During the road trip to Denmark that I mentioned in a post back in September – see “There ain’t no b(ee) in Starbucks” – my wife Karin received a special gift from her sister Pia. It was a small jar containing a starter culture for sourdough bread, a culture that Pia has been using since she received it from a friend, who long ago received it from another friend. I didn’t know much about sourdough bread and did some reading. That Wikipedia link is a good introduction but don’t be put off by the complexities of “refreshment” – we’ve kept the starter culture in the fridge since early September and it’s been fine. Karin used the culture for the first time this morning and made the rye bread you see above.
But on to the biology. In essence the sourdough culture is a mix of wild lactic acid bacteria and wild yeasts, plus flour and water. When added to the bread mix (which in our case contained water, salt, seeds and molasses, as well as rye flour) the yeasts feed on some of the sugars within the mix and the lactic acid bacteria feed on other sugars that the yeast cannot metabolise. During that bacterial fermentation, byproducts are also produced on which the yeasts feed. The yeasts in turn produce carbon dioxide which serves to leaven the dough, and the bacteria produce lactic acid as another byproduct, which gives the bread its slightly sour flavour. This lactic acid also lowers the pH of the environment and, together with the production of anti-fungal chemicals, the lactic acid bacteria prevent the growth of other bacteria and moulds. The yeasts, however, can tolerate these conditions and they thrive.
At least six species of yeast and 25 species of lactic acid bacteria have been shown to be involved in this process, often as multi-species mixtures. The exact biodiversity of the culture is dependent upon its source: micro-organisms vary a lot across the world. But the heart of the relationship between yeasts and bacteria is always the same: they each facilitate the growth and reproduction of the other, and so the relationship is mutualistic, much like (most) relationships between plants and pollinators, birds and berries, and sea anemones and clownfish.
Of course there is a third organism involved in this mutualism: Homo sapiens. By producing the resources on which these organisms feed, and then distributing the starter culture, we are providing the right conditions for the yeast and lactic acid bacteria to increase their populations. In turn the yeast and bacteria play an important role in producing food for us, and in fact this way of making bread is thousands of years old. Microorganisms and people all benefit: what could be more mutualistic than that? Indeed, these interactions could be classified as a rare example of a ménage à trois mutualism.
There’s also a social-cultural dimension to all of this as the passing of gifts such as the starter culture binds friendships. If any of our local friends are reading we’d be happy to share the sourdough culture once we’ve bulked it up. The bread that it makes is delicious and from now on we’re going to try to give up buying the shop-bought kind.
If you want to read more about all of this, and have a try at making your own starter culture from scratch, there’s some great information and links on the Microbial Menagerie blog.
Many thanks to Pia for sharing the starter culture, and to Karin for baking the bread!