Fermentation: art, science and sourdough

As the world ground to a halt some months ago, many of us (this author included) sought solace in the slow serenity of fermentation. Whether in the gentle bubbles of fresh sourdough or in the crisp fizz of new beer, we accessorised our lives in abeyance with life on the microscopic. Maybe it was the control: in fermentation, we regulate decay to create something new. Maybe it was the speed: there is no rushing a home ferment, which is inevitably sort of an adagio of growth. Maybe it was even an appeal to the still-unfound vaccine: microbial fermentation is among the many complex steps involved in producing a vaccine in useful quantities.

Whatever the reason for its recent ubiquity, fermentation is an interesting art and science in itself, bringing microbiology and biochemistry into your home kitchen. Today, we’re going to take the opportunity to dig a little deeper into the science beneath the bubbles in a (hopefully calming) meditation on the science and art of fermentation, as seen in sourdough bread-making.

Let’s start at the beginning of the process. Sourdough begins with just a mixture of water and baker’s flour called a starter. This starter is home to all sorts of yeast and, most importantly, bacteria, which come from a combination of the baker’s hands, the flour they use, and the air around them. When you feed the starter by adding more flour and water, it grows, becomes bubbly, and gives off a tangy, warm smell: it is a living, breathing thing. This is the fermentation in action: because there is no oxygen in the starter, the bacteria and yeast consume the carbohydrates in the flour anaerobically, converting the carbs into other useful products.

For sourdough, the key is the bacteria that produce lactic acid, an organic acid that gives the bread its famous tang. In fact, it’s the same acid that turns milk sour and creates kimchi, sauerkraut, and yogurt. Yeast and bacteria also produce other chemicals, like carbon dioxide, which gives the all-important bubbles in the starter and eventual bread, and acetic acid and eventually acetone, the core ingredient in nail polish remover, if you don’t feed the starter enough.

The exact science depends on the culture of microbiota in the sourdough. In fact, there are over a hundred species of lactic acid bacteria that might be involved in a sourdough process; depending on the method, location of baking, and materials, a starter will develop totally different bacterial cultures. Traditional sourdough cultures are made up of fast-acting bacteria like Lactobacillus sanfranciscensis because the starter is replenished frequently, as the faster bacteria are selected for in a microscale artificial selection process. Slower, warmer sourdough processes select for more diverse cultures. The proportion of carbohydrates available to the bacteria—how much sucrose, maltose, and fructose in particular—and the length and conditions of fermentation affects everything from shelf life to volume to texture.

With these many variables, getting exactly what you want out of a batch of sourdough is a fine art and a tricky science. Indeed, it takes years to perfect the experimental technique of feeding and fermenting and folding to make a light, bubbly, chewy, and just-sour sourdough, and to develop a desired microbial starter culture. Making a good-enough loaf (like this author did!) is a matter of patience and having a solid recipe. It is slow, and scientific, and meditative—a gentle exercise in observation and control, in making mistakes and learning to do better each time. As the world slowly re-opens, I recommend that you give sourdough (or any fermentation process) a go, with an eye to the experimental side. For science, of course.

References and more information:

• "Secrets of Sourdough,” Gastropod episode
• “It’s Alive with Brad,” Bon Appétit show
• “How to Make Sourdough Bread,” Claire Saffitz via New York Times Cooking
• “Sourdough” in Encyclopedia of Food Microbiology. 2nd ed. San Diego: Academic Press [Imprint], 2014. Print.