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.
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.
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 AskHank@culinate.com.
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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