Sourdough is a type of bread that is baked using naturally-occurring yeasts and given a long, slow rising period so that it develops a characteristic sour flavour. In a more general usage of the term it is any bread that is made from sourdough starter, and may not have the sour flavour of true sourdough bread. Mass-produced versions of sourdough bread are baked with traditional baker's yeast and have acids and flavourings added to simulate the sour flavour. However, these are barely qualifiable as sourdough bread1 even under the loosest of definitions.
Sourdough starter is flour mixed with water that has an active yeast culture in it. It can be purchased in dried form and reactivated by adding flour and water, but the simplest way of obtaining starter is to have someone give you some of theirs. Another alternative is to make your own.
To do so, mix some flour2 and water in a bowl or jar with a large mouth, let it sit 24 hours loosely covered at room temperature, then add more flour and water doubling the quantity. Continue doing this every day until you have a yeasty, bubbly mixture. The yeast is found naturally in the flour (as well as the air in your kitchen) so this method generally but not always works, and may sometimes take a week or longer. If quantity becomes a problem, just throw away a portion of the starter prior to adding more flour and water.
Adding commercial baker's yeast to your starter is not a good idea. Commercial yeast has been selected for its highly active, fast-rising properties and it is not well-suited to the slow process of sourdough, it may consume all the nutrients in the flour faster than you can feed it, interfere with the development of the natural yeast, then die off.
Do not add sugar either for much the same reason. You may encounter 'friendship starters' that you treat the same way as sourdough but with the addition of sugar; these tend to grow very rapidly and are more appropriate for cakes. If your starter should get mouldy, remove the mould as soon as you notice it and keep going, but if the mould continues to grow or the starter turns colours throw it out and begin again. A healthy starter has a yeasty, sour smell and taste, but it should not smell or taste bad.
Sourdough starter also contains symbiotic bacteria, and it is the bacteria that gives sourdough bread its sour flavour. The specific strain of both the yeast and the bacteria will vary according to where you live, giving different sourdoughs very different qualities. If you make your sourdough starter from a dried sample or inherit it from someone else, your yeast will likely be the same as in the original starter, but the bacteria will still change, giving your breads a unique flavour.
You can keep your starter on the countertop or in the fridge, depending on how often you will use it. Kept on the countertop, the sourdough starter should be fed with flour and water every day. If you end up with too much starter, simply divide it up prior to feeding and either give some to a friend or throw some away. If you keep it in the fridge the yeast becomes semi-suspended, it only needs feeding once every few weeks or when you want to use it. Take it out of the fridge a day or two before you want to bake and feed it immediately, continue feeding every day until it is fully reactivated and bubbly.
Sourdough starter can be kept indefinitely3 using either method. When kept in the fridge, the liquid will sometimes separate, this should be stirred back in. However, on the countertop the presence of a liquid means that your starter has gone too long without being fed - it's not a problem but you may want to consider storing it in the fridge, and you may find that your starter is a little sluggish for a few days.
The liquid contains the natural alcohol produced by the yeast (and it is therefore often called 'hooch'), but this alcohol slows the growth of the yeast until the proper balance is restored. You may notice that your starter has its own rhythm and does better if you feed it more or less often. Follow your starter's lead.
How much water and flour do you feed your sourdough starter? In terms of keeping your starter active, it really does not matter; the yeast will be happy with any quantity of flour and water. Generally you want to add equal parts water and flour, although some people prefer a looser starter with double the water to flour, and you will add enough to at least double the quantity of starter that you are feeding. In the end, the exact quantities will depend on how much starter you use when baking and how you measure the ingredients in your recipes.
The essential element is to know how much water and flour are in the starter that you add to your dough. If you determine proportions mainly by feeling your dough, you have nothing to worry about. If you need to measure, always add your flour and water in the same proportions so you can estimate how much is going into the dough.
As a practical example, let's say you will use 6 oz (by weight)/ 200 grams of starter in your bread dough. After using the 6 oz./200 grams, add 3 oz/0.1 litres of water to the remaining starter and then 3 oz/100 grams of flour. This replenishes the starter that you used up, and you can be reasonably sure4 of how much flour and water you are adding to your dough (3 oz flour, 3 oz water).
To make sourdough bread, use your favourite recipe and keep in mind the myths of breadmaking. It can help to start with a basic recipe that you are very familiar with and practice on that until you get it right before proceeding to more complex recipes.
Make your bread as you normally would, but reduce the amount of water and flour to compensate for the starter that you will add. Of course you will leave out the commercial yeast entirely and substitute with starter. Beyond those simple adjustments to the ingredients, the only major difference in the process lies in the rising time. Sourdough yeast will take longer than commercial yeast to rise, so at a bare minimum expect to leave your dough to rise an hour or two longer than usual, possibly more. Additionally, if you want to develop a sharper sour flavour, you will need to retard the rising.
Retarding the dough means slowing down the bread's leavening process, and that is done by controlling the temperature. Allow the bread to go through its first rise and punch it down5, then put it in a cool place covered with plastic wrap to prevent it from drying out.
Bakers often have custom built 'retarding chambers' to get the exact temperature that they feel is best, but for most people a cool room or the refrigerator is more than adequate. The length of time that you retard the bread depends on the temperature. In the refrigerator you can retard the bread for twelve to forty-eight hours, but in a cool room it will generally need to be a shorter length of time. During the retarding period, the yeast slows down but the symbiotic bacteria continue to reproduce. The longer the bacteria can grow, the more pronounced the sour flavour will be.
When the dough has doubled it is ready for the oven. If it is has not quite doubled, put it in a warmer spot to finish doubling, then bake as usual.
It is possible to bake sourdough with a bread machine6, but bread machines are designed to work with quick-acting commercial yeast and have pre-programmed rising times that are in general too short for making bread with sourdough starter. You must try to subvert the machine's programming. Again, start with your most basic bread recipe; once you figure out how to get your machine to make it reliably as a sourdough recipe you can start experimenting with more complex recipes.
Use your favourite basic bread machine recipe, but reduce the flour and water to compensate for the sourdough starter. Use about 6 oz/200 grams of starter. You will have to check the dough during the kneading process and add more flour or water depending on the consistency of the dough, since it is difficult to be accurate with the measurements when using starter.
One of the simplest ways to make a successful loaf is to include commercial yeast as you normally would, decreasing the amount by about half. You will end up with a regular loaf of bread, but it will have a more hearthy flavour, and probably be a little crustier.
A more genuine way to proceed is to leave the commercial yeast out entirely, but then you must compensate for the short rising times. It helps to know how quickly your sourdough yeast rises, which you can determine by watching your starter after you feed it and observing how quickly it bubbles up and how long before it subsides again.
Set your machine to the longest cycle (usually the 'French bread' or 'whole wheat' cycles), and if your sourdough yeast will rise within the amount of time that the machine allows, simply using that cycle should produce a loaf of bread that is far better than ever came out of your machine before.
However, if the yeast takes longer to rise than the longest cycle on the machine or if you want to develop more of the sour flavour, put in half the flour and half the water with the sourdough starter, allow it to knead the dough, then unplug it. You may want to put the pan in a cool location or in the refrigerator. Let it sit until it doubles, then plug the machine back in, add the remaining flour, water, salt and other ingredients, and start the machine again on the longest cycle. You will have to experiment with this process to adjust for your machine and your specific sourdough starter, but eventually you will turn out an excellent loaf of bread.
More questions? Consult the Sourdough FAQ.