Sourdough. Bread. Few things rival a fresh slice of homemade sourdough - and since the start of the pandemic last year (ugh) there’s been a rapid increase in amateur bakers experimenting with establishing their own sourdough starter - the living mixture of flour, water, and microbes that give sourdough bread its flavor and texture. Bakers have depended on starters for thousands of years to make bread, and recently researchers dove into the complexities of the microbial communities in sourdough starters on a large scale. Today, we’re going to summarize a recent study (1) that investigated the sourdough microbial community - which microbes are there, what dictates which microbes are there, and how do microbes influence your sourdough’s flavor.
Which microbes are present in sourdough starters?
To make your own starter you mix water with flour, and then the microbes in the mixture start to metabolize the compounds in the flour. After a couple of weeks, the result is a community of bacteria and yeasts that will confer aroma, taste, and texture to your sourdough loaf. Which is pretty amazing if you think about it because that means that each loaf of sourdough bread is made just using flour, water, salt, and of course a dynamic community of microbes.
The study we’re discussing today characterized the bacteria and yeast in over 500 sourdough starters from all over the world (see map), and the researchers found that on average a starter community contains around 7 types of bacteria and 35 types of fungi.
In general, only a few microbes dominated the sourdough community. For example, the yeast, Saccharomyces cerevisiae made up more than 50% of all the single-cell fungi in over two-thirds of their sourdough samples. On the other hand, lactic acid bacteria were the most commonly encountered bacteria, while acetic acid bacteria were found in only a handful of samples. Yeasts are primarily responsible for making the bread rise by producing carbon dioxide - also called leavening in the baking world (2) -, while lactic acid and to a lesser extent acetic acid producing bacteria give sourdough its sour flavor.
What dictates which microbes are present?
One of the most surprising findings in this paper was the lack of a correlation between the microbial community in sourdough starters and geography - as it was thought for a long time that starters are unique to their geographic area (3). However, whether the researchers looked at cultures just from the US or at starters from around the world, they only found a weak statistical link between the structure of the microbial community and location. They also looked for ties between the feeding frequency, starter age, climate, and thirty other factors that could have influenced which microbes were present in different sourdough starters. But all of these things could only explain about 10% of differences between starters. So if external factors (geography, climate, etc) aren’t major players in the assemblage of microbes in a starter...then what is?
...external factors (geography, climate, etc) aren’t major players in the assemblage of microbes in a starter...then what is?
The statistical data in this paper suggest that biotic interactions (interactions between microbes) are the main factors that determine the structure of the microbial community in sourdough starters. To test this further, the authors took some of the most abundant bacteria and yeast, grew them in the lab both alone and in pairs, and monitored them.
They found that how well a microbe gets along with other microbes is one of the most important things determining if it will thrive in a starter. First of all, bacteria who can grow well alone tended to also reach high concentrations when competing with other bacteria, while bacteria that grow poorly alone could only thrive when paired with the right partner. Take Lactobacillus sanfranciscensis for example, it did not grow very well alone, in fact it only grew well when it was coupled with the yeast Kazachstania humilis. This specificity lowers a microbe’s chance to outcompete other species, thus making it less likely to flourish in your starter. A useful analogy for these specialized relationships between bacteria and yeast would be a talented athlete and his team. Say Lionel Messi for example, he’s undoubtedly a very gifted football (soccer) player. However, what takes his talent to the next level and lets him fully demonstrate his skills is his teammates who know how to support him and let him shine. This is what happens when L. sanfranciscensis has K. humilis backing it up in a sourdough starter.
...how well a microbe gets along with other microbes is one of the most important things determining if it will thrive in a starter.
What do these microbes do for our bread?
To sum up what we have learnt so far, (i) only a few different microbes dominate the sourdough microbial community, (ii) interactions between microbes and their flexibility determine which of them will stay and flourish in the final starter; but at the end of the day the important thing is what are the microbes doing for your bread’s aroma and flavor...right? Right. Well before determining that, researchers needed to know what were the flavors/aromas present in the sourdough.
Using a subset of samples they identified 14 common ‘notes’ (smells) “including yeasty, vinegar/acetic acid/acetic sour, green apple, fermented sour, and ethyl acetate/solventy” and a total of 123 volatile compounds that confer both flavor and aroma to the starter.
Unsurprisingly, the overall composition of microbes (beta diversity) in the starter culture was significantly correlated to the different types of volatile organic compounds in the starer. In fact, one type of bacteria - Acetobacter malorum spp. - was strongly linked to a vinegary-smelling-slow-rising sourdough starter. This acetic acid generating bacterial group was also determined to be a major driver in structuring the microbial community in the culture.
Take home message
This is the first time researchers have investigated the microbes in different sourdough cultures at this scale - and their data go against the long standing belief that where a starter originates impacts the flavor of the bread you make with it. They also found that how you maintain your starter can impact which microbes will thrive in it; and that some acetic acid producing bacteria heavily influence the smell and activity of your sourdough starter. At the end of their paper the researchers emphasize how unique sourdough starters are -they are a contained/easily sustained microbial ecosystem that you can keep on your kitchen counter- and that understanding the way the microbes interact in this ecosystem can result in better bread as well as provide key insights into how microbiomes assemble in fermented food.
Edited by Alexey Vorobev
References:
Landis et al The diversity and function of sourdough starter microbiomes. Elife. 2021
http://microbialfoods.org/yeast-profiles/. Accessed 4/3/2021
http://www.bbc.com/travel/story/20200402-why-san-francisco-does-sourdough-best. Accessed 3/4/2021
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