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The Science of Sourdough and How a Jar of Microbes Could Help Keep Your Bread Fresher Longer

Making sourdough starters is one of the latest hobbies we're taking up to pass the time during the pandemic. They serve a practical purpose (leavening bread without the need for yeast) but also serve as sort of surrogate pets. As we feed and care for our starters we are unknowingly taking part in some complex chemical and biological reactions, that may even help our eventual loaf of bread stay fresher longer!

Its catapult to popularity may have been triggered by the pandemic-induced yeast shortages, but even months later, when instant yeast is once again available at most grocery stores, sourdough’s contemporary stardom is barely starting to fade. Sure, many of us turned to making a sourdough starter to simultaneously combat yeast scarcity and our newfound fear of going to the grocery store. But lots of us have kept up with our strange new hobby of mixing water with flour and leaving it on the counter for reasons beyond just the practical.

Taking care of a sourdough starter takes time, especially when it’s first coming to life, and we have lots of it now. If you had asked me to add a new daily task to my routine 7 months ago, I think I’d have answered with a big sigh and a polite “no thank you”. But now, amid a global pandemic, when time seems to simultaneously move lightning-fast and terrifically slow, taking a few minutes a day to care for a little jar of microbes seems like a great idea to me.

North Carolina State University professor Rob Dunn and his lab led a project that saw 560 samples of sourdough starters from around the world sent to and analyzed by his lab. They found tremendous variation in what species of microbes inhabited sourdough starters, but they also found that people treated their starters as so much more than just ingredients. “People talked about them like they were pets,” wrote Dunn for, “Most starters had names”.

Personally, I named my starter ReeRee. I made her from scratch with just flour and water by following this guide. She lives in a mason jar covered by a cloth, and now that I’ve returned to my usual schedule of going to the lab every day and don’t have time to bake bread so often, she lives in the fridge. She gets one meal a week of 50 grams of white flour, 50 grams of whole wheat flour and 100 grams of water.

If you don’t have a sourdough starter of your own, the personification of our lumps of gluten can seem pretty strange. But once you dig a bit into the science of sourdough, and what that lump is actually made of, treating it like a pet starts to make more and more sense. After all, a sourdough starter is essentially a colony of microbes.

While most breads are baked using yeast (specifically bakers’ yeast, S. cerevisiae), sourdough starters contain more bacteria than yeast, sometimes up to 100 times more. Bacteria that produce lactic acid are overwhelmingly dominant in starters, and accordingly, it’s mostly lactic acid that makes a sourdough starter sour!

There are some fungi in most starters, but they’re different species than bakers’ yeast. They have to be since they need to be tolerant of extremely acidic conditions (a sourdough starters’ pH can range anywhere from 3-6).

Just like bakers’ yeast, the microbes in sourdough starters consume sugars from the flour and produce carbon dioxide gas. This gas becomes trapped in the dough, forming little bubbles and producing the airy bread texture we know and love. But where do these CO­2-producing machines come from?

A better question may be where don’t they come from. It seems that almost every process of making and maintaining a sourdough starter contributes some microbes to the mix. Some bacteria and fungi come from the flour itself, some from the water, some from the air in our home, and many from our hands when we knead the dough. Because no microbes are purposefully added, the process of a starter coming to life is called spontaneous fermentation.

With such a wide variety of potential microorganism-sources, it’s easy to see why sourdough starters can vary so wildly. Even though most of the bacteria that thrive in a starter are lactic acid-producing, that only narrows it down to several hundred different species. The previously mentioned survey led by Rob Dunn found several hundred different species of both yeast and lactic acid-producing bacteria in starters around the world.

It also found that certain microbes were confined to certain parts of the world, giving credence to the idea that sourdoughs from different places truly do taste different. But even within a certain region or country, no two sourdough starters are liable to be exactly the same. The type of flour, where that wheat was grown, the history of the soil, the type of water, the microbes present on the baker’s hands and in their kitchens, all change the delicate balance of the sourdough starter colony.

Rob Dunn’s lab did another experiment where they sent 15 different bakers in different homes identical sourdough starter ingredients and instructions, and after several weeks characterized the microbes present. The starters all had some species of bacteria and fungi in common, specifically the ones that were also found in the flour itself. However, they all also had microbes unique to each starter. In an attempt to help solve the case of the spontaneous fermentation, the researchers swabbed the 15 baker’s hands, and found some really interesting results!

Many of the microbes present in the starters but not in the flour indeed seemed to have come from the bakers’ hands. More interesting, however, were the microbes that appeared to have jumped not from the bakers’ hands to the starter, but vice-versa! While previous research showed that Lactobacillus made up between 2 and 8% of the bacteria on human hands, the bacteria on the hands of these bakers were on average 25% Lactobacillus (and up to 80% in one case). Similarly, almost all of the fungi on their hands were yeasts common in sourdough starters.

It seems that even while our sourdough starters are colonized by the microbes in our environments and on our bodies, the inverse is true as well. If you’re worried about getting sourdough starter microbes on you when baking bread, you shouldn’t be. For one, the vast majority of bacteria and fungi found in starters are non-pathogenic. But more importantly, you are already covered in billions of microbes of your own!

As Rob Dunn explained, “If you sample the microbes on someone’s hands, have them wash them, and then sample the microbes again, no change in the overall composition of the microbes occurs.” We are covered in so many microbes that washing our hands can’t even meaningfully impact the population, so I highly doubt that a few more or less from your sourdough starter will make any impact. Though I guess you could always wear gloves.

Whether you make it yourself or buy it at a bakery, sourdough bread is undeniably unique tasting and delicious. But did you know that it may also spoil slower than bread made with bakers’ yeast? Research has shown that the bacteria in a sourdough starter produce a variety of chemicals (such as various organic acids and hydrogen peroxide) that can inhibit the growth of other microorganisms like moulds.

I tested the preservation power of sourdough bread for myself, by baking two loaves of bread (one made with ReeRee, one made with bakers’ yeast) and leaving a slice of each in a container in my kitchen. The conventionally made bread showed its first spot of mould after three days, and while it didn’t take the sourdough bread much longer to mould, only about 24 hours longer actually, the sourdough did last slightly longer. This is obviously far from a properly designed and controlled study, but if you’re looking for something to do while at home in quarantine, why not try it out for yourself! I’ve included photos of my loaves when I made them and three days later.

As if you needed more reason to switch to sourdough bread, studies have also shown that it stales slower than conventional bread, and it may even increase the bioavailability of certain minerals like iron, magnesium and zinc, although it doesn’t seem like this last assertion has been tested in humans yet.

If you don’t have a sourdough starter of your very own yet, this website doubles as a handy guide, and a citizen science project from the Rob Dunn lab that you can take part in! Just follow the instructions to make your own starter, and report on its characteristics in the linked webform. If you’re feeling really ambitious, you can even create more than one starter to compare either different types of flour, or what happens if you keep your starter inside or outside. Personally, I think I’ll be making ReeRee a friend, all in the name of science of course.


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