So I started doing some research, and it turns out that sourdough starters are fascinating!
Everybody knows that yeast is what bakers use to make their bread rise, and the increased popularity of artisan breads has introduced all of us to the concept of sourdough; made from a starter containing wild yeast, cultured in kitchens and larders the world over. It's super easy, if not lengthy, to get your starter going: you simply mix strong bread flour with water to a batter consistency, leave it somewhere to get contaminated, then feed it every few days with more flour to keep the microbes growing big and strong.
I always assumed that the yeast in a sourdough starter was Saccharomyces cerevisiae [sac-a-row-my-sees ser-eh-vis-ee-ay] (commonly known as bakers' yeast). This is the stuff you buy in the supermarket for bread baking. To the best of my knowledge, it comes in three forms: fresh, dried and fast action. It is also sometimes called Brewers' yeast, because it's the same species which ferments the sugars in grapes or malt to produce wine and beer. At home, we put this work horse of the food industry to good use often, with bread baking and home-brewing regularly on the go. As we all know, when bread is baked, the yeast digests sugars in the dough and produces carbon dioxide. This gas gets trapped in the strands of gluten in the dough, and expands as it heats in the oven. This is how bread rises and what gives the dough its tiny holes.
|I'm a very lucky girl, to have a boyfriend who makes great beer and bakes delicious bread, using the yeast Saccharomyces cerevisiae. Shame none of it ever lasts long...|
However, I was wrong in my assumption about the species found in a sourdough starter. It turns out that it's impossible to introduce S. cerevisiae into a sourdough starter, because it dies out after about a week. Instead, the wild yeast which most commonly falls into the flour and water mixture and becomes established is Saccharomyces exiguus, also called Candida holmii. So this S. exiguus lives happily in the batter, feeding on the carbohydrates from the flour. As the cells eat, they grow larger. Then when they've doubled in size they go through a process called binary fission; they split into two new cells. These smaller, new cells are called daughter cells. Each daughter then eats, grows, divides, and so it goes on...
Actually, it's more complex than that!
The carbohydrates in the flour are often indigestible by the yeast, so it needs help to access the food locked up in the flour. That's where Paul Hollywood's Lactobacillus comes in. Sourdough starters are actually a mixed population of yeast and lactic acid Bacteria. The Bacteria break down the complex carbohydrates, turning them into smaller, simpler sugars which the yeast can sink their proverbial teeth into. The two species end up in a stable relationship, because if one grows a bit too quickly to out-compete their neighbour, they run out of food quickly and then their growth slows down, allowing their neighbour to catch up.
The most commonly cited lactic acid bacterium in a sourdough starter is Lactobacillus sanfranciscensis, which was discovered in a starter in, believe it or not, San Francisco! Luckily for the yeast, the way Bacteria digest their food is somewhat messy. Instead of taking the complex carbohydrates into their cells and breaking them down there, they secrete digestive enzymes into the environment to do the digesting outside the cells, and then they take up the products. This means that the long, complex carbohydrates in the flour get broken down while they're floating around in the batter, and the smaller sugars are left lying around for the yeast to gobble up. It's the same as the way my Mum used to cut up fish fingers for me when I was too young to use cutlery. Lactobacillus, like Saccharomyces, is also a genus with several applications in the food industry. For example, this group of Bacteria are used in the dairy industry to help convert milk into yoghurt and cheese. They do this by breaking down lactose, the sugar found naturally in milk. As a result of the break down of lactose, they produce lactic acid (yes, the same stuff your muscles produce when you push yourself hard at the gym). The lactic acid curdles the milk, making it thicker and more sour.
|Cheese, made when Lactobacilli break down lactose and produce lactic acid (http://www.morguefile.com/creative/Alvimann)|
Can you see where we're going here? We've got Lactobacillus in a sourdough starter. Yes! You see, that's where the characteristic flavour of sourdough comes from. Cool, huh?
So the yeast needs the Bacteria to help cut up its food for it; the yeast is required to produce the gas which makes the bread rise; and the Bacteria give the bread its delicious flavour. It's very clever! And the real beauty of all this is that it just happens by accident. The microbes are trying to get on and live their lives, and we've stumbled upon a way to use that to our advantage, again!