The Highly Diverse and Nutrient-Rich Inner Life of Sourdough Bread

Bachmann H.-P., Stergiou-Gekenidis M. T.

Original article appeared in Swiss Agricultural Research

07.11.24

Photo: Hans-Peter
Bachmann, Agroscope

The microbial composition of the sourdough culture defines the character of the bread baked from it. A sourdough can increase the health-promoting characteristics of bread when combined with specific types of flour and suitable processing parameters.

The ancient craft of baking bread has changed considerably over its ten-thousand-year history. While leavened bread was originally made with a naturally occurring microbiota, nowadays the fermentation process is very strictly controlled in most cases. However, sourdough bread has seen a resurgence in popularity in recent times. It has a better flavour, better structure, longer shelf life and higher nutritional value than conventional yeasted breads.

Every sourdough has its own community

Sourdough cultures consist of microbial communities of bacteria and yeasts whose complexity varies enormously depending on the raw materials and process control systems. However, the general physical and chemical characteristics of the sourdough mean that these communities are ultimately dominated by lactic acid bacteria and yeasts (Fig. 1). Mature sourdough starters can therefore be used to make highly safe sourdough products.

Figure 1: Production of mature sourdough ‘French style’ (left) and graph showing the typical dynamics of the different groups of microorganisms within it (right). For each microbial group, the number of dots in the bar is directly proportional to the cell density in a specific phase of production (based on Minervini et al. 2014).

Spontaneous and starter culture-initiated sourdough

There are basically two main types of sourdough. Type 1 sourdoughs are colonised spontaneously by bacteria and yeasts present in the flour and the environment, are continually refreshed with flour and water for up to 15 days and are normally kept at room temperature. In contrast, type 2 sourdoughs are produced by inoculating a flour-and-water mixture containing selected bacteria and/or yeasts and are normally fermented at higher temperatures. Regardless of the method, sourdoughs can develop a high level of diversity over time. Their composition is influenced in particular by the type of flour, hydration, temperature, time and frequency of replenishment with flour and water.

The secret lies in the diversity of sourdough communities

Cereal products are staple foods that have long been a key component of the human diet. However, their popularity, especially in industrialised countries, is declining as they contain gluten proteins and other constituents (e.g. amylase trypsin inhibitors) which can cause bowel inflammation and other gastrointestinal symptoms in some consumers. The use of sourdough in bread production can largely break down the gluten network and reduce other undesirable constituents. Acidification by lactic acid bacteria and yeasts is key to sourdough fermentation. However, very little is known about the precise role played by the microorganisms. Some studies have demonstrated that specific strains perform a certain function, but more research is needed to better understand the role of the diverse sourdough communities.

Conclusions

Microbial sourdough communities can be simple or complex.
Spontaneous fermentation leads to more complex systems, while the addition of pre-selected starter cultures creates simpler microbial systems.
The microbial composition of a sourdough is mainly influenced by flour type, hydration, temperature, time and frequency of replenishment with flour and water.
The use of sourdough can increase the digestibility of cereal products. More knowledge about the role of specific microorganisms would enable optimal results to be obtained through their targeted use.