Science and Sourdough

Acccording to Dr. Google, there are 82,900,000 entries for sourdough on the internet. Here’s my contribution to that impressive list.

Making sourdough bread can be very satisfying. Even while you are falling asleep the night before you make the bread, with your levain ripening, you’re thinking, those little yeasts and lactobacilli are working away down there and in the morning we’ll all get together and make a really satisfying loaf of bread. I’ve done a little research, which I will describe; but for the definitive treatise buy a copy of Nathan Myhrvold’s epic series ‘Modernist Bread’. It’s 2,642 pages of deep study and gorgeous photography about bread, and sourdough enjoys a goodly share of that space.

Myrvold came to Microsoft when it purchased his own company in 1986, and he became Microsoft’s first Chief Technology Officer, my old job. In 1995 he left to devote himself to other projects, including photography and food. ‘Modernist Bread’ is one of the outcomes. Another is ‘Modernist Cuisine’. The bread missive, in 5 volumes, rings in at 510.23 US$ on, which is 169.57 US$ off the usual price, in other words, a bargain. You should know that Myhrvold holds a culinary diploma from ´Ecole de Cuisine La Varenne’ in France. He has also won the James Beard Foundation Award. Anyway, enough about Myhrvold.


One of the appealing things about sourdough bread (you can also make pancakes, biscuits, and even chocolate cake with sourdough starter) is that it contains only 4 ingredients: water, salt, flour and starter. Like the blues, a very simple foundation which can give rise to a lot of variety. Calling the starter a single ingredient, of course, is a deception (as is calling the blues ‘simple’). The starter, basically just a smaller sourdough, contains yeast, which will provide the leavening for the dough, and lactic acid bacteria, ‘lactobacilli’, which will convert sugar to lactic acid and other trace ingredients that give the bread its tangy sour taste. As to which yeast and which lactobacilli . . . ah, there’s the rub. Each person’s starter may differ from most others, and the one you make, if you choose to make one, will probably change over time. The only microbiological analysis that counts is the taste of the bread that comes out of the oven.

So the process is, mix sourdough starter with flour, salt and water. Watch it ferment and expand. At some critical juncture pop it into the oven. Then take it out and face a great temptation, to which the only correct response is, do not cut into the loaf for at least an hour.

The Science

Flour contains starch, a polymerized form of glucose, which the wheat plant (or other grain) lays down in the endosperm of the wheat seed. It’s there to feed the little embryo that’s also in the seed, which will produce the next generation (unless its growth is interrupted to yield sprouted flour, currently a thing). The starch needs to be broken down to the simple sugar glucose, which is what the yeast and lactobacilli will consume.

Flour itself contains enzymes that break down starch to maltose when it is soaked in water. Maltose consists of two glucose molecules linked together. Yeast secretes an enzyme that cleaves maltose to glucose, which is the fuel for the microorganisms.

Both the yeast and the lactobacilli consume glucose by a common pathway, called glycolysis, in which the sugar glucose is converted by a sequence of metabolic steps to two molecules of pyruvate. In yeast, pyruvate is then converted, by a couple of additional steps, to ethanol and carbon dioxide (‘fermentation’). Under other circumstances, that would lead to the production of beer, or wine. In breadmaking, the ethanol and carbon dioxide escape during fermentation or are baked off, leaving behind holes that define the bread’s ‘crumb’.

The lactobacilli (bacteria that produce lactic acid) turn pyruvate into lactate, which is also what happens when an athlete is propelling herself down the track too fast for her oxidative metabolism to catch up, and experiences muscle cramps from the lactic acid produced by glycolysis. Glycolysis, the most primitive part of energy metabolism, is essentially the same in the athlete’s muscles, or any other of her cells, and yeast and bacteria.

Both processes, those of the yeast, and particularly those of the lactobacilli, also produce a variety of other metabolic products, and these give the bread its individuality.

If you know something about energy metabolism, you may be puzzled that the yeast produce carbon dioxide and ethanol, rather than oxidizing pyruvate all the way to carbon dioxide, which yields a lot more energy. The answer is, we’re not sure. It’s the same thing that many cancer cells do — they are profligate in their consumption of energy supplies, excreting a product, ethanol from yeast, lactic acid from the cancer cell, that results from utilizing only a fraction of the potential energy of the original glucose. And not all yeast do this — there are some that oxidize glucose all the way to carbon dioxide, without producing alcohol. The yeast for bread making, Saccharomyces cerivisiae, carries out fermentation and ethanol production.

The Starter

You can get starter commercially, or online , or from a friend. Or, if you like to experience it all, you can make your own. Sourdough starter contains yeast and lactobacilli. The yeast (species Saccharomyces cerevisiae) causes most of the rise; the bacteria provide the acid, and aromatics, that characterize a sourdough. The yeast probably comes from the flour used to make the starter. Some variations are known; it is said that in San Francisco, there’s a substrain of yeast called Candida Milleri that makes sourdough extra tasty. This may in fact be the principal yeast in most sourdoughs. I don’t know.

The lactobacilli, which probably originally come from the body of the person making the starter, produce lactic acid, as they do in yogurt making. The pH goes down to about 3.8 during bread fermentation. Only some yeast can grow at this pH; this is probably why making starter takes a while — selection is necessary.

In San Francisco, there are unique bacteria called Lactobacillus sanfranciscensis in at least some of the sourdoughs. I don’t know what it is in other places, but variation in the lactobacilli is probably responsible for the difference in sourdough made with different starters and in different places.

The basic science of sourdough is nicely described in Discover magazine, at:

It took me quite a long time to make starter. I mixed white and whole wheat flours with enough water to make a smooth dough (roughly 50:50). I put it on top of the fridge, where it’s a little warmer than room temperature. Then I fed the culture every couple of days for three weeks. Feeding consisted of removing about half the material (into the compost, not the sink) and adding 30-50 grams of flour (any kind – the yeast didn’t seem to care) to the culture and an equal weight of water, and mixing vigorously with a knife.

In my case, small bubbles and a thin, unappealing liquid formed on top early in the three-week odyssey. This was probably because the lactobacilli were doing most of the fermenting, the yeast having not yet been selected for the ability to grow at the low pH produced by the bacteria. You’ll know when you’ve arrived — the culture can expand three or four fold overnight. In my case, the culture I eventually obtained probably only got started in the second week or so. By passaging it for several weeks it reached a stable balance between the yeast and the bacteria. In any case, it’s a vigorous little monster.

Starter should be kept in the fridge. Most online sources will tell you to feed it twice a week. In my experience, that’s not necessary — I get good results even feeding it every 14 days. To feed it, throw out most of the starter, add 30 grams of flour and an equal weight of water, mix with a knife, and let it grow for a few hours at room temperature. When it’s going well, put it back into the fridge.

If the starter hasn’t been fed for a while it may look downhearted, but you can almost always cheer it up by feeding. If the starter, or your bread dough, stops rising, it won’t start again without feeding. It has probably run out of the sugar, or maybe it got too acidic because the lactobacilli outgrew the yeast.


There are more methods for making sourdough than you can shake a stick at. My method is a mashup, plus some of my own tricks.  A good description of some of the technique involved is at:

Some notes

  1. Flour. You can use almost any white flour – all purpose flour, bread flour, or so-called ‘strong’ flour (which is the same as bread flour). But it should contain at least 13.5% protein. I add whole wheat flour and dark rye flour in the recipe below. Sift both of these – the sharp little husky bits in these flours are said to  break up the bubbles in the rising dough. I noticed that the sourdough rose faster once I started adding dark rye flour to the mix. Apparently that’s because rye flour contains more of the enzyme, called amylase, that liberates carbohydrates from flour. But rye also contains less gluten-forming proteins, so I add vital wheat gluten to the recipe. You can probably ignore that step.
  2. Salt. Bread made without salt is inedible. But salt slows the growth of yeast, and so slows the rising. I’ve experimented with salt, and the amount in the recipe below is the result.
  3. Temperature. Cold temperature, like salt, slows the rising. I have a ‘warm place’, which is at 24ºC (about 75ºF). In summer, it may be hard to find a place as cool as this.
  4. Time. At the temperature I use (24ºC, 75ºF), it takes about 6 hours to go from the initial mixing to transfer to a banneton for the second rise. The dough goes into the fridge at about 8-9 hours.

The total elapsed time for my approach is 3 days. Which sounds oppressive, but really isn’t. On the first day, you work for two 10-15 minute stretches. On the second, you have to do things, and you need to be around, in and out, for most of the day. But the amount of time you work is modest. On the third day, you bake the bread, whenever you want (baking takes about 1.5 hours in total).

The equipment

sourdough baking tools
The gear I use. In the centre, the handy ‘Escali’ kitchen scale. Beginning on the left and going clockwise, a 2 Liter mixing bowl with cover. A very usefull little jar (just over a cup in volume; the Bon Maman jam jar shown is perfect for keeping the starter in the fridge). Banneton and cloth liner. Brush for removing excess flour from the dough. The lame (razor blade). Bowl scraper to help encourage the dough to drop onto the counter from the mixing bowl. Bench scraper. Not shown: baking dish and a larger glass or measuring cup that will hold about 500 mL.

I use a r0und banneton for the second (“proof”) rising. This makes a round loaf (“boule”). You can use a different shape, or a bowl, as described below.

The levain

Around 10 AM on the day before you want to start the dough, mix 10 grams of starter from the fridge with 20 grams of warmish water and 20 grams of white flour. Mix with a knife. Cover, and put it in a warm place. The levain should greatly increase in size during the day, and the top should have a convex shape. In the evening, throw away all but 20 grams of the starter and add 90 grams of white flour and 90 grams of water. Mix and put it into the warm place. Typically, it will expand to about 500 mL overnight.

Making the dough

This is a 1000 gram, 72% ‘Baker’s percentage’ loaf. (‘Baker’s percentage’ is the amount relative to the amount of flour.) Best moisture content may vary with the flour used.

312 grams of warmish water

11 grams salt

200 grams active levain

252 grams white flour

116 grams of sifted whole wheat flour

116 grams of sifted dark rye flour

1 Tbl vital wheat gluten (optional,but good to use, in my experience)

On the morning of day two, put the water into a 2 liter glass bowl. Mix the salt in the water until it’s all dissolved. (I’m a mix addict. Most web sites show sloppy mixing. I think it’s important at each step to mix certain things well.) Add the levain and cut it up with knives, as in making pastry. The levain must float initially. It isn’t necessary to completely break it up. Let it rest a few minutes.

Wash your hands.

Weigh and mix the flours (and gluten, if you’re using). Mix them! Add the mixed flour to the water. Mix thorougly, first with a spoon, then by squeezing with your cold wet hand. Cover the bowl and put it into the warm place.

The dough is now undergoing hydration – the flour is absorbing the water and changing its consistency (as you will notice when you go back to it after an hour).

Bulk Fermentation

After about 60 minutes, mix the dough a little more, then carry out the first stretch-and-fold. The internet site above, and a million others, shows how it’s done. This is part of the famous ‘Tartine’ method of sourdough making, from San Francisco. Basically, just grab one edge of the dough, pull it up and out, then stick it back onto the doughball. Rotate 90 degrees, repeat 3 times.

Do 4 (or more, if you feel like it) stretch-and-folds at 30 minute intervals.

At the level of hydration I’m using here (72%), the dough may come out of the bowl as you try to stretch it. Let it. I like to take the dough ball right out of the bowl and stretch it in all directions, so it looks like an emerging pizza dough, and then fold it before putting it back into the bowl.

Now let the dough ‘bulk rise’. Knowing when to stop the bulk fermentation is critically important. To help with that, you can cut about 100 grams of the dough off after the last stretch-and-fold and put it into a small glass jar with vertical sides (a small Bon Maman jam jar works beautifully), which you put into the warm place beside your bulk rising dough.

When the dough has increased in volume by 50-80% (as seen in the small test jar), you’re ready for the next step, the ‘shaping’.

Shaping and Proofing

Let the dough fall out of the glass bowl onto a dry, unfloured counter top. Use a bowl scraper, the orange tool in the picture above, to loosen the edges before turning the bowl upside down.

Stretch the dough into a rough square. Treat it gently, you don’t want to deflate the little carbon dioxide bubbles. Grab the corners, pull them out quite strongly, and then over, and stick them to the top side. Repeat about 8 times. Then flip the doughball over, using a bench scraper if necessary. Shape the doughball into round using your damp hands or a bench scraper.

After 10 minutes, flip the doughball over so it’s top down and do the stretching again. Turn it right side up and shape.

The dough should now be showing some toughness. If it still slumps, do a third stretch-and-rounding.

Lift the dougbhball, top down, into a large container of white flour. Then lower it into the banetton which has been covered with rice flour. Sprinkle a little flour on the bottom of the dough (which is facing up in the banetton).

(If you use a bowl to proof the dough, line it with parchment, then place the dough ball, right side up, into it.)

Cover it and put it into the warm place. When it rises 30-50% – this may take a few hours – either bake or refrigerate.

Refrigeration option: put the proofed dough into the fridge overnight, covered. Even if you want to bake the same day, it’s a good idea to refrigerate the dough for a couple of hours first. I prefer to let it sit in in the fridge overnight.


A baking dish favored by a lot of sourdough bakers is a 4L dutch oven (must have an oven-proof handle on the lid), but you can use any ovenproof pot with a good lid. Preheat the oven, with the baking dish in it, at 450-500º for 40 minutes.

Remove the dough from the banneton by placing a large square of parchment over it, and a plate over the parchment, then inverting. Trim the parchment paper, leaving two “ears” on opposite sides for handling the dough.

If you’ve put the dough into a bowl to proof, simply remove it by the parchment paper. It’s already right side up.

Score the top of the dough about 1/2 inch deep with a lame or razor. Scoring facilitates “oven spring”. For fancy scoring, check out the Internet. The commercial lame I have curves the razor blade, and makes a better gash in the dough.

Take the hot baking dish from the oven, lower the dough ball into it using the parchment. Place the hot lid on top (essential!).

Cover and bake 20 minutes at 450-500º. Remove the lid and bake for another 10-15 minutes, until the top has a nice dark tan colour.

Most important step: Resist the temptation to cut the bread for at least an hour after it comes out of the oven.

The finished loaf will have about 2000 Calories (if you took off 100 grams to watch the bulk fermentation).

Raisin bread option

Add 80 grams of raisins that have been soaked in water for a couple of hours. Easiest is to add them at the first stretch and fold. Some people also add cinnamon. Know that these raisins will add about 240 Calories to the bread.

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