Science and Sourdough

(Revised August 27, 2023)

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 in the kitchen, 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 you can 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 tech 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  usually several kinds of 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. The engaging feature of the sourdough starter is that it will probably contain lactobacilli derived from the skin of the person who made the original starter. And manipulating the dough with your hands, as suggested below, will further ‘contaminate’ it with bacteria derived from you. Each loaf unique, yet bearing a history.

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’). Depending on the circumstances, that can 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 in a sprinter’s muscles when she is propelling herself down the track too fast for her oxidative metabolism to catch up. The lactic acid buildup in the muscle tissue is responsible for ‘the burn’ sometimes experienced by athletes during anaerobic exercise. Glycolysis, the most primitive part of energy metabolism, is essentially the same in the athlete’s muscles, or in any other of her cells, and in 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 all of the pyruvate 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 want to have the whole experience, you can make your own. Sourdough starter contains yeast and lactobacilli. The yeast causes most of the rise; the bacteria provide the acid, and aromatics, that characterize a sourdough. The yeast possibly comes from the flour originally used to make the starter, or, I think more likely, from the air. Some variation in sourdough yeast is 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 skin 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 the lactobacilli are in other places, but variation in these bacteria 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 now add dark rye flour to the recipe. Gives a more interesting flavour. I tried adding whole wheat in various proportions. This may make the bread more appealing to the health conscious, but I didn’t think it added to the flavour (even the heritage strain Red Fife didn’t do anything for me).  If you are adding whole grains, sift them – the sharp little husky bits in these flours are said to  break up the bubbles in the rising dough. Dark rye flour may increase the rate of rise. Apparently that’s because rye flour contains more of the enzyme amylase that liberates carbohydrates from flour. But the gluten-related protein in rye is feckless with respect to forming structure in the bread, and using rye flour means that there will be less rise (smaller holes) in the final product. Adding vital wheat gluten didn’t make any difference, in my hands.
  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 gives the best result.
  3. Temperature. Cold temperature, like salt, slows the rising. I have a ‘warm place’, which is at 24ºC (about 75ºF).
  4. 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 day you have to do a number of things spread throughout the day. But the amount of time you actually work is modest. On the third day, you bake the bread, whenever you want to (baking takes about 1.5 hours in total). Working at 75ºF (24ºC), the time after first mixing is: to preshaping – 4 hours; to placing it in the banneton – about 4.5 hours; placing the banneton into the fridge – 5.5 hours.

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 cover. 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 round 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 ‘fridge starter’ with 20 grams of warmish water, 4 grams of dark rye flour, and 16 grams of white flour. Mix with a knife. Cover, and put this ‘working starter’ in a warm place. It should greatly increase during the day, and the top should have a convex shape. (Throw out most of the ‘fridge starter’ and feed it with 30 grams water and 30 grams flour.)

In the evening, add 20 grams of the ‘working starter’ to  90 grams of water, 18 grams of dark rye flour, and 72 grams of white flour (it’s easiest to add in that order, water first). Mix this levain and put it into your warm place. Typically, it will expand to about 500 mL overnight.

Making the dough

This is a 1000 gram, 80% ‘Baker’s percentage’ hydration loaf. (‘Baker’s percentage’ is the amount relative to the amount of flour.) The dark rye flour increases the water requirement; the hydration for straight white bread can be less.


345 grams of warmish water

14 grams salt

200 grams levain

365 grams white flour

91 grams of dark rye flour

rice flour (to flour the banneton later)

On the morning of day two, put the water into a 2 liter glass bowl. Dissolve the salt in it. Add the levain, which should float. Cut it up with knives, as in making pastry. It isn’t necessary to completely break it up. Let it rest a few minutes.

While it’s resting, weigh and mix the flours. Yes, mix them! Add the flour to the water-levain mix. Mix first with a spoon, then by squeezing with your clean 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 by hand, 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’ sourdoug method, 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 5 stretch-and-folds at 25 minute intervals.

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.

After the last stretch and fold, let the dough ‘bulk rise’. Knowing when to stop the bulk fermentation is important. To help with that, you can cut off about 100 grams of the dough 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). Mark the dough level with a rubber band around the jar, and put it into the warm place beside the 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 ‘pre-shaping’.

Shaping and Proofing

Let the dough fall out of the glass bowl onto a dry, lightly-floured (or 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 square. Treat it gently, try not to burst too many of the little bubbles. Grab the corners, pull them out and over, and stick them to the upward-facing part of the doughball. Repeat about 8 times. Then roll the doughball over, using a counter scraper if necessary and shape it into round using your hands or a bench scraper.

After 20 minutes, flip the doughball over 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 doughball, top down, into a container of white flour. Sprinkle some flour on the upper surface (which will become the bottom of the loaf). Dust off lightly, then place the doughball, top down, into a rice-floured banneton. (If you use a bowl to proof the dough, line it with parchment, then place the dough ball in it right side up.)

Cover the banneton and put it into the warm place. When it rises 50-60% – this may take an hour or two – put it into the fridge overnight, covered (a banneton cloth cover works nicely). You can bake the bread on day two, but it’s still a good idea to refrigerate it  for a couple of hours first.


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 425ºF for 40 minutes.

Remove the dough from the banneton by placing a large square of parchment paper 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 25 minutes at 425º. Remove the lid and continue baking until the top has a nice dark tan colour (probably 20-25 minutes).

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 monitor 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 raisins will add about 240 Calories to the bread.

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