All about sourdough

It need not taste sour

March 25, 2009

What makes sourdough sour?
— Michael H., Los Angeles, California

A concise question deserves a concise answer, so here you go: It’s bacteria poop.

I’m paraphrasing a little; it’s really “acids resulting from bacterial metabolism of disaccharides,” but that’s not as much fun to say.

If you’re not grossed out yet, stick with me, because yeast burps are involved too.

Sourdough — dough leavened by wild microscopic organisms (or microbes) — is the original leavened bread. Grain (and everything else) growing in the field naturally collects a huge variety of bacteria and yeasts (two different categories of microbes), and the process of harvesting and milling the grain into flour doesn’t get rid of those bugs.

Making a sourdough starter is easy and fun.

When the flour gets wet, the dormant microbes wake up, and if conditions are favorable, they reproduce and thrive. Enzymes in the flour break the starches down into two sugars — glucose and maltose — that microbes eat. There’s only so much sugar to go around, so not all of the microbes can thrive.

Some of them are nasty bugs that could spoil food or make us sick, so we don’t want those to grow. But this is where it gets cool: The yeast and bacteria that almost invariably end up thriving in wet flour happen to be not only safe to eat, they also make breads rise and taste better.

How does this happen? It turns out there’s an unusual type of wild yeast that loves to eat glucose, but unlike most yeasts, it won’t touch maltose.

And there are some wild bacteria that happen to require maltose to survive, and also need certain amino-acid chains that are produced by yeasts. The problem is, when these bacteria digest maltose, they poop out lots of acid (yes, I am essentially 12 years old), which kills most yeasts.

As it turns out, the wild glucose-loving yeast is also very acid-tolerant. In addition, the acid inhibits the growth of other yeasts (and bacteria), thereby reducing competition. See where this is going? These particular microbes — let’s call them “sourdough yeast” and “sourdough bacteria” — have a collection of traits that would ordinarily make them average performers at best, but they are so perfectly suited for each other that together they thrive and crowd out all the other microbes. They don’t compete for food, and their waste products actually help each other thrive.

Biologists call it symbiosis, but I call it delicious.

Granted, this is a simplified account of the love story (I’ve read stacks of scientific papers with romantic names like “Functional Characterization of the Proteolytic System of L. sanfranciscensis DSM 20451T during Growth in Sourdough”), but you get the idea.

It’s a pretty magical system. Of particular benefit to bakers, the sourdough yeasts burp lots of carbon-dioxide gas (hey, I got this one directly from bread guru Peter Reinhart — the teacher may need to separate us), which makes the dough rise.

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The acids and other substances generated by the sourdough bacteria add that distinctive sour flavor to the dough. They also protect the finished bread from mold and spoilage, naturally improving shelf life.

So it seems that ancient bakers needed only to accidentally smash some grain and get it wet, and this may be how leavened bread was discovered.

But obviously there’s a lot more than that to good sourdough, and mastering it takes some work. The wild microbes are pretty slow-growing, so it takes them days to reach their gas-burping, acid-pooping peak. In the meantime, they slowly destroy the gluten in the flour that’s necessary for a successful loaf. Thus, bakers maintain a starter culture — a highly active colony of sourdough yeasts and bacteria (kept active through regular feeding) that can be mixed with fresh flour, water, and salt to make a viable bread dough.

The active microbes can aerate and flavor the dough the same day; then the dough is baked before the flour loses its structure. The baker controls the flavor and texture of the final loaf by monitoring and adjusting the fermentation temperature, starter-feeding schedule, and other factors.

It’s a fair amount of work. Leave it to the modern world to trade flavor for efficiency as follows: The beer-brewing industry also relies on yeast, but a much faster-growing type whose scientific name (Saccharomyces cerevisiae) means “beer-sugar fungus.” This brewing workhorse became a crossover star in the baking world, and this is the active dry yeast (ADY) you can buy at the grocery store in packets of brown granules.

Compared to sourdough yeast, ADY is incredibly fast. You can inoculate a dough with ADY and be ready to bake bread or pizza within a couple of hours. Just don’t expect it to taste like much except flour. ADY gobbles up all available sugars very quickly, crushing the hopes and dreams of any wild microbes in the area. You can make good bread with ADY, but it can’t match the flavor of a well-made sourdough.

For the curious, making a sourdough starter is easy and fun. It’s like a science-fair project, minus the cheesy poster-board presentation. There’s a lot of waiting, but not much active work, and the results are worth it.

Even if your first (or fifth) loaf isn’t a home run, as Nancy Silverton of L.A.’s La Brea Bakery says, “in bread baking, mistakes are edible.” (And usually pretty good.)

There are plenty of expert instructions on how to make a starter — see the sidebar for some suggestions — and there’s more than one way to do it successfully, so don’t get too hung up on which method you use. Just avoid methods that suggest adding yeast; as explained above, ADY will dominate the culture and prevent real sourdough development.

Lore about “attracting wild yeasts from the air” is unproven, as far as I have been able to determine, as there are sufficient microbes in the dough and the water to get the job done. Similarly, adding grapes or extra sugar isn’t necessary, but probably doesn’t hurt.

I made a viable starter using only rye flour and water in a covered bowl, following the method in Maggie Glezer’s Artisan Baking. I’m not baking very often right now, so my starter lives in the refrigerator, and it takes a few days of refreshment feedings before it’s ready to rock.

Despite the name, sourdough bread doesn’t have to taste particularly “sour,” and many versions don’t. The sourdough yeast and bacteria contribute many complex flavor elements beyond mere sourness. A more accurate name might be “extra-tasty wild-fermented bread,” but the marketing department would probably nix that.

Certain varieties, such as San Francisco sourdough, are famous for a very tangy loaf, but that’s a deliberate choice of the baker. The researchers who first identified the sourdough bacteria worked in the Bay Area and named the bacteria L. sanfrancisco, but many of the same microbes are found in sourdoughs throughout the world.

So how does the baker modulate the sourness of the loaf? Herding billions of microbes isn’t an exact science, but bakers have developed a few techniques. The biggest factor is the amount of time the acids are allowed to build up in the starter between feedings, which dilute the acids with fresh flour and water. So more frequent feedings generally mean less sour flavor.

Refrigerating the starter also slows down the yeast activity and lets the bacteria dominate and produce more sour acids.

Acetic acid (vinegar) has more sour taste impact than lactic acid, and stiff (dough-like) starter is said to produce more acetic acid compared to a wetter batter-like consistency, which favors the milder lactic acid. Working oxygen into the starter by mixing or kneading during feedings should also promote more acetic-acid sourness.

Finally, the baker can control how much of the starter goes into the final dough. More starter means more sour (and other) flavor.

Honestly, I haven’t paid too much attention to these factors in my sourdough baking, and my flavors have been all over the map, but always delicious. I see my starter like a child: Neither of them are entirely predictable, but I’m OK with that.

Based in Portland, Oregon, Hank Sawtelle has engineering, legal, and culinary degrees. Email questions for the Ask Hank column to

There are 16 comments on this item
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1. by Hank Sawtelle on Mar 25, 2009 at 2:18 PM PDT

I really wanted to work in a reference to the scene from Wargames where Matthew Broderick gets in trouble for his joke about yeast reproduction . . .

2. by James Berry on Mar 25, 2009 at 2:59 PM PDT

Hi Hank. Great article—I finally had a chance to read it. I’ve been baking all this year, so it was good to get a new innoculation of sourdough lore.

3. by Matthew Amster-Burton on Mar 25, 2009 at 3:15 PM PDT

I used to work in a lab where we grew both S. cerevisiae and E. coli. One of them smelled like baking bread. The other smelled like, well, you know where E. coli lives, right?

This has nothing to do with sourdough, I just wanted to see if I could get sent to the principal’s office to get the latest password.

4. by anonymous on Mar 26, 2009 at 8:43 AM PDT

Yay for Sourdough. I have been baking up a storm lately, but haven’t tried to make a wild starter of my own yet. Thanks for the article. I’ll get it going here some day soon.

5. by Tracy Mathey on Mar 30, 2009 at 11:15 AM PDT

Thank you Hank, for the inspiration to make a wild starter! I cheat most of the time when baking bread. I use packaged yeast or a quick method and have been wanting to make a more authentic loaf. You also made the topic very clear and a fun read!


Tracy Mathey

6. by Scotty on Mar 31, 2009 at 6:54 AM PDT

I have to respectfully disagree on one point. I have made successful starters from grapes and various whole grain flours, but it will work with commercial yeast. I have no scientific data to back this up, but my hunch is that flour, water, and a pinch of commercial yeast starts the fermentation process. At some point the ferment Ph becomes inhospitable to the commercial yeast microflora, and the wild yeasts in my house take over.

It has worked for me. YMMV.

7. by dgreenwood on Apr 1, 2009 at 11:21 AM PDT

Thanks Hank for a great article. I have been making sourdough breads with a grape based starter for the past 10 months. I find the degree of sourness has more to do with how long after feeding your starter you use it. Use it 4 hours after feeding for hardly any sour taste. !2 hours gives you more character. Also how long you proof the bread.

There is an excellent website (well...more than one actually) with good videos about bread making, especially sourdough:

And I want to point out that a starter is not a finicky as people think - I have left mine in the fridge for 3 weeks without attending to it and it comes right back.

Happy baking!


8. by Richard Yarnell on Apr 1, 2009 at 5:18 PM PDT

Three weeks? Is that all? I’ve had the same starter for almost 40 years now (gasp). There have been times when I didn’t/couldn’t use it regularly. On the road with a show, in the military overseas, moving from one place to another, or, as now, building a house and camping out where bread baking just isn’t in the cards.

I stop feeding the stuff, crank down on the jar it’s in after the danger of exploding dough is past and leave it in the back of the fridge. The longest dormancy was about 5 years and it did take about 4 feedings to get it back in shape, but the flavor, as best as I remember, is about the same. Surely, nothing else could horn in to replace my sleeping friends.

For Hank. I didn’t believe this, but being a sport I tried it and against all logic it seems to have worked: I’d been told that one can recover a strain (we now know, two) of sourdough critters from the soft interior of a loaf you like.

I can imagine that the interior of a loaf doesn’t kill 100% of the organisms that are active when the loaf hits the heat. I can also imagine that dormant organisms have a higher tolerance for heat of baking, but there really shouldn’t be any sleeping cells in the batter. Any ideas, or is there another agent at work - luck?


9. by dgreenwood on Apr 2, 2009 at 9:13 AM PDT

ry -

5 years!!! Wow! I’m not sure I have 40 more years of bread baking left, but I won’t worry about the starter ever again. Thanks.


10. by Leta on Apr 8, 2009 at 4:47 PM PDT

mmmm...I am a huge fan of the San Francisco microbe as it is the “sourest” sourdough I have ever tasted. But I also like bitter beer, so go figure. I tried starting a starter once, and the bread was not good. Any tips for a non-baker to take with her as she starts her bread making journey?

11. by Hank Sawtelle on Apr 16, 2009 at 4:26 PM PDT

@Leta - what was “wrong” with the bread? There are some starter instructions from Peter Reinhart on Culinate that I linked to in the sidebar above. Or according to this guy you can basically do whatever you want :)

12. by jo_jo_ba on Apr 29, 2009 at 8:32 PM PDT

Ah, I think I need to start making my sourdough boule again... I’ve kept my starter alive (barely, and in the fridge) for almost a year now and the loaves I’v made have been decent, but nothing compares to the gorgous round boule I made the first time. The crumpets from Baking Bites are a winner too for using up “toss off”, FYI, and freeze exceptionally well!

13. by Hank Sawtelle on Apr 29, 2009 at 9:23 PM PDT

@jjb: let us know if your starter revives. I’ve kept mine sleeping in the fridge for 3-4 months (sad but true) and it seems to bounce back after a few feedings.

14. by Jacqueline Church on May 2, 2009 at 6:19 AM PDT

I love this article! I rediscovered bread baking when my husband got me the King Arthur 200 y.o. starter and a crock. Was rocking for months then left the starter unrefrigerated a bit too long and got the wrong color stuff on top. Pink, I was told anything in the orange-pink hues means it must be disposed of. Later I was told I could have scraped off the top safely and begun again with the bottom portion?

Anyway, it’s too late for that batch. I cannot get a bakery to give me any of their starter, either...

I really have to create some new starter. I find it’s much more tolerant than most would assume. Curious about the mold issue. (PS I’m a recovering atty too so you can skip the disclaimers about risk ;-) )

15. by dgreenwood on May 2, 2009 at 11:50 AM PDT

Jacqueline -

Check out Breadtopia - he sells starter that is apparently very active. The site has lots of inspiring bits about sourdough as well as great how-to videos and a lot of viewer feedback/forums. It’s a great resource.

Happy baking!

16. by Jacqueline Church on May 2, 2009 at 10:24 AM PDT

Thanks for the quick and helpful response! I shared this article with my hubs. We’re in separate cities this weekend. His email back: “I love yeast shit and burps.” And I love that man...almost typed “loave” LOL bread on the brain!

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Ask Hank

Hank Sawtelle has engineering, legal, and culinary degrees, and an intense curiosity about food and cooking. Follow Hank’s blog, Sous Vide Jones.

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