At some point, during my workshops, comes the moment that I demonstrate how to feed/refresh a sourdough culture. The participants are watching me transfer a teaspoon of old sourdough into a small bowl and mix it with fresh flour and water. Then, quite often, someone asks me how much of each component I’m using.
My spontaneous answer is that it doesn’t really matter!
As long as you keep your culture happy, feeding it regularly, every 1 or max. 2 weeks (if stored in the fridge), the exact amounts of sourdough, fresh flour, and water that you’re going to use don’t play a significant role.
Typically, sourdough is kept at 100% hydration, meaning that the same amounts (in grams) of water and flour are used in each refreshment. This doesn’t mean that sourdough activity would be compromised by using different hydrations, resulting in stiffer or more liquid consistencies. For instance, a well-known type of stiff sourdough (50% hydration) is the Italian ‘pasta madre’, which is traditionally used to make brioche-like breads, like panettone.
For simplicity, let’s stick to 100% hydration, which means that the same amounts of fresh flour and water are added to the existing sourdough. Then, the question that still remains is how much of the existing (old) sourdough we take.
This is usually described by referring to ratios.
So, a sourdough feeding ratio is the relative amount (referring to weight) of old sourdough compared to fresh flour and water. Typical feeding ratios are 1:2:2 or 1:3:3 (old sourdough: fresh flour: water). However, even extreme ratios like 1:50:50 would still work. In that case, the freshly fed sourdough would just require more or much more time to grow and reach its peak, as judged by the maximum volume increase in the jar (at least doubled).
Based on the above, the initial question can now take the following form.
What sourdough feeding ratio shall I use?
Again, a simple answer is that it doesn’t really matter, the smaller feeding ratio faster the sourdough would grow and reach its peak, higher the ratio slower the growth.
However, if we know approximately the growth rates under different feeding ratios we could probably plan our baking schedule better because we could estimate for how much time we have to wait after feeding our sourdough until it reaches its peak, which is the point that we would ideally like to use it in the final, bread dough.
(At this point, you might be interested in reading my post on the sourdough life cycle which describes the four stages of sourdough growth).
For the purpose of the current post, I did an experiment, measuring the growth rate of my sourdough under different feeding ratios. I started by using an active sourdough at room temperature (19°C for this winter). Then, I performed four feedings using different ratios, namely 1:1:1, 1:2:2, 1:5:5, and 1:10:10. I transferred the four freshly fed sourdoughs into glasses, placed a rubber band that marked the initial height around each of them, and measured growth (height increase) in each glass for every hour.
I recorded growth for twelve hours in total and plotted the data on a graph that depicts the sourdough growth curve for each of the four feeding ratios.
As expected, higher the feeding ratio slower the sourdough growth.
Now, can this graph help me somehow with my baking routine?
The answer is yes!
For instance, if I would like to have my sourdough activated (ready-to-use) around the afternoon, I would feed it in the morning using a low feeding ratio, for example, 1:2:2. In this case, based on the data it'll take ca. 6 hours to reach its peak.
On the other side, if I prefer to assemble the bread dough in the morning, I would feed my sourdough the previous night, before going to bed, using a feeding ratio of 1:10:10.
Keep in mind that the mentioned baking schedules based on the above data will change if for instance the room temperature also changes (higher in summer) or if I start with a sourdough that lies in the fridge for days. In those cases, the growth rates would be higher and slower respectively.
And that means I have to conduct the equivalent experiments to have a better idea of what to expect in each case!!!