You may have noticed that your sourdough loaves remain free of mold on the shelf for much longer than a comparable yeasted loaf. It is indeed true that sourdough has a longer shelf life than typical yeasted breads.
Before we go any further into describing why this is, let’s tak a moment to discuss the difference betwen shelf-life and freshness.
Shelf-life vs. Freshness
Shelf-life refers to the length of time bread remains safe to eat without presenting health risks, such as mold or bacterial growth. Beyond this period, even if the bread appears fine, it might not be safe for consumption. Freshness pertains to the period immediately after the bread is baked, where it retains its optimal taste, texture, and aroma. It’s when the bread is at its best, offering a delightful sensory experience with every bite.
Although sourdough can remain mold-free for a long period of time, it is best enjoyed while as fresh as possible.
Comparing Sourdough Fermentation to Yeasted breads
The key to understanding why sourdough bread remains mold free for longer than yeasted bread lies in how it is fermented.
In a yeasted bread, commercial yeast is added directly to the dough. This yeast can effectively convert the carbohydrates in the flour into carbon dioxide and water. The carbon dioxide gas causes the dough to rise, and it is then baked.
In comparison, a sourdough bread starts from a “starter”, which is a mixture of flour and water that has been allowed to ferment over time. This starter captures wild yeasts, lactic acid bacteria (LAB) and acetic acid bacteria (AAB) from the environment. It’s these microorganisms that ferment the dough, giving sourdough its unique characteristics. In addition to yeasts that generate carbon dioxide, the various LAB in the sourdough starter generate a variety of flavorful compounds.
Among these compounds, two stand out: lactic and acetic acid. These aren’t just any acids, they’re the chief contributors to sourdough’s trademark tang.
- Lactic Acid: This is the milder of the two, offering a gentle tanginess that’s reminiscent of yogurt. Apart from its flavor contribution, lactic acid modifies the bread’s internal environment, making it slightly acidic. This slight shift in pH is not very favorable for many spoilage bacteria, thus acting as a line of defense against them.
- Acetic Acid: Recognized for its sharper, vinegar-like taste, acetic acid is another byproduct of LAB’s diligent work. In addition to providing flavor, acetic acid has strong antimicrobial properties. It acts as a deterrent for molds and certain bacteria, ensuring they don’t get a stronghold in the bread, thereby contributing to its longer shelf life.
In essence, as LAB ferments the dough, it’s not only crafting a unique flavor profile but also setting up a protective barrier. This natural preservative system is one of the reasons sourdough can often outlast other bread varieties in the freshness game.
pH Level and its Impact
The tangy taste of sourdough is because of the presence of acids in the bread. In scientific terms, sourdough has a low pH.
The pH scale runs from 0 (most acidic) to 14 (most alkaline), with 7 being neutral. Most fresh foods, including many bread types, naturally fall on the slightly acidic to neutral side of this scale. Sourdough, however, because of its fermentation process, has a notably lower pH, making it more acidic.
This acidic environment is unwelcoming for many spoilage organisms:
- Bacterial Inhibition: Many bacteria that can cause spoilage or even foodborne illnesses thrive in neutral pH environments. The acidity in sourdough makes it challenging for these bacteria to grow and reproduce, offering a natural form of preservation.
- Enzymatic Activity Reduction: The acidic environment can also slow down the activity of certain enzymes that might lead to the degradation of bread components over time.
The Role of pH in Mold Prevention:
Molds, a common culprit behind bread spoilage, have specific environmental preferences, and pH is a crucial part of this equation.
- Mold Growth Deterrence: Most molds prefer slightly acidic to neutral pH levels for optimal growth. Sourdough’s pronounced acidity creates an environment less conducive to mold proliferation.
- Synergy with Other Factors: It’s not just the pH alone that deters molds. The combination of sourdough’s acidity with other factors, such as its unique crumb structure and moisture content, collectively creates an environment where molds find it hard to gain a foothold.
While the acidic nature of sourdough helps in mold prevention, it’s essential to note that it doesn’t make the bread mold-proof. Other factors, like storage conditions and exposure to mold spores, can still introduce mold to the bread. However, the pH level gives sourdough a fighting edge in the battle against spoilage.
This was demonstrated well by Katsi et al. (2021), who took sourdough and yeasted breads and measured the number of bacteria (left panel) and pH (right panel) of the bread. Here we reproduce some of their data
It is clear from these charts that the sourdough bread lasts for more time (out to 15 days) before the number of bacteria begins to rise, and that the pH of the sourdough bread is acidic (around 5.7) and stays there. Conversely, the yeasted bread has an initially very slightly acidic pH, but as bacteria rapidly grow in that bread the pH drops precipitously.
To Sum It Up
Sourdough bread stays fresh longer because of how it’s made. The way it ferments creates natural acids that keep bad things like mold away. But even with this advantage, it’s always best to eat bread when it’s fresh to enjoy the best taste. Remember, sourdough might last longer without mold, but that doesn’t mean it tastes as good as when it’s fresh out of the oven! So, enjoy your sourdough soon after baking and store it right to keep it at its best.
Citations
- Katsi, P., Kosma, I. S., Michailidou, S., Argiriou, A., Badeka, A. V., & Kontominas, M. G. (2021). Characterization of Artisanal Spontaneous Sourdough Wheat Bread from Central Greece: Evaluation of Physico-Chemical, Microbiological, and Sensory Properties in Relation to Conventional Yeast Leavened Wheat Bread. In Foods (Vol. 10, Issue 3, p. 635). MDPI AG. https://doi.org/10.3390/foods10030635
