Exploring the Sourdoughome

I love it when my interests intersect, so this new report from researchers in Italy and Belgium, on the microbiota of sourdough breads, definitely caught my attention. As the authors explain:

This study aimed at the identification of the [lactic acid bacteria] (LAB) and yeast microbiotas of 19 Italian sourdoughs used for the manufacture of traditional/typical Italian breads. The dominating LAB and yeasts were monitored by culture-dependent methods. Multivariate statistical analyses were performed in order to find the correlation between ingredients and the composition of the sourdough microbiotas, as well as the effects of the latter on the biochemical characteristics of sourdoughs.


My own applied microbiology project.

It seems that Italian sourdough is a particularly good subject for this, as the country is home to about 200 different types of bread, many of them leavened with regionally unique sourdough starters. These distinct bread recipes also use different types of flour and employ different procedures for “back-slopping,” or propagating the mixed bacterial-yeast culture. If there’s anything Italians love more than bread, it’s disagreeing about how to do things.

To see what the different starters look like microbially and chemically, the team took samples from 19 different sourdoughs, then cultured and identified their bacterial and fungal constituents to the level of species and strains. They also analyzed such parameters as pH, lactic acid concentration, and levels of free amino acids (FAA), gamma amino butyric acid (GABA), and other byproducts of fermentation.

Sure enough, the diverse baking techniques have led to diverse microbial and biochemical traits in the sourdough starters. There are a few dominant species of lactic acid bacteria and yeasts – Lactobacillus sanfranciscencis is the top bacterial species and the venerable Saccharomyces cerevisiae dominates the yeast communities – but they’re joined by a large supporting cast of related microbes, and each region seems to have its own specific combination of sub-strains. The resulting breads are equally varied, from Pane di Altamura (pH 4.03, 82mM lactic acid) to Pane Casareccio di Genazo (pH 4.14, 63.7mM lactic acid) to Pagnotta del Dittaino (pH 3.70, 83mM lactic acid).

The differences might affect more than just the flavor of the bread. As the researchers comment in the paper:

In addition, FAA and GABA produced by LAB may increase the nutritional value of the breads. For instance, the amount of GABA in 150 g of Pane di Matera PGI represents the minimum effective daily dose to get positive effects in humans.

The “positive effects” they’re talking about include lowering blood pressure in people with mild hypertension. Perhaps that’s another way the famous “Mediterranean diet” offsets the effects of that region’s delicious meats and cheeses.

One potential limitation of the study was that it relied on culturing the sourdough microbes in order to identify them. As metagenomic studies have recently revealed, the culturable part of the microbial world is just the tip of the iceberg. There’s a whole universe of bacterial, fungal, protozoan, and viral life out there that just can’t survive in any of the relatively small number of culture media available in the lab. That said, sourdough starters have been selected for a certain type of culturability, so there probably aren’t too many unculturable organisms in these samples. If it grows in a deliberately maintained culture in the kitchen, it will probably do so in the lab, too.

I hope the authors are planning to do follow-up studies on other sourdoughs, and perhaps on some beers. Belgian beers should be particularly interesting, as that country’s brewers have pursued their art in as many ways as Italian bakers have theirs.

Meanwhile, I’ll keep doing my own work in this field, which consists of baking a loaf of sourdough every few weeks. My starter allegedly originated in 1847 along the Oregon Trail, though it’s been passaged by many people in different parts of the US since then. The really charming thing about it is that a dedicated group of volunteers still distributes this starter to anyone who asks, for the cost of a self-addressed stamped envelope. I don’t know whether it produces enough GABA to lower anyone’s blood pressure, but it sure does taste good.

ResearchBlogging.orgMinervini, F., Di Cagno, R., Lattanzi, A., De Angelis, M., Antonielli, L., Cardinali, G., Cappelle, S., & Gobbetti, M. (2011). Lactic Acid Bacterium and Yeast Microbiotas of 19 Sourdoughs Used for Traditional/Typical Italian Breads: Interactions between Ingredients and Microbial Species Diversity Applied and Environmental Microbiology, 78 (4), 1251-1264 DOI: 10.1128/AEM.07721-11