The Science of Sourdough

Sarah Meyer, Columnist

When I first gained interest in baking sourdough in 2012, it was little more than a niche artisan bread. But now, in 2021, making sourdough is unquestionably the hottest quarantine baking trendand for good reason. Beneath the crackling, golden-brown crust, it’s more than just a fad: it’s a complex microbial community of wild yeasts and lactic acid bacteria (LAB). 

I remember visiting the Boudin Bakery Museum, the undisputed home of San Francisco sourdough, with my family when I was 7 years old. As soon as I walked in, my nose was assaulted by the sour, tangy smell of fermenting sourdough and the warm, toasty smell of fresh-baked bread.

They had set out a big basket of still-warm sourdough slices for museum go-ers to try—it was love at first bite. The lacy, bubbly structure of the sourdough went hand in hand with the glutinous, chewy texture in a way that defied my limited experience of store-bought white bread. Though  the smell of sourdough was initially aggressive, the taste was well-rounded and tangy in an unobtrusive way that complimented everything I paired with it. 

That was only the beginning of my many sourdough escapades. As it turned out, recreating their sourdough was hardly an easy task, and I failed many, many times. However, I learned a lot in the process. 

Since sourdough predates commercial yeast, it utilizes other methods of leavening. The process dates back to the ancient Egyptians, who discovered an overlap between the fermentation of alcohol and the fermentation of bread, thus inoculating their bread with wild yeast. 

The fermentation of bread for leavening can also be found in other cultures as well. Ethiopia is home to a sour pancake-like flatbread, called injera, traditionally made out of teff flour. Sudanese kisra, a crepe-like sorghum bread, is produced in a similar vein. Even as far as India, where they make a type of steamed rice and lentil bread called idli, fermented bread is well-documented. 

Fermented bread was so popular in part due to its vastly improved taste and texture. It wasn’t until the 19th century when commercial yeast was made readily accessible, that bread could be leavened by other methods. Although commercial yeast provides a fast and reliable way to leaven bread, commercially leavened bread just can’t compare with sourdough in terms of flavor and texture.

This flavor and texture are owed entirely to the diverse microbial communities of wild yeast and LAB found within sourdough. LAB in sourdough can generally be categorized into two typeshomofermentative and heterofermentative. Homofermentative strains by definition only produce lactic acid, giving a flavor palette similar to the mild sour-ness found in yogurt and sour cream. 

On the contrary, heterofermentative strains of LAB produce acetic acid, lactic acid, ethanol, and certain gases. This gives a more aggressive sour taste similar to vinegar, and potentially contributing to a lighter texture.

These microbial communities are so fascinating, in fact, that researchers have devoted a significant amount of time to analyzing what factors impact them.

One study, Influences of Ingredients and Bakers on the Bacteria and Fungi in Sourdough Starters and Bread by Aspen T. Reese et al., suggests microbial communities may come from the flour used to make them, and the bakers’ hands themselves. 

Another study, Characterization of sourdough bread ferments made in the laboratory by traditional methods by Mohamed Faid et al., suggests that the consistency of sourdough is key in the flavors of the finished product. Firmer sourdoughs may host more heterofermentative strains, while thinner sourdoughs may contain fewer heterofermentative strains. Therefore, even within the same environment, sourdoughs can vary drastically depending on consistency and hydration. 

These microbes, according to the article Potential of sourdough for healthier cereal products by K. Katina et al., can make cereals more nutritious. Firstly, it makes them more tasty, but it also inhibits starch bioavailability while increasing mineral bioavailability, therefore allowing your body to absorb more of the right nutrients.

The benefits don’t stop there. The quarantine baking trend has many social scientists wondering about the social benefits of sourdough during the pandemic. 

The article By Bread Alone: Baking as Leisure, Performance, Sustenance, During the COVID-19 Crisis by Gwyn Easterbook-Smith, provides a unique commentary on how quarantine baking offers sustenance in a time of food shortages, while also filling in new leisure time and providing opportunities to utilize social media. 

Some social scientists go as far as to label baking sourdough and other artisan bread as a new assertion of socioeconomic differences between social classes. Especially in the face of flour shortages, many see it as a luxury to be able to waste flour in the upkeep of sourdough starters, which commonly scrap a certain portion per day. 

Easterbook-Smith suggests that white bread can be associated with homogeneity and lower socioeconomic groups, and by baking artisan bread, the middle class differentiates itself. 

Perhaps they have a point, but there’s no arguing against the nutritional benefits of sourdough. Moreover, making this finicky baked good is strangely meditative and rewarding, especially if you have a lot of time on your hands. Consider making your own sourdough with my recipe below (adapted from the Youtube channel “Pro Home Cooks”).


-100% Hydration Sourdough Starter-

Initial Ingredients

70 grams – unbleached flour, either white, whole wheat, or rye

70 grams – water, preferably unchlorinated


New Ingredients 

35 grams – unbleached flour, either white, whole wheat, or rye

35 grams – water, preferably unchlorinated


Day 1: Mix together initial ingredients in a small jar or tupperware, rest at room temperature

Day 2+: Scrap ½ of the starter mixture each day, and incorporate new ingredients to “feed” the starter

Ready to use when 8-12 hours after being fed, the mixture doubles in size and floats in water.


-Sourdough Bread-

300 grams – any blend of whole wheat, rye, or spelt flour

300 grams – all purpose or bread flour 

512 grams – water, preferably unchlorinated

63 grams – starter 

10 grams salt 

Any desired mix-ins or toppings (must be dry- consider dried fruits, nuts, or seeds)


Keep in mind that these times are guidelines. If the bread has not risen enough overnight, or looks underbaked, adjust accordingly.


3:00 pm – Combine flour and water into a shaggy mass and let rest in a covered bowl. Feed starter if not already fed today. This step can be shortened to an hour, if necessary.

8:00 pm – Add the starter to the dough, and rest in a covered bowl. 

8:00 pm to 10:00 pm: Every 30 minutes for the first two hours, repeat the following process, referred to as a “stretch and fold”:

Wet your hands with tap water and reach under the dough and pull it upwards from the bowl until it detaches and stretches out somewhat. Place it back down so that the ends tuck under the rest of the dough. Turn the bowl 90 degrees, and repeat 3 more times. 

On the last iteration of this process fold in any mix-ins, if necessary, and let dough rest at room temperature overnight in a covered bowl.

8:00 am – Move the loaf onto a lightly floured work surface, repeat stretch and fold one last time, and let rest.

8:30 am – Form the loaf into a round shape by dragging the outside of the dough downwards and pitching at the bottom repeatedly. Line a loaf sized bowl or proofing basket (if you have it) with a clean dish towel, and flour aggressively. Sprinkle any toppings in the bottom before transferring the loaf into the receptacle seam side up. Let rise, covered.

10:10 am- Preheat a sheet tray at 500 degrees and boil ~2 cups of water in a small, oven safe pan. Adjust your racks in order to accommodate the water in the bottom rack of the oven and the bread tray in the top rack.

10:30 am – Flip the loaf out onto a sheet tray, score the loaf with a sharp knife, and place it in the oven alongside the pan of water.

11:50 am – Remove the pan of water, turn the oven down to 450 degrees. 

12:10 am – Remove your deliciously baked bread from the oven.