbles were visible; however, it far from doubled
Day 3, ~2:00pm: Added another 50 grams each of flour and water.
Day 4, Discarded all but 25 grams of my starter by pouring it out into a new, larger jar. Once in a new jar, I added 50 more grams each of water and flour. When pouring out the starter, I noticed that it was much thinner and runnier than the previous days.
Day 5 &6, ~2:00pm: Added 50 more grams each of water and flour. The starter began to smell more richly of bread rather than just flour. The air bubble activity was slight but increasing.
Day 7, ~8:00pm: Discarded all but 25 grams of the starter by pouring it out into a new jar. Once in the new jar, I added 50 more grams each of water and flour. Because the starter’s texture felt similar to what I was used to and there was a fair amount of air bubble activity on Day 7 morning, I put a rubber band around the jar to visually measure the amount of activity.
Day 8, ~8:00am: The starter doubled! Despite the activity, the starter smelled much less sour than the mature starter that I’d been using. It smelled like a loaf of baked traditional bread rather than sourdough.

Step 2- Bake 

Now that I had a ripe, active starter, I was ready to bake bread. I did make one loaf of bread with the new starter prior to the experiment, as I wanted to be sure the dough would rise properly. In order to eliminate as many opportunities to impart differences in the two loaves of bread, these are the precautions that I took.

Experimental variables:

  • Each sourdough starter was fed at the same time with the same ratios: 65 grams each of starter, flour, and water into clean jars.
  • The water for each starter’s feed was from the same large measuring cup to ensure the water temperature was consistent.
  • The starters rose in the same location and in the same type of jar.
  • The loaves were prepared simultaneously, with similar equipment, and from the same recipe by Bless this Mess. While the proving baskets (a strainer and a mixing bowl) had different depths, they were both 7” in diameter.
  • The recipe I followed does not contain any sugar or spices that would alter the sourdough’s flavor profile.
  • Both loaves were baked in Lodge Dutch ovens at the same time, in the same oven, for the same length of time.

When preparing the loaves, the only difference that I noticed was the firmness of the dough after the second rise. When I tried to score the loaf made with my young starter, the dough was too loose for the knife to make a clean score. In contrast, the same motion for the dough from the more mature starter easily sliced through the dough. This resulted in the biggest difference between loaves.

Step 3- Taste

Surprisingly, the loaf from the mature starter did not create a much “stronger” tasting loaf. The only difference in flavor, albeit slight, was that the sourness of the loaf from the young starter was a little brighter. In contrast, the sour flavor from the mature starter was more deep and full tasting rather than thin and sharp. That said, this was me and my husband being very critical. Both loaves were clearly sourdough and of the same recipe with identical texture. 

Step 4- Refeed the starters ( 5 days later)

To confirm that the inconsistencies in the firmness and overall shape of the dough were due to the starter and not outside factors, I decided to bake the loaves again, reverse the order of scoring, and swap the proving baskets. This second time, I made sure to compare the starters’ activity post feed as well. 

The day before I wanted to start the dough, I took the starters out of the fridge at 8:00pm and let them come to room temperature overnight. Surprisingly and as you can see, the young starter had more bubbles and “activity” than the mature starter upon taking it out of the fridge. At 11:15am the next day, I fed both starters with the same proportions (65g each) as the previous bake.

Left: Mature Starter & Right: New Starter

Here is what the starters’ activity looked like just fed and at the 2, 6, and 10-hour marks. While they rose very similarly, the mature starter appeared to have risen slightly more.

Left: Mature Starter & Right: New Starter

Step 5- Bake Round 2

I began preparing the bread dough moments after the final picture,10 hours post-feed.I was curious to learn if the two proving baskets (strainer versus mixing bowl) produced the same results as the first bake, namely causing the loaf in the strainer to be flatter; however, my second round of loaves turned out almost identical to each other despite the different baskets. The flour patterns were the only indicators to visually tell them apart once out of the oven. 

Step 6- Taste Round 2

While my husband and I both agreed that there was a slight difference in flavor, we disagreed on how we perceived each “sour” flavor. Oddly enough, we both described the flavor by telling what part of our mouth the “sour” flavor hit first, whether it be the top or all encompassing. While our descriptions were the same, they were about the opposite loaves. Other than the flavor, the texture and crust of the breads were identical.

Left: New Starter & Right: Mature Starter

The Results:

With both rounds of baking in mind, I’ve found the answer to my question and come to the following results.

Question: Does the microbiome of a “new” versus “mature” starter impart differences in the same bread recipe? 

Answer: Long story short, no

Even though every sourdough starter has a unique microbiome, those that are cared for in the same manner and in the same location yield almost identical results and flavor despite their age. Had I just gotten my mature sourdough starter from my aunt that had been fed on a different cycle and with a different type of flour, this experiment may have ended up with vastly different results. What I really demonstrated was that there are more factors that play as big of roles, if not larger roles, as a starter’s age (type of flour, feeding schedule, environment, etc).

Sourdough is a complex flavor that people’s palates perceive differently. The flavor comes from a scientific wonder and is much different than an artificial flavor made to taste a certain way. My husband and I could identify slight nuances in each loaf, but our experiences contrasted each other.

For my experiment in particular, the difference in firmness between my first set of loaves was likely due to how well I performed the folds and shaped the dough into a tight ball. Switching the bowls, order of preparation, and Dutch ovens didn’t impart changes in the second batch of bread. Rather, the way that a loaf is scored has the potential to vastly change the appearance of two otherwise identical breads.


While I learned a lot about the intricacies of sourdough by doing this experiment, I find that I only have more questions. Would I notice a difference in the starters if I fed them exclusively with a different brand or type of flour (organic versus not) or fed them on a different cycle. There is so much more of the science and microbiome of sourdough starters that is yet to be unlocked, and I find that fascinating. 

I thoroughly enjoyed documenting this experiment, and I hope you learned a little something along the way too. Let me know if there are any other experiments you would be interested in!

Until next time, Gabriela


  1. https://asm.org/Articles/2020/June/The-Sourdough-Microbiome
  2. https://www.seriouseats.com/sourdough-starter-science
  3. https://www.blessthismessplease.com/how-to-make-a-sourdough-starter/#tasty-recipes-44665-jump-target
  4. https://www.bonappetit.com/story/gluten-sensitive-sourdough

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