Science and Sourdough

Acccording to Dr. Google, there are 82,900,000 sourdough recipes on the internet. Clearly, what the world needs now is another sourdough bread recipe. So here it is.

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 you are free to ignore. 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 lactobacilli, bacteria that 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 produces alcohol.

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 garbage, 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.

The literature

There are more methods for making sourdough than you can shake a stick at. The closest to my own approach is described at:

The Tartine Bakery in San Francisco is a sourdough shrine. It’s recipe, as adapted in the NY Times, is at:

A description of kneading techniques is at:

My method for sourdough, which is the result of trying this and that, follows.

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 and making the levain). 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.

The levain

The process starts when you make a small dough called a ‘levain’ (it leavens). In the morning of the day before you want to start making the bread, mix 10 grams of starter from the fridge with 15 grams of warmish water and 15 grams of flour (any kind). Mix with a knife. Cover, and put it in a warm place (aim for 25ºC, which is 77ºF). The levain should double in size during the day, and the top should have a convex shape (higher in the middle) indicating that it’s growing and healthy. That evening, add 30 grams of flour and 30 grams of water, mix, and put a saucer under it (in case it gets too feisty). If the levain doe not look rarin’ to go next morning, feed it and try again the next day.

Making the dough

Pour 310 grams (1.3 cups) of warmish water into a mixing bowl that can hold at least 2 liters (8 cups). (Note 1)

Add 1.5 tsp (9 grams; OK, sometimes imperial is easier) salt and dissolve it in the water

Add 100 grams (in other words, all) of your levain to the salted water. It will float if it’s active. If it doesn’t float, call it a day.

Cut up the levain with 2 knives, like cutting butter into pastry flour.

When the levain has been somewhat dispersed (don’t worry if there are still some blobs floating around),

Add 350 grams (2.75 cups) organic, white, bread (‘strong’) flour and 100 grams seived whole wheat flour. (Note 2)

Wash your hands. Mix the dough ingredients briefly with a spoon, then get in there with one wet hand and squeeze the dough ball repeatedly until it is mixed. The only way to avoid this messy (but not unpleasant) step is to have an electric dough mixer, which I don’t. When it looks thoroughly mixed, scrape most of the dough off your hands and into the mixing bowl.

Notes on making the dough

  1. There’s a lot of chat on sourdough internet sites about hydration. Hydration is the amount of water in the dough, compared to the amount of flour, expressed as a percent. I suggest that at least initially you go with my amounts (72% hydration, if you include the levain). If you later want to go to high hydration, that’s fine; higher hydration produces bigger holes in the bread. (But butter might fall through the holes.)Many authorities suggest mixing most of the water with the flour and leaving it for an hour before adding the starter and salt and remaining water. This is called an autolyze (‘auto-lease’) step. I don’t find any advantage in doing it.
  2. White flour comes in a variety of types. I use ‘strong’ bread flour, which has a higher protein content than multipurpose white flour. But multipurpose will also work. Bread flour is not always easy to find — I get mine from a health food store. Whole wheat flour adds flavour. I use ‘Red Fife’, a heritage strain. Seive whole wheat flour to remove bran, which may puncture holes in the bubbles when the bread is rising. You can also make great sourdough with whole wheat anywhere up to 100%. I suggest you don’t do that your first time. It will, for one, be a little harder to manage the dough, and will definitely require more water at the beginning.

Working the dough

When the dough is mixed, put it into a warm place (around 25ºC, 77ºF). After an hour squeeze the dough a few more times with one wet hand, just to insure it’s fully mixed. Then, the stretch and fold.

The Tartine stretch and fold method is described in the internet link above. What I do is a little different: I grab one side of the dough and lift it out of the bowl. I then stretch the dough into a thin round, as if I was making a pizza. Then I fold it together and put it back into the mixing bowl. The stretching and folding helps to get the developing gluten fibres lined up, and develops ‘toughness’. It replaces kneading.

I do 6 stretch-and-folds at half-hour intervals. The published methods will tell you a variety of numbers, usually no more than 6. You will notice the dough getting smoother and stronger as you go.

After the last S+F, let the dough rise. When it has expanded by about 30%, and there are encouraging bubbles, turn the dough out onto a very lightly floured counter. The bottom surface at this point will eventually be the top of the bread. Stretch it gently into a square. Then pull the corners out and fold them over. When you’ve done all the stretching and folding you can, turn the doughball over (a bench scraper is handy) and shape it into a round with your hands and a scraper. Do all of this latter stretching and folding a little reverently; you don’t want to destroy too many bubbles.

After 10-20 minutes turn the doughball over (again, top down) and repeat the stretch and fold. Turn it right side up and repeat the rounding.

Finally, place the doughball, top down, into a banneton for final proofing. (Bannetons come in two basic shapes. I use the circular one.) Lining the banneton with a cloth sprinkled with rice flour makes things a little easier, but largely erases the nice circular pattern of the banneton on your bread. (You can use a bowl for this proofing. Line the bowl with parchment and place the doughball into it, top up.)

Cover the dough with cling wrap and put it into a plastic bag to keep it from drying out. Let it rise (‘proof’) until it almost fills the banneton or bowl, or from experience, about 80% of the total rise you can expect. This rise may take 3 hours or so.

Now, options: either bake immediately, or put it into the fridge overnight. I prefer the latter. Even if you want to bake the same day, it’s a good idea to chill it in the fridge for a few hours first.


I make a round loaf (‘boule’), for which my 5L Staub dutch oven works very nicely. But any dish that is oven proof and has a good cover (Corning dish, for example) can work. Having the baking dish closed at the start of baking generates steam, which leads to a good crust.

Preheat the dish, lid off but also in the oven, at 230ºC (450ºF) 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 with a lame (razor). Scoring facilitates ‘oven spring’, the extra rise you get from the expanding bubbles. For fancy scoring, go to 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.

Cover and bake 20 minutes. Remove the lid and bake for another 10 minutes or so, until the top has a nice dark tan colour.

Repeat: most important step is to resist the temptation to cut the bread for at least an hour after it comes out of the oven.

715-gram loaf, 1800 Calories (2.5 Cal/gram)

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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