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  • Alex Cloherty

Bubble, bubble, toil and double

Lately, I've gone a bit crazy about sourdough bread. However, when I started out with it a couple of weeks ago, I felt a bit like I was floundering in my kitchen (or home lab, one could call it).

Me with my latest loaf, including the giant bubble up top (this wasn't an intentional bubble)

You see, I was following a recipe, but I didn't really know exactly why I was taking each step. I knew it was important to add the starter at the beginning of the process, for instance, but I didn't know exactly what was in that starter. Was it really just yeast? Even though I was sticking my hands in there?

Because I'm a biologist at heart, and also in the kitchen, I set off to find out. Today's post aims to take you on a simple biological walk through the sourdough starter. I definitely wouldn't call it a recipe - but maybe rather something like an annotated protocol.

To begin, let me introduce you to my sourdough starter. Her name is Elizabeth.

MM readers, meet Elizabeth.

Like other so-called "wild" sourdough starters, Elizabeth started out as just flour and water mixed together in a jar. I left the lid of the jar wide open, with nothing on top, because I had heard that the yeast needed to start the fermentation of the flour would get into the jar by floating about on the air streams in my house and eventually landing in the jar. However, I forgot about the most important source of yeast: the flour! The flour that you choose to start your starter with brings lots of little yeasty beasties with it, which can kick-start the fermentation process.

Indeed, within a day or so, Elizabeth was bubbling away happily. Bubbling is a sign that fermentation is happening: those bubbles are carbon dioxide produced from the fermentation of sugars. This is the moment at which people usually think, "Great! My yeasty starter is doing a great job!" But, the yeast is not the only type of microbe at work here.

Just like a kombucha SCOBY, a sourdough starter is a microbial community consisting of not only yeast, but also bacteria! In fact, lactic acid bacteria ("LAB") can outnumber the yeasts in a starter by 100 to 1. It's these lactic acid bacteria that help give sourdough its distinctive sour taste.

But wait a minute - we learned in the Microbial Monday article on kombucha fermentation that sugar is an important starting product for fermentation. In other words, bacteria and yeast like to eat sugar. But flour isn't sugar, is it?

It all comes down to the flour again. The very flour itself actually contains enzymes that break down the starch into simpler sugars that can be eaten up by the bacteria and yeast. From this food, the yeast can then produce ethanol, which helps prevent the growth of moulds and unwanted bacteria. Likewise, the lactic acid bacteria make (you guessed it) lactic acid, which creates an acidic environment that's unwelcoming to other bacteria. Taken together, this means that your sourdough starter will stay somewhat constant: only a limited number of bacterial and yeast species will be able to grow in such an acidic environment. This is good news for the bacteria and yeasts already living in the starter, because they have now monopolized on their environment and built defenses against intruders. They can now multiply away, happily singing "bubble, bubble, toil and double" as they undergo binary fission. It's also good news for you, because intruders could be "bad" bacteria for humans, and furthermore, it tastes good.

Of course, these acidic defenses are not the only things your sourdough microbes will produce. They also make a wide variety of other compounds, like additional acids, alcohols, and aldehydes, which taste pretty good to us humans. And, of course, they make the carbon dioxide that literally gives rise to bubbles.

The magic of the sourdough starter is that it can be used as an "inoculate", which is basically a spoonful of microbes, to kick-start a larger scale fermentation in your dough. When you add some sourdough starter to more flour and water to make a loaf of bread, the fermentation process will be basically the same. To take an even wider view, the fermentation process is pretty much the same in all of your favourite microbe-rich foods. The take-home message can be summed up simply: fermentation is yummy.

Til next week, eat up!

~ Alex

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