About sourdough

Sourdough is a grain and water mixture populated by microorganisms that consume the nutrients in the flour and produce flavour and rising effects as a byproduct. A stable population requires a variety of organisms that support each other in an ecosystem. When a stable balance is achieved, the sourdough can be perpetuated indefinitely. The baker can use that to flavour and rise their bread without adding commercially produced agents. Sourdough is the only way leavened bread could be produced until commercial yeast was developed in the 19th century.

The sourdough culture consists of two main kinds of microorganisms—yeasts and bacteria. The yeasts produce most of the carbon dioxide that raises the bread, and also produce small amounts of alcohol and other flavours. The bacteria produce lactic acid. The acidity is hostile to most other microorganisms that might contaminate the environment, but the strain of yeasts that are good for bread can tolerate it. My understanding is the yeasts and bacteria also can consume some different sugars from the flour so don't completely compete with each other. Therefore they form a symbiotic relationship that stabilizes the sourdough.

Yeast and bacteria grow at different rates. When there is an influx of nutrients, yeast go through a rapid growth cycle and become a larger proportion of the population. Bacteria grow at a more steady state with less peaks and dips of activity. The consequence of this is that acidity from the bacteria develops later in the growth cycle. A freshly activated sourdough has only a little acidity, which contributes slight sourness and flavour. An older starter has more acidity, creating more of the identifiably sour flavour some associate with sourdough and also helping to preserve the batch.

Yeast and bacteria can metabolize in the absence of oxygen, but yeasts require oxygen to reproduce. To perpetuate the starter, therefore, oxygen must be introduced. This comes when flour is stirred into the mixture, and can be enhanced by the stirring action itself. When the introduced oxygen is used up, the yeast switch to anaerobic metabolism and the population no longer increases.

Sourdough starter

The focus of this article is on working with sourdough starter, which seems most practical for a home baker. That isn't the only way to work with sourdough—"old dough" can be perpetually cycled through an operation that makes bread regularly. But starter is more practical to work with and keep at home, and it can be made into an old-dough style if desired.

One of the big variables and personal preferences on starter is the hydration—the ratio by weight of water to flour. Hydrations I've seen range from 60% for a very stiff starter to 150% for a very liquid one. In a stiff starter, the yeast will grow more slowly, and the bacteria proportionally even more so, resulting in less acidity. A more liquid starter is easier to work with and promotes greater acidity, aiding in preservation. However, the microorganisms grow faster and can exhaust the nutrients more easily, so it is more important to use the starter at its peak, and to refrigerate promptly to encourage the microorganisms to hibernate while the environment is still suitable.

Balancing these considerations with my own workflow, I have settled on 125% hydration for my starter. This yields a stirrable batter that can grow fairly quickly. I refrigerate the leftover just as it reaches the peak of activity. It seems to remain good for bread baking for a week this way, and remains fairly easy to activate for a couple months.

Another factor to consider is temperature. Bread yeasts work optimally at around 30 °C (85 °F), while the bacteria work across a wider range of temperatures. Although yeasts work best at warmer temperature, they work reasonably well at room temperature. I find it easiest overall to work with starter at room temperature and encourage it to adapt to that. Besides practicality, at higher temperature the yeast can outbalance the bacteria and result in insufficient acidity for the population. This makes nutrient exhaustion and contamination from ambient organisms more likely.

Making the starter

I haven't successfully made a starter from ambient yeasts, so can say less here than I would like. If I manage it I will describe more about the process.

In theory, one makes a paste or dough of flour and water, using proportions by weight to achieve the desired hydration. A good amount might be 200 g flour and 250 g water). Cover it loosely to prevent drying but allow exposure to air. Once per day, feed the starter by adding flour and water in the same proportions. In general add enough flour and water to double the weight of starter; discard some when the total amount is getting unwieldy. Only a few hundred grams of active starter is really needed, though some like to work with larger amounts.

Over the course of a week, the starter should show signs of life. It will take on a yeasty and sour aroma, show bubbles, and towards the end of the process, visibly rise over each ripening period. When it has gotten to the point that it will double in bulk over the 1-day ripening period, it can advance to the next stage, Activating the starter. It may take a week or two for the starter to reach this point.

In my own efforts, it hasn't worked this easily. Sometime I simply never achieved signs of life. Once I did indeed achieve life, but it smelled putrid. Reading up on this, I learned that in the early stages of a starter's growth, when not much acidity has formed, undesirable yeasts can colonize the batch. As bacteria also grow, they gradually increase the acidity, making the environment hostile to the undesirable yeasts while tolerable to the desirable ones. In theory, simply giving time for the acidity to develop will resolve the situation, but my experience was that the carryover putrid smell remained in the batch even after it had matured. It would probably have eventually been diluted out but I gave up before verifying that. There are suggestions to add acid to the dough at the start to avoid this problem, but it requires the right degree of acidity, most easily achieved by using fruit juice, and I didn't like the flavor that left behind either.

The starter I currently use comes from a freeze-dried sample I mail-ordered several years ago. Prior to that I used a starter handed down to my by my grandmother, which had been kept continuously active for who knows how long. I unfortunately stored that starter too long during a move and it lost its viability, though if I knew then what I know now I might have been able to restore it. Sourdough enthusiasts like starters that have storied histories like this, though it's not clear to me that it really makes a difference in practice. Biologists say that between evolution of the microorganisms (a natural adaptation to the particular conditions you apply to it) and unavoidable contamination with ambient organisms, any starter is always pretty contemporary.

Activating the starter

A new starter that will double in bulk in about 24 hours, or a starter that has been stored in the refrigerator for up to a couple months, needs to be activated in order to use it to make bread. This process builds up the liveliness of the yeasts (which may be dormant from storage) and moderates the acidity. A fully active starter should double in bulk in 4 hours.

Activating the starter is the same process as developing it, but with more precise measurements and timetables. My basic method is to take:

• 100 g (3/4 cup) starter
• 125 g (1/2 cup) water
• 100 g (1 cup) flour

Add water to the starter and stir together well until the starter is fully dispersed. Add the flour and stir in. Beat vigourously for a minute to add air. The mixture will look like a light pancake batter. Cover and let ferment. It can be helpful to occasionally beat the mixture, to work in some more air as well as to remix the microorganisms with available nutrients. Doing this, however, masks the visible rising that is a key sign of prorgess; smell is the next best guide but takes more experience.

Generally I repeat this build process three times to bring a stored starter to full activity. I withdraw 100 grams from the fermentation batch and mix in another measure of water and flour. The first build I ferment for 12 hours, the second 8, and the third 4. If at the end of the first 12-hour period it doesn't seem to be very lively, I might give it another 12-hour period or two before the final shorter fermentations. If it is still not looking good, follow the procedures to revive an old starter below.

After the final four-hour build, the starter should look like a bubbly batter and smell pleasantly, not overpoweringly, yeasty and sour. It is then ready to use for bread, or be stored for future use. The most recently activated starter is used for baking and continual perpetuation. Older starters left over from the process can be discarded in the compost or used in recipes that use the acidity more than the yeast; see Using excess starter below.

Sometimes, to bake bread while creating less excess starter from the activation process, I activate just a very small amount. Use 10 g starter, 12.5 g water, and 10 g flour, and activate as above, using these smaller amounts for each feeding. This creates much less excess from what will be needed for bread baking. I do this when the recently activated starter in the refrigerator is not too old, and when I have enough to take a 10 g sample and still have a good amount left. But when the starter in the refrigerator is more than a month or two old, it is best to put the entire starter through an activation cycle to restore its viability, in spite of the excess this generates.

Making the dough

Once the starter is active, it is ready to make bread. Much of that process is generic to bread, covered in a separate in-depth article, so here I cover only the particular considerations of sourdough bread.

The recipes I use require only a small amount of active starter - 30 g (1 oz) for two loaves. The starter is added to about 1/3 of the total flour and water for a 12-hour build phase. This allows a good population of microorganisms to develop and help raise the final dough. This has a mild acidity and complex flavour from the fermentation of the grain. Depending on the recipe, this build can be stiff like a final dough, or more batter-like as the original starter, or in between. Some recipes specify that the original starter must itself be at a particular hydration. While a starter that was built and maintained at that hydration will be optimal for the particular use, I simply adjust recipe proportions, taking into account the hydration of the starter, to arrive at the hydration required by the recipe.

One of the biggest difficulties I had with early sourdough bread attempts related to excess acidity. The sour flavour in commercial sourdough comes from added citric acid, not from the sourdough process. Recipes trying to duplicate this called for a cup of active starter, which brought too much acid to the dough.  Acid weakens the gluten structure of wheat bread, so the bread can't rise as much and produces a heavier loaf, although rye bread uses higher-acid doughs to help set the gums and compensate for lower gluten. Natural sourdough wheat bread is only slighly sour and still able to form a strong gluten matrix. I have also learned to make best use of kneading, folding, shaping, and baking techniques, described in the bread article, to help the slightly weakened dough rise better.

Maintaining the starter

Sourdough starter is alive and needs to be kept so. A bakery that makes bread every day can simply store their starter at room temperature and continually perpetuate it. At home, though, I can only reasonably make and consume bread once every week or two. To store starter this long between baking, it is necessary to refrigerate it. This slows down the metabolism of the microorganisms so they don't consume all the available nutrients, then starve and die off. Yeasts enter a hibernation state that allows them to stay alive without consuming resources, so the population is somewhat protected. Use a container with a reasonably but not hermetically tight lid, to minimize contamination from other organisms in the refrigerator but to allow any excess gas to escape. I like clamp lid jars for this.

Refrigeration of the starter does cause changes that affect how to treat it when it is time to reactivate it. One of the most important factors is that the bacteria, while slowed down by refrigeration, do not stop metabolizing available nutrients. This means acidity continues to increase in storage, and can reach a level that is no longer a good environment, and the microorganisms begin to die off. The hibernation state of the yeasts protects them from this to an extent, but the acidity reduces the population and makes it more difficult to reactivate them. The continued growth of the bacteria during storage also results in an imbalance of the proportion of yeasts to bacteria, affecting the stable symbiosis of the culture.

This progression is visible through a series of changes. When a fresh starter is first refrigerated, it is still full of bubbles and may even continue to rise. Gradually the bubbles subside and it looks quiescent. Up to this point, it can still be treated as an active starter in a bread recipe even though it has been refrigerated, but when it settles down, it will need at least one activation cycle to use again. After the settling, a liquid layer forms on top, colloquially called "hooch". Over time the hooch gradually darkens. Underneath it, the paste changes as well, losing the springy texture and becoming softer and easier to stir. The smell becomes ever more pungent as these changes progress.

During storage, it is best to leave the starter undisturbed. When it is time to reactivate it, stir it back together. I've tried removing the hooch layer but it seems to result in an unbalanced culture. Even if it has darkened it is safe to stir together. But the darker the hooch, the more acidity has accumulated, which will impact the reactivation. A starter that has been stored for up to one to two months and has a light layer of hooch can be reactivated as above. A starter stored longer, with a darker hooch layer, needs extra treatment to reactivate, as follows.

In the past, I followed widespread instructions to feed a refrigerated starter by adding a small portion of flour and water and returning it to the refrigerator. The idea was that it helped prevent the microorganisms from exhausting available nutrients and going dormant. From my reading and experience in recent years, I have concluded that the dormant cold yeasts are unable to benefit from this until returned to room temperature. Therefore, I no longer feed starter to prolong its refrigerated life. Instead, if I think it is getting too old, I take it out and put it through a reactivation cycle and return the freshly activated starter to the refrigerator.

Reviving an old starter

There are two inter-related issues with an old starter: the acidity is too high for the microorganisms to proliferate, and the yeast population has dwindled and won't rebuild as easily. These are both addressed by some extra builds in reactivation that include diluting acidity as a goal. Diluting acidity also means diluting the microorganisms, making them hard to build to a full population and increasing the chance of contamination from ambient organisms, so this factor must be considered as well.

For most old starters, my procedure is to add twice the amount of flour and water to a measure of starter, for a (12-hour) build or two. This means for 100 g of starter, 250 g water and 200 g flour. For a very old starter, using four times the measure of flour and water may be needed. But I worry this over-dilutes the microorganisms, and instead prefer to do a few successive builds with twice measures, so the acidity is gradually worked out while still allowing the yeasts to start to proliferate. While the starter still smells sharply acidic after its ripening phase, it may need another dilution for the next build. When it returns to the smell of ordinary starter, I continue with the standard activation process.

These dilutions of the starter mean the yeast in particular has been reduced in comparison to the bacteria (which kept growing in the starter for longer). To help the yeast along, I may do the early builds at a warmer temperature. This is most easily achieved in an oven with just the light on, or a gas oven with just the pilot light. Once the starter has revived somewhat, I continue working with it a room temperature.

Using excess starter

The procedures to maintain starter described here tend to result in a lot of excess starter, and after all that, the bread recipes use very little of it. Excess starter can be washed down the sink or put in the compost. But I hate waste and prefer to find uses for the starter. I have found that old starter can be used much like buttermilk in grain-based recipes that are leavened by baking soda or baking powder. If a recipe calls for buttermilk, I substitute an equal volume of old starter. If a recipe calls for baking powder, use 1/2 tsp baking soda in place of 2 tsp of baking powder, add a cup of old starter, and reduce the liquid proportionally. I have refined a number of sourdough recipes to use up old starter.