By Ellie Fung
Before lockdown, baking sourdough bread was seen as an intimidating and finicky art mostly reserved for professional bakers and committed enthusiasts. Now, thousands of quarantined individuals have taken on the challenge to create their own artisan loaves. Like commercial bread, yeast is a crucial factor behind a successful bake, but what sets sourdough apart is that instead of a simple Saccharomyces cerevisiae monoculture, a complex symbiosis between diverse wild yeast and bacterial species exists in the dough to provide not just lift, but also irreproducible flavours, distinct textures and even enhanced nutritional qualities to the final product (Gänzle, 2014a).
Behind every loaf is a mature sourdough starter, a mixture of equal parts flour and water, home to a flourishing ecosystem of fermenting microorganisms. The initial combination of flour and water creates an incubator of sorts, as it immediately provides a rich source of growth nutrients – attracting microorganisms from a variety of sources (Gänzle, 2014b). The flour itself harbours endophytes found within the grain, as well as microorganisms naturally residing on the grain surface and others introduced through the processing and storage of the flour. Bacteria in the dust of the surrounding air may also be introduced into the starter ecosystem, as may the inhabitants of the individual baker’s skin microbiome (Reese et al., 2020). In essence, a classic tale of interspecific competition and natural selection is unravelled as the starter matures into a stable ecosystem. The baker’s role is to optimise the conditions for microbial growth, through keeping the starter at a certain temperature and by regular “back-slopping”, a process where a portion of the starter mixture is discarded and replaced with fresh flour and water. Eventually, the starter will rise and fall predictably, indicating its ripeness for baking (Leo, 2013).
The maturation of the starter is a 3-phase evolutionary process, where each phase is characterised by a predominance of certain species, much like the stages of ecological succession. The first few days are mostly populated by opportunistic microorganisms that are atypical of sourdough microbiota, such as lactic acid bacteria (LAB) genera Enterococcus, Lactococcus and Leuconostoc. As the name suggests, LAB produces lactic acid, among other by-products, as a result of carbohydrate fermentation (Van Der Meulen et al., 2007). In the next two to three days, more sourdough-specific LAB, namely Lactobacilli, Pediococus and Weisella, gradually increase in prevalence due to a higher acid-tolerance and individual metabolic adaptability to the flour environment. From the fifth day onwards, the acidic culture would have excluded many bacterial genera except for the highly competitive Lactobacilli (De Vuyst et al., 2014). Unsurprisingly, yeasts are also characteristic of a stable sourdough system, specifically acid-tolerant ascomycetes (Weeks & Gadsby, 2003), but the predominating Lactobacilli outnumbers them by as much as 100:1 (Chin, 2020).
Ultimately, this process culminates into a stable ecosystem typified by a pH of below 4, oxygen limitation and high carbohydrate concentrations (De Vuyst et al., 2014). A complex interplay of biochemical conversions is evident, involving enzymes endogenous of flour and intracellular yeast and LAB enzymes. One of the main processes is carbohydrate metabolism. Grain flours harbour amylases and glucoamylases that degrades starch into maltodextrins, maltose and glucose substrates to be fermented by LAB enzymes, such as highly maltose-specific maltose phosphorylases (Gänzle, 2014). Although grain amylases are inhibited by acidification, yeasts and some amylolytic LAB strains can produce their own amylases to provide a continuous source of sugars (Van Der Meulen et al., 2007). The stable mutualism between yeast and LAB is also reflected in the preferential use of substrates. For instance, maltose-negative, glucose-fermenting Candida humilis populations tend to be accompanied by the maltose-positive Lactobacillus sanfranciscensis (De Vuyst et al., 2014). Via the fermentative production of lactic acids, acetic acids and alcohol, these species also collaborate to maintain ecological dominance – by creating an environment inhabitable for unadapted competitors (Chin, 2020). Lipid oxidation in some Lactobacillus strains may even produce antifungal fatty acids that help prevent moulding and enhance the shelf-life of the bread (Gänzle, 2014).
Metabolic conversions in the sourdough ecosystem are also the key to the reason why sourdough loaves are so coveted. LAB-secreted lactic acid offers a mild acidity reminisce of dairy products, whereas acetic acid imparts a sharper tang (Chin, 2020). A complex array of aroma and flavour volatiles is generated from phenol degradation and lipid oxidation, while bacterial proteolysis of wheat gluten contributes softness (Gänzle, 2014). Since individual species engage in distinctive metabolic pathways, a wide spectrum of resultant by-products elevate the gastronomical qualities of sourdough bread, at a level far beyond that of their commercial counterparts (Holmes, 2020).
Even if bakers follow the same sourdough recipe with the same ingredients, each starter community would still vary in species composition and yield bread with unique flavour profiles and textures (Reese et al., 2020). Hundreds of LAB and yeast species have been found in starters worldwide, and some are even region-specific (Dunn, 2018). Many factors have been suggested to contribute to the variation seen in sourdough communities, including flour type, relative hydration, the bakers’ skin microbiome, frequency of “back-slopping” and the ambient temperature, but their absolute and relative influence on stable community composition remains unclear (Rob Dunn Lab, 2020b).
What has intrigued microbiologists, however, is that despite differences in specific strains, various starters worldwide are dominated by the same main groups: heterofermentative LAB and yeasts. Species identified in the starters of 15 different bakers were shown to be a mere subset of all the species found in a separate survey of starters from across the globe (Dunn, 2018). Even more surprisingly, certain strains of these groups are almost always found, such as LAB strains L. fermentum and L. plantarum and yeasts S. cerevisiae, Candida humilis and Kazachstania exigua (De Vuyst et al., 2014).
An ongoing effort to study sourdough microbiota thus aims to understand the biological mechanisms underlying the formation and composition of starter communities. The Wild Sourdough project invites individuals worldwide to observe their developing starter cultures and record their aromatic and rising characteristics, aiding in the study of the effects of geography and flour type on microbiota and the resulting bread (Rob Dunn Lab, 2020a). The recently expanded community of sourdough bakers provides a rich source of data, but there is still much to learn about the intricacies of the sourdough microbiota, particularly in regard to the unique contributions of each species, the complex biochemical pathways and broad range of interspecific interactions. For many, the Instagram posts of gloopy starters and freshly-baked boules may seem dull and unremarkable for the time being, but as we hear all too often, there is more than meets the (naked) eye.
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