I hesitated about writing this post. The reason? I have never used sourdough. Usually, I prefer to write about things I tried, but, in this case, mea culpa, I can’t provide a direct experience with the sourdough. Anyway, I read a lot about it. Paradoxically, it was all the information I got that made me thinking if I really wanted to go on with sourdough. Until this week. I eventually thought it would have been useful to write a post on yeasts for all those people who, as me, are debating on which one to use.
In this post, I am going to explain the differences between regular yeast and sourdough, along with pros and cons of using one or the other.
Originally, it was extracted from the beer culture. Nowadays, it is produced by industries and consists of a single strain of yeast, known as Saccharomyces cerevisiae.
You can find it on the fridge counter of every grocery store, in single-portion blocks. Fresh yeast is also available in a dehydrated versions (not “fresh” anymore), which can be stored at room temperature and has a much longer shelf life compared to regular fresh yeast. I actually love the dehydrated yeast. I know, the purists might be skeptical, but I ensure you it is easy to dose, long lasting and as efficient as the fresh yeast. Moreover, as dehydrated yeast is more concentrated than the fresh one, you will need less. Just to give you an idea, dehydrated yeast is at least 3 times stronger than the fresh one.
Hard to find in grocery stores, although I heard it is now available in a few Italian markets, is made from water, flour and, sometimes, a “starter” (fruit puree, honey or yogurt), which helps with the growth of bacteria and yeasts. The batter is exposed to the air and the microbes populating the surrounding environment start to colonize it. Sourdough, thus, is a mix of yeasts, among which the above cited Saccharomyces cerevisiae, lactic acid bacteria (LAB) and acetic bacteria.
However, the simplest way to develop a sourdough is finding someone willing to give you a bit of his batch. The sourdough must then be stored in fridge, where it grows as a blob, and, as such, demands to be fed with water and flour
How does leavening occur?
Yeasts convert the simple sugars of the dough (e.g. glucose) into water, carbon dioxide (CO2) and ethanol. The alcoholic fermentation is employed in the production of beer. Apparently, a temperature around 4 ˚C would favor the alcoholic fermentation, but the aromatic bouquet of the dough still lacks of complexity.
Regular yeast, which only contains the strain Saccharomyces cerevisiae, gives the simple fermentation reported above. The final product has a standardized flavor and a poorly varied aromatic bouquet.
On the other hand, sourdough develops more complex flavors, thanks to the presence of LAB and acetic bacteria. Just to give you some numbers, more than 500 volatile compounds responsible for the fragrance of sourdough bread have been characterized. Furthermore, the LAB and the acetic bacteria produce lactic and acetic acids, respectively, which acidify the dough, preventing the growth of molds and extending, thus, the shelf life of the product.
Which are the optimal condition for leavening?
Yeasts work well in specific conditions of humidity and temperature. Doubtlessly, their activity is promoted with the temperature. However, there is an optimum temperature interval which is between 30 and 35 ˚C. Above this value, certain strains start to suffer, while, below that, the yeasts activity is limited.
What inhibits the activity of the yeasts?
Surely, salt limits the activity of yeasts. This is why salt should never be put in direct contact with them. Also fats limit the activity of yeasts, because they act as sealing agents and hamper the procurement of the sugars from the yeasts. Complex dough as those of panettone and colomba require hours to leaven!
Why did I never use sourdough?
1. I can’t find someone who can pass me a batch. Apparently, no one in Ottawa grows sourdough ;P Here, people are more into kumbucha, therefore, it is easy to find a good batch of skoby, but good luck with sourdough. The other option was starting from scratch, but it sounded difficult, also because I had a hard time finding a good recipe…till now :)
2. Maintenance of sourdough. Sourdough requires care, feedings and cuddles (OK, I was kidding on the last point :P). However, sourdough maintenance could be tricky and I am already struggling keeping my orchid in a good shape.
3. Reproducibility of recipes. Regular yeast is a standardized product, sourdough is not. This means that if a recipe calls for, e.g., 10 g of fresh yeast, you can be sure that you can succeed with that amount. With sourdough is a different story. The composition of your sourdough might different from the one used in the recipe. Don’t you think so? Do you really believe in Ottawa there are the same bacteria my parents have in their places in Tuscany?
4. Variability in strength. Unless expired, regular yeast has a standardized efficiency. But sourdough strength might vary. The activity of sourdough can decrease if the batter is not well maintained. Still, sourdough is not a standardized product, therefore, you have to test it a few times to understand how to dose it.
5. Long leavening time. Sourdough bread might take a full day to leaven, while the one made with regular yeast needs 1-2 h. With that said, a long leavening time improves the quality of the dough. How? The enzymes present in the dough break the molecules in simpler compounds. For instance, the enzyme invertase breaks sucrose into glucose and fructose, while proteases degrades gluten. This allow a) supplying yeasts with more simple sugars to ferment and b) obtaining a more digestible product.
Eventually, I decided to be brave and try the sourdough. Also because I am curious about the type of results I can get, compared to regular yeast.
Thursday we will see a recipe with regular yeast, while next week we will face the preparation of sourdough.
Leonardo di Carlo, Tradizione in Evoluzione, 158-170, Chiriotti Editori
C. Pétel at al., Sourdough volatile compounds and their contribution to bread: A review, Trends in Food Science & Technology, 2017, 59, 105-123