Sourdough is generally considered wild yeast, though the flavor of sourdough is actually from a mixture of yeast and lacto-bacteria. Once a stable culture has been created, the amount of leavening gas is produced roughly equally by the yeasts and bacteria.
The flavor and nature of a given sourdough has been claimed to depend strongly on the location. The famous San Francisco sourdough grows only in (and near) the city of San Francisco; if taken elsewhere local yeasts and bacteria will soon grow, and in a few months the cultures will no longer be the same. For this reason, sourdough fans every few months trade batches with San Francisco locals, to get the original flavor or leavening action.
Because each local variation is different, rising times and flavors will also diverge. Some batches, can even take longer to rise, up to a full day. These alterations will then result in a local specific flavor.
Cooking with sourdough must be planned days or even weeks in advance to ensure sufficient starter is on hand. Bread can take 3 days between starting and baking, not counting time to prepare the starter.
Obtaining starter 
Sourdough starter is alive. Thus the easiest way to get starter is find someone with it, and borrow a small amount and increase it. If you wish to have a starter other than whatever is local, this is the only way to get it. You can get a free sourdough starter that has been kept alive for 150 years from the Friends of Carl Griffith.
Making your own starter 
If you cannot find a source of starter it is easy to make your own.
Key concepts 
Several key concepts are needed for understanding the sourdough production process. In what follows, recipe by volume measures are probably familiar. Measurement of the ingredients by weight, and any notation system used may be new and unfamiliar. Such notation systems are actually very simple, and it is recommended you familiarize yourself with them.
Two types of sourdough starter 
In the most basic sense, there are two types of sourdough used in baking. One type is used in acid-base reactions to form leavening gas, such as sourdough quick breads or pancakes. Such doughs or more liquid batters might be combined with baking soda for the production of leavening gas. These sourdoughs may be kept in an imbalanced or unstable state of abnormally high acidity. The other type of sourdoughs are ideally kept in stable-culture states. Breads made from such starters are often associated with places like San Francisco and French sourdoughs, or German-rye sourdoughs, etc. These sourdoughs are leavened by the gas production of yeasts and lactobacilli during fermentation.
Unstable sourdoughs can easily be created from stable ones, all that's needed is to stop refreshing it as frequently, or to refresh it with a too-high ratio of old:new flour. For both types, sufficient time is required for the organisms to initially grow, and with respect to the latter type, balance themselves into what is called a stable culture. Patience is required for creating both types, and with respect to stable cultures, more attention to details are required.
Refreshment occurs when new dough is mixed with older dough. New dough is made by mixing fresh flour with water. New dough becomes older dough when it has sufficient age. New dough is essentially new food supply for the yeasts and lactobacilli existing in the older dough. That food supply comes from the flour in the dough.
When older dough is mixed with new dough, any organisms in the older dough inoculate the new dough. With sufficient time the populations of organisms in the older dough increase. Over multiple refreshments, a stable culture emerges. If the objective is maintenance of a stable culture suitable for leavening bread, one which is stored or fermented at room temperatures, refreshments that occur once per day are acceptable.
According to sourdough researcher Micheal Gänzle, optimum growth for one strain of Lactobacillus sanfransicensis occurs at 32 °C (89.6 °F), and for the yeast Candida milleri, 27 °C (80.6 °F). That yeast stops growing at 35–37 °C (95–99 °F). Some enthusiasts think that starter-storage temperatures over 27 °C (80.6 °F) are too hot. At temperatures lower than that, the generation times of both organisms are increased.
According to Van der Meulen, Scheirlinck, Van Schoor, et al., when starting a spontaneous starter, enterococci and lactococci establish themselves before lactobacilli, which establish themselves before the yeasts. The emergence of heterofermentative lactic acid bacteria follows, and is a marker of having a stable culture. These bacteria produce CO2 leavening gas.
Thus, a tactic to speed the establishment of a stable culture may be to use optimum lactobacilli temperatures in the first 2-3 days, then slightly lower temperatures better suited to yeast growth afterward. Once a stable culture is established, the mother dough is often kept at room temperature, and sometimes is refrigerated to slow growth. Calvel cautioned against the use of storage temperatures lower than 10 °C (50 °F).
Until bacteria and yeast populations establish themselves, the pH of the dough tends to be sweeter or more alkaline. The "sour" of sourdough essentially means "acid" dough. As new dough ages it becomes more acidic. The bacteria growing in sourdough produce lactic and acetic acids. Sourdough starter has preservation effects in the bread made with it, decreasing its rate of staling. The pH of a dough can be measured at anytime; however, from a dough kneader's viewpoint, a more acidic dough is stickier.
In his correspondence with Dan Wing, Gänzle said the optimum pH for lactobacilli is in the range of 5.0-to-5.5. The lactobacilli colonies stop growing at 3.8, they stop producing acid at 3.6. New dough has a higher pH than sourdough, so every time mother dough is refreshed with new dough, the pH is increased or made more alkaline. As the organisms' populations grow following refreshment, they produce more acid, lowering the dough's pH as time passes. The yeasts are less sensitive to acidic conditions than the lactobacilli, so by manipulating the older:new dough ratio, in cooperation with the time interval between refreshments, conditions more or less favorable for each set of organisms are created.
Typical ingredients 
Bleached flour or tap water can be used, but may give undesirable results. Bleached flour has most of the yeasts killed. Chlorine in tap water can inhibit the wild yeast that you wish to grow. For typical, artisan-style lean-dough breads, the wheat flour used should be of a protein level suitable for bread. For quick breads, lower protein flours are commonly desired.
Flour manufacturers may not have added barley malt or enzymes, thus diastatic malt may be a desired ingredient in the initial dough. Diastatic malt is generally finely ground malted barley or other sprouted grain that has not been subjected to high temperatures which inactivate its enzymes. One function of the enzymes is to snip apart the starch matrix into food the organisms can consume. If the flour's ingredient label doesn't indicate enzymes or malt were added, then add powdered, diastatic malt at a rate of 0-2% of the flour weight. Calvel used 0.5% based on the flour weight in the initial dough of his primary-culture levain.
Firm starter by weight 
The following formula was inspired by the former Larraburu Company of San Francisco. Inspiration was also provided by the works of sourdough researcher Micheal Gänzle, as well as many others, found by following the links in the references section. What follows is a simple, basic process, and thus should be beneficial for beginners' understanding. There are many instructions by excellent bakers that use more complexity in their sourdough formulas, often for very good reasons. Unfortunately, those very same embellishments tend to obscure the basic process.
Bakers' percentages 
Professional bakers measure by weight due to its higher accuracy compared to volumetric measure, even though some professionals publish volumetric recipes. Professional bakers also tend to work with larger dough amounts in commercial settings. One advantage of bakers percentages is that they may be easily scaled to larger or smaller quantities. Baker's percentages are based on flour weight, and are calculated so the flour percentage equals 100%. However, this is not an article about how to calculate them.
A brief review of how to use a typical formula composed of bakers' percentages:
- ingredient weight = flour weight × baker percent(ingredient)⁄100%
Sourdough has a flour fraction, as aged as it may be, and it has a water fraction. This means the starter or older dough would need to be proportioned in order that the new flour and the old flour (in the older dough) would add up to 100%. An example of such a notation system is presented by Kulp and Lorenz in their Table 10, "Formulas of U.S. Sourdough French Bread and Panettone Bread".
In the notation system of Firm starter by weight, the new flour is considered the 100% ingredient. Such a notation system is similar to the one Gisslen used in his formula for Rustic Sourdough Bread, a final dough. The starter from the prior refreshment and its flour, the old flour, is not included in the 100% new-flour value. Thus, the above formula has been altered with a minor change:
- ingredient weight = new-flour weight × baker percent(ingredient)⁄100%
Scales or balances 
A scale of 1/10 gram readability or better is required for measuring the ingredient weights that follow. The error range of a 1 gram readability scale when measuring 5 grams is 20%.
If your scale reads with only 1 gram of accuracy, the weights given will need to be increased. Simply multiply the flour weights by 10 and recalculate the other weights from the bakers' percentages. You could also multiply the given weights by 10.
Initial dough 
grams wheat flour 100% 13.33 water 50% 6.67 formula 150% 20
The bottom line's numbers are known as the formula percentage and formula weight.
Combine 13.33 grams of flour with 6.67 grams of water in a rather small bowl, about the size of a custard cup. Lightly mix it with a spoon until the water has just combined with all or most all of the flour. A second spoon is helpful for scraping the dough from the first spoon. Let it sit or rest for a few minutes to hydrate. Next, finish the mixing process by hand kneading it. It is too small of a dough amount for mixing machines, and it won't need much kneading. If it gets hard to knead, and the dry flour is not fully incorporated, let it rest for 15-20 minutes to relax the dough, then finish kneading it. Form it into a small dough ball. Be sure to have prepared a small jar or container half full of flour, with a slight depression in the center as viewed from above. Place the dough ball into the depression. The purpose of the flour and its depression is to keep the dough ball from rolling and contacting the container and sticking to it: it also raises it higher in the jar, easing daily access. Loosely cover the container with a lid, and ideally store it in a location with a temperature in the range of 27 to 32 °C (81 to 89.6 °F), slightly warmer than room temperature, for about 24 hours. If fermentation occurs at temperatures lower than that, the organisms' generation times are increased.
Maintain-weight refreshments 
Days 2-7 (or longer)
refresh every 24 hrs
grams wheat flour 100% 10 water 50% 5 starter 50% 5 formula 200% 20
The flour weight equals the formula weight divided by the formula percentage: 20 g ÷ 200% = 10 g.
Lightly mix 10 grams of flour with 5 grams of water, making 15 grams of new dough, and let it rest for at least a few minutes to hydrate. Hand knead the result, making a new-dough ball. Weigh 5 grams of starter from the total of 20 grams made the previous day, and discard the remainder. Knead the starter and new-dough ball together well, and form into a refreshed-dough ball. Place it back in the jar, and cover. Repeat this process once per day for at least 7 days. Until you have a stable sourdough and one that rises or swells reliably, continue to ferment it in the range of 27 to 32 °C (81 to 89.6 °F), refreshing once per day. Once your dough rises reliably, fermentation may occur at room temperatures. If you don't have such a warm place, and only have room temperature available, expect the process to take a bit longer.
Presuming you originally used a small jar half-filled with new flour, and haven't changed the flour, you will probably observe that it moistens somewhat after a few refreshments, and once a firm sourdough culture is successfully created and fermented in this flour, it will begin to develop its own set of faint odors. It is no longer fresh, new flour.
You may decide to change the mother dough's flour weight for various reasons. However, the ratios of the newly added fresh flour to the older flour already fermented are rather important. This ratio is somewhat occulted in bakers' formula notation, so figure and weigh carefully.
Increase-weight refreshments 
grams wheat flour 100% 40 water 50% 20 starter 50% 20 formula 200% 80
To increase the starter weight the same abstract formula is used to determine the flour weight, but the values used are different. You'll note that in the matrix the 20 g figure occurs on an ingredient line, the starter's line instead of the formula line. To determine the flour weight, the existing starter weight from the prior fermentation is divided by the bakers' percent(starter): 20 g ÷ 50% = 40 g. This is the maximum amount of new flour that can be used for this increase-weight refreshment given the available starter weight.
The formula claims you have 20 grams of starter from the prior day. However, weighing it and calculating ingredient weights based on the actual weight is a consideration due to losses, such as dough that sticks to the mixing spoon, or storage container, etc. With a small dough ball, more precision is required.
If you use less flour and the prior day's starter weighs 20 g, there will be some starter discard when measuring to the calculated starter weight. If more starter is needed than the formula weight of 80 g, it is best to use only 40 g of flour, let it ferment for 24 hours, then increase it once again.
Presuming the next step's formula calls for a bakers' percent(starter) of 50%, and presuming you have no dough losses, the table above shows a formula weight of 80 g. Thus 80 g ÷ 50% = 160 g. This means 160 g is the maximum amount of flour to use on the next increase. This increase method maintains the starter:flour ratio and the implied ratio of old:new flour.
After you have increased the mother dough to sufficient weight for your final dough, it is important to weigh and reserve your mother-dough starter before adding the other portion to the final dough, otherwise you will lose your stable culture. In the formula weights calculated above, the starter weight is 5 grams, thus you need 5 grams more starter at the end of increase refreshments than the final-dough formula demands. You may also decide to maintain a different weight mother dough, particularly so your increase weight iterations synchronize better to the final starter weight needed.
Starter amount for final dough 
The Larraburu Company is reported to have used a baker's percent value of 15% for the final dough. Let's say you have increased your original 20 g stable-culture sourdough ball to 80 g of starter (because of losses, you'll actually have a little less than that). Remember to reserve 5 g for the mother-dough ball. 80 g − 5 g = 75 g. How much bread can you make? 75 g ÷ 15% = 500 g. This means when you have made 80 g of starter and reserved 5 g for the mother-dough ball, you already have enough starter for a final dough of 500 g new-flour weight. Any other non-flour ingredients in the final dough are irrelevant to that calculation. The formula weight of the final dough will be greater than 500 g.
Advanced topics: altering the formula 
One beauty of baker's percentages is the ease with which an ingredient's values may be changed, thus making a new formula. In the mother dough formulas above, two values that may be changed are the bakers' percent(water) and bakers' percent(starter). The new flour is fixed at 100%.
Liquid starter 
If you desire a more liquid starter than the firm starter described above, increase the bakers' percent(water), and recalculate the ingredient weights and formula percent. More liquid starters would generally have a water value of around 100%, though it could be increased further.
Liquid starter ingredient bakers
wheat flour 100% water 100% starter 50% formula 250%
However, by only changing the water value, the old:new flour ratio is also changed.
When fermenting a liquid starter, do not store it in a container filled with flour, just use a container with a suitable opening. A wide-mouthed glass jar with a loose-fitting or breathable lid works well for small amounts. Only fill the container partway full, otherwise the starter may overflow as it ferments. How much it rises is somewhat dependent on its liquidity and other factors.
Starter ratio 
The other value which may be changed is the starter percentage, a value which is related to the old:new flour ratio. With the water value fixed at 50%, starter values in excess of 100% generally result, just after refreshment, in too low of a dough pH for the lactobacilli to grow well. As that value is increased even further, the lactobacilli will be more inhibited. If you've increased the starter percentage too much, decrease it some, and refresh a few times with the new ratio.
wheat flour 100% water 100% starter 100% formula 300%
Water and starter interrelationships 
There are interrelationships between the water percentage and the starter percentage that are not quite as obvious. When the starter's percent is a fixed value relative to the new flour, increasing the water percentage decreases the relative amount of flour in a given weight of starter, thus changing the old:new flour ratio. So, when the bakers' percentage(water) is changed, it will be necessary to adjust the bakers' percentage(starter) to maintain the desired ratio.
True percent 
While baker's percentages are based on flour weight, true percentages are based on formula percentage or weight, frequently phrased as total weight. The formula percent is the sum of the bakers' percentages.
- true percent(ingredient) = baker percent(ingredient)⁄formula percent × 100%
The ratio underlying the true percent is useful for calculating various old:new ratios.
- true ratio(ingredient) = baker percent(ingredient)⁄formula percent
If an ingredient's bakers' percent remains constant, the true percent decreases as formula percent increases, and the true percent increases as formula percent decreases. When any of the other bakers' percents change, the formula percent changes.
Consider the following two formulas:
wheat flour 100% 100% water 50% 200% starter 100% 200% formula 250% 500%
What are the old:new flour ratios?
Highly-liquid starter by volume 
Combine 1/2 tablespoon of flour with 3 tablespoons of water and let sit overnight at room temperature. Each day add 1 tablespoon of water and one tablespoon flour and mix until the total volume is about 1 cup (240ml). The symbiotic combination of yeasts and lactobacilli can be encouraged to reproduce more easily by controlling the pH (acidity) of the mixture using pineapple juice instead of water for the first three days and maintaining a temperature of 75 °F (about 23 °C, roughly international room temperature). Make sure that you have a working starter by observing whether the dough bubbles and rises. If not, then leave exposed to the air and test again. Thereafter, dump out ½ cup (120ml), and mix in ½ cup (120ml) water and ½ cup (120ml) flour.
A method of decreasing the dough's pH faster, one used by The Baker's Manual author Joseph Amendola, is to have an excessively large starter percentage, based on new-flour weight, on the second and third days (first and second refreshment), roughly 214% and 433%, respectively, before dropping it back down to 66.7% on all remaining days of a 7-day-long process.
When making sourdough for the purposes of acid-base leavening, such as pancakes, a more-acid dough is often desired. A simple way for a baker to achieve that using accurate measurement and formula notation is to use an excessively high starter percentage.
Generally, sourdough improves with age. There is a noticeable difference between a 1-week and a 1-month starter, and some can tell a difference between 1 month and 1 year. Some will claim it takes 40 years to get a good starter, though nobody waits that long to use it.
The sourdough method sometimes takes patience when starting anew. You may have to refresh your dough for longer than 7 days, much depends upon the flour and water used, as well as any yeasts and bacteria that exist in the environment, the starter ratio, the time between refreshments, and the temperature. There is no guarantee that you will pick up good yeasts floating in the air (there are many yeasts, but most will not make for good bread). The only way to tell is to wait until you get a critical mass of yeasts and see if it makes good bread. Generally, bad yeasts will smell strange; and so too will starter that uses too high of a starter:flour ratio, and thus is too acid for the lactobacilli to grow. Your starter should always smell clean, perhaps with undertones of alcohol and a sharp touch of acid if it is getting old.
There are other methods that are more likely to succeed, but you may lose the locality effect. Take some organic grapes. Wash them to knock off any dust or dirt, and immerse them in a bowl of potable water for 2 minutes. Remove the grapes from the water, which now contains the yeast that was growing on the outside of the grapes. If the grapes are local, then you have a local starter. Use this water to mix with the flour to make a dough, place in a small cup, cover and wait. If the dough rises, then you have a working starter.
A third method is to use a little kefir to initiate the sourdough starter. As with grapes, you will not get the effect of using a localised starter, but it is a reliable and effective method. Give it a stir every eight hours or so. It should be very active within 3 hours, possibly as few as 2.
Mixing 100 g flour with 100 g buttermilk or natural yogurt, plus 15 g water is also very effective for beginning a sourdough starter. This usually takes about 72 hours at room temperature to get going.
Or if you really want to cheat, you can mix flour with water to a desired hydration ratio and add a pinch of shop bought yeast. This should be left for 48 hrs at room temperature before using. You won't get the same natural taste initially, but the local yeasts in the air and in your flour will take over in time.
Keeping starter 
Sourdough starter is alive, and thus it must be regularly refreshed. Exactly how often this should be done depends on storage temperatures and the local strain. An active starter should be fed daily, if not multiple times per day depending on temperature, starter ratio, and other conditions. See the note below about dormant starters.
Sourdough is best stored at room temperature or slightly warmer. Anything outside of this range will change the proportions of the bacteria and yeast, which affects the flavor of the baked product.
Starter can be safely stored in the fridge, and if so, let it sit out several hours after feeding before returning it to the fridge. This allows the yeasts to get active and feed. Longer than this and your starter will begin to smell boozy and have a sharper tang to it than you might want. To fix this, just discard 90% and start the feeding cycle again.
According to Amendola, starter stored in the refrigerator needs refreshment only once per week. When removed for use, it requires two refreshments at room temperature to recover before adding to a final dough.
Long-term storage can be done by drying some starter, causing the yeast to go dormant. A thin layer of liquid starter spread on glass or ceramic plate speeds drying. If you have access to a vacuum dryer, that's even better. Exactly how long yeast can be stored this way varies, but it is enough for trading starters.
Freezing the dried-out starter allows it to be stored for over a year. Just dry the starter out, place it into a sealed jar and put it in the freezer. If you need it again, take one part of frozen starter, crush it, and mix it with one part of fresh flour. Add water and it should get active in a few hours.
If you do not bake daily, then your starter will go dormant as the yeasts shut down from hunger. You may see a separation occur in the starter vessel, where a yellowish clear liquid rises to the top and the white doughy starter falls to the bottom. The liquid is rich in yeast metabolic by-products. You can mix it back in when you feed, but it sharpens the finished bread flavor to a degree that you may find distasteful. Simply throw out the liquid before feeding. But you should note that once your starter has gone dormant, you must re-invigorate by multiple feedings to get it back to a healthful vigor (see below).
Using starter 
Using starter is easy, just take out the amount you need, and then increase the starter to replenish your supply. Avoid using all the starter on hand, though if you must, enough will cling to the sides of the pot to get the starter going again.
If using starter often you should keep your batches large enough so that enough is on hand. If you rarely use your starter you should keep just a small amount on hand, and increase it before it's needed.
You need to make sure that your starter is full strength before committing it to a dough. That means that it should quadruple if fed and left for an hour. If not, then it needs to be fed. Accelerate your feeding schedule until it passes the above test.
Many recipes call for more starter than is kept on hand. Starter is easily increased. At its most basic level, simply add more new dough and let it ferment for a day. However, the amount of new dough added affects the pH, this change in pH in turn affects the culture. If not enough new dough is added, it may not raise the pH sufficiently to insure vigorous lactobacilli growth. If too much new dough is added, the amount of inoculum is reduced, and according to Karel Kulp and Klaus Lorenz, starter values less than 20% of total weight correlate to less stable cultures. In either case, a previously stable culture can become unstable, requiring multiple refreshments and fermentation intervals to restabilize. A formula for increasing starter precisely to a stipulated ratio is covered in the by weight sections.
- A.M. Galal, J.A. Johnson, E. Varriano-Marston (1978). "Lactic and Volatile (C2-C5) Organic Acids of San Francisco Sourdough French Bread" (PDF). Cereal Chemistry 55: 461-468. http://www.aaccnet.org/publications/cc/backissues/1978/Documents/chem55_461.pdf. Retrieved 2012-Apr-05.
- Lorenz, Klaus J.; Kulp, Karel (2003). Handbook of Dough Fermentation. New York: Marcel Dekker Inc. p. 130. ISBN 0-8247-5527-8. http://books.google.com/books?id=SlLLsoHmedUC&pg=PA130&lpg=PA130. Retrieved 2012-04-11.
- Gisslen, Wayne (2009). Professional baking (5th ed.). New York: John Wiley. p. 158. ISBN 0-471-78349-8.