Like many, I am fascinated by the chemistry behind baking, and in this blog I want to talk about bread—sourdough in particular. There is a well-known saying in the baking industry: «The pH value bakes and the total titratable acidity tastes». Why are these two parameters important for baking bread, and how can they be determined in the best way? This is what I want to discuss here.
A brief history of sourdough
Bread has been part of the human diet for several thousand years, although not necessarily in the forms we are familiar with today. One exception to this is sourdough bread. Wild yeast and bacteria (lactobacilli) ferment the dough naturally, creating a tangy loaf full of crevices. Despite originating in the Fertile Crescent, one of the oldest physical examples (at nearly 6,000 years old) was excavated in Switzerland, showing how widely it spread by that point already.
Currently, one of the places most well-known for its sourdough bread is San Francisco, in California. Why California? Bakers from France brought their techniques there during the Gold Rush in the mid 1800’s, and it has since become ubiquitous with the city. In fact, San Francisco has its own eponymous strain of sourdough bacteria: Fructilactobacillus sanfranciscensis.
Figure 1. Cross section of a sourdough bread loaf.
Many home bakers try to make sourdough at some point, since the ingredients are simple and no leavening agent is used, except for what nature provides. However, with so many people at home during 2020–2021, it was an ideal time for many people to see what they could produce. The development of the starter is of key importance—if there is not sufficient wild yeast and bacteria (or they do not have enough nutrients), then the dough will not rise, and you are left with a dense, chewy result. (While much has been written about how to make the best homemade sourdough, I cannot contribute to this topic, as my own baking spree focused on the Swiss Butterzopf.)
Click here to download the recipe and try it out yourself!
Lactobacilli: helpful bacteria
As their name suggests, lactobacilli produce lactic acid (Figure 2) and also acetic acid, and these give the sourdough bread its characteristic tangy, sour taste. The sourness of the bread also has positive effects on its shelf life, making it possible for our ancestors to preserve the bread for a longer time to supplement their diet.
There is another reason why the presence of this helpful bacteria is important. Without the lactic and acetic acid, it would be impossible to bake bread made from rye flour, which is commonly used in sourdough bread of northern Europe. How come?
Figure 2. Chemical structure of lactic acid.
Starch is the key component within bread and influences the shape, crumb consistency, and overall flavor. During the baking process, gelatinization occurs between the starch within the flour and the water added to the dough. However, flour also contains the enzyme amylase, which catalyzes the hydrolysis of starch into sugar. During the gelatinization process, starch is more prone to hydrolysis by amylase. Strong amylase activity at this point will have detrimental effects on the bread crumb. For wheat, the amylase is already denatured at the temperature gelatinization begins within the dough. This is not the case for rye, which gelatinizes at a lower temperature when amylase activity happens to be the highest . By making an acidic (sour) dough, the amylase activity is inhibited and it becomes possible to bake bread made from rye flour.
So how much acid is necessary and when is it too much? This question brings us back to the two key parameters, pH value and total titratable acidity (TTA), I mentioned in the introduction.
Figure 3. Fermenting sourdough starter in a glass jar.
pH value regulates enzyme activity
The pH value is important to inhibit amylase in an optimal manner. Every enzyme has an optimal pH range in which it functions the most efficiently. For amylase, the optimal pH value (highest enzyme activity) ranges from pH 5.4 to 5.8. At a lower pH value its activity will be reduced.
The pH value can be easily measured using a pH electrode. For dough analysis, an electrode such as the Spearhead electrode which can pierce into the sample is the best sensor. As the pH value is temperature dependent, the sensor measures the temperature as well.
What is the pH value?
The pH value is the negative logarithm of the hydronium concentration. Therefore, the smaller the pH value, the higher the hydronium concentration.
Pure water itself contains a small amount of free hydronium ions, and its pH value is therefore 7.
As acids release hydronium ions when they are in solution (dissociation), acidic solutions have pH values between 0 and 7.
Contrary to this, alkaline solutions and products have even less hydronium ions than pure water. They have pH values ranging from 7 to 14. An example of an alkaline solution is lye, which is used to produce lye rolls.
Total titratable acidity helps assess the taste
Why do we need to determine the total titratable acidity (TTA) if measuring and controlling the pH value is sufficient to regulate the amylase activity? This is because the pH value does not provide any information about the ratio of lactic acid and acetic acid present in the dough. While the amylase activity is not dependent on the ratio of the two, the composition is important for the taste. For optimal sourdough flavor, the ratio of lactic acid to acetic acid should lie between 3:1 and 4:1. If the ratio shifts towards containing more acetic acid, the taste usually becomes too sour.
Weaker acids such as lactic acid and acetic acid do not completely dissociate, meaning not all acid molecules present will release their hydrogen ion. As lactic acid is a stronger acid in comparison to acetic acid, more lactic acid will dissociate and thus contribute more to the pH value. By determining the TTA, it is possible to find out what the total amount of acids is within the dough.
For the determination of the TTA, dough is homogenized with water to obtain a suspension. It is then titrated to a pH value of 8.5 with 0.1 molar sodium hydroxide solution. The use of an automated titrator provides reliable results without human interference (Figure 4).
Before the titration starts, the pH value of the suspension can be determined easily, so you get both parameters (pH value and TTA) without needing to do double the work. For more detailed information on how the analysis is done, download our free Application Note.
If you would like to know why using an automated titrator is preferred over performing titrations by hand, read our blog post «Why your titration results aren’t reproducible: The main error sources in manual titration».
pH value and TTA for the perfect sourdough quality
By combining the information about the pH value and TTA it becomes possible to assess the quality of sourdough and thus maintain a constant quality in the final product, especially if delays in the production process occur.
It also becomes possible to detect changes in the sourdough starter which might occur if storage conditions cannot be maintained, and thus provide critical information when it is time to prepare a new starter.
Table 1. Common pH and TTA values for various kind of breads .
|Bread type||pH value||TTA|
|Wheat mixed bread||5.0–5.3||6–8|
|Rye mixed bread||4.5–4.8||7–9|
|Rye bread (coarsely ground)||4.2–4.6||9–14|
I hope this blog post on the chemistry of sourdough has given you some new insights on this fascinating kind of bread.
As for myself, I will probably not venture into the sourdough baking arena but stick with my homemade Butterzopf.
The chemistry of bread
Straightforward determination of pH value and total titratable acidity (TTA) in dough
Post written by Lucia Meier, Technical Editor at Metrohm International Headquarters, Herisau, Switzerland.