Sourdough life cycle
The most common concepts that people find difficult to understand when they start growing and using a sourdough culture are those ones that have to do with the feeding protocols. Things like, when exactly to feed the culture and with how much food?
The short answers to these questions are, first, whenever it’s hungry, and second, depending on how much hungry it is!
To better understand those answers let’s take a look at the life cycle of sourdough.
Sourdough is a mixture of wild yeast and bacteria species which as all living organisms follow a life cycle, are born, grow, multiply, and eventually die. This cycle resets every time the food sources are depleted and new ones are introduced, basically whenever we refresh/feed our culture with flour and water.
Yeast and bacteria cells might go through different metabolic routes and multiply using different mechanisms. However, for simplicity we could consider all of them following a similar growth pattern like this sigmoidal curve that splits into four distinct stages.
It’s important to mention that the speed of curve evolution depends highly on environmental conditions, namely temperature and humidity. As home bakers, we care most about the temperature which is the easiest to adjust and which gives us the ability to control how fast or slow things are evolving. We do things either at room or ‘warmish’ temperatures so as the microbes can grow faster, or we use the fridge when slowing down the growth is our aim.
Because at home we don’t have the means to measure cell activity to know in which stage at any given time our culture is, we could just use as a proxy the height of the culture in the jar, which is a measure of how much gas is produced, therefore of how active the yeasts are. Moreover, by tasting the culture for sourness we could also assess how the bacteria are doing.
If we assume that yeast and bacteria live in harmony together (which might not be always the case), we can think of them as a unity of species that undergo similarly the four life stages.
After fresh flour and water are mixed into the culture the microorganisms go through an adaptation period. This, depending on temperature/humidity can last a couple or several hours, and is characterised by the lack of microbial growth. Simply, since the environment is suddenly changed, yeasts and bacteria require some time to realize and adjust to the new conditions.
During this stage, the culture stays still in the jar and tastes nothing, or to describe it better, it tastes like raw flour. At this point, we should do nothing with it, other than wait to see it growing!
At some point, the microbes feel very comfortable in the new medium which has plenty of fresh food and they start to grow and multiply rapidly, in an exponential fashion. The culture begins to rise in the jar. It is when we feel excited and start dreaming of gorgeous breads. However, the culture is not ready yet to be used for bread making. There are obviously many more active cells present than before, but still not enough to inflate a bread dough.
We should better wait for some more time!
If we taste the culture at this point we might begin to sense the presence of some new flavors but rather mild ones, nothing pronounced. If our initial aim was just to refresh the culture and not to make bread at a later point, now it’s a good moment to transfer the jar in the fridge and forget it there until the next feeding, after a week or two.
After some more hours, the mixture in the jar has risen even more and reached its maximum level. There are plenty of bubbles around the jar walls and on the top of the culture. This is a sign that a mature and highly active sourdough culture has been formed. Depending on the exact room temperature (winter vs summer), a healthy sourdough culture should reach this level (at least having doubled in height) after a time window of 4 to 8 hours from the moment of its last feeding.
Tasting the culture at this point reveals a pleasant but not very aggressive mixture of flavors and aromas. Depending on the type of flour used (different flours provide different flavor characteristics) you might sense notes of yogurt, beer, and/or honey. No matter what, for sure a mild sourness should be present.
This is the perfect time to use the sourdough for bread making. This bread will have all the potential to develop the great structural and flavor characteristics known for a sourdough bread.
However, if we do not care at this moment to make bread, we can still transfer the culture in the fridge for storage, or feed it again at room temperature to reset the growth cycle.
A typical and moderate feeding protocol at this point would include a per weight sourdough:flour:water ratio of 1:2:2.
The growth curve reached its peak at the maturation stage simply because the food resources are now scarce. Our tiny pets had consumed most of the food, enjoying a happy life but now things are not looking good. Under the threat of starvation, the microbes will activate their survival mechanisms, some of them will turn to become dormant and many others will eventually die. The culture will start losing its proud look in the jar and will begin to deflate.
Now it tastes very sour, more like vinegar, and after long periods at this stage even alcoholic. If we try to make bread using this sourdough we will probably obtain one with evident sour flavors. Additionally, the crumb might show a denser and less airy consistency because of gluten structure deterioration due to the high acidity.
This is the sign that it’s high time for the next feeding. Here, a good amount of food can work better so as to dilute the sour character and the unwanted metabolites that had accumulated. Therefore, it’s a good idea to use a 1:5:5 or even 1:10:10 feeding ratio because our pets are craving food!!