Lactic acid bacteria, magnified 100x. Retrieved from

Sitting at home, revelling in an after-finals serenity the likes of which is usually only possible in a Buddhist temple, the scent of freshly baked sourdough wafting through the apartment, one may ask oneself: “What is the origin of this wonderful bread? How does nature produce such incredible, scrumptious delight?”


Well, (if you study biology, then it’s time to brace yourself, because here comes an explanation of a biological process given by a physics student who actually doesn’t really know that much about biology): sourdough is created through a beautiful symbiosis between lactic acid bacteria and one (or possibly multiple) types of wild yeasts, whereby the yeasts produce alcohol for the lactic acid bacteria to munch on, and the lactic acid bacteria create a nice and comfy, low pH environment for the yeasts. In the end, the yeasts produce carbon dioxide, which helps the bread rise and the lactic acid bacteria produce (surprise suprise!) lactic acid, which gives sourdough it’s unique flavor.


But wait, where are these critters coming from?! Actually, they were there from the start! Unlike when one bakes “conventional” bread with baker’s yeast (where you add yeast that’s been harvested in a factory somewhere), baking with sourdough is all about the art of creating a homegrown bacterial soup– which we called a “sourdough starter” in the last post—from the organisms that were already there. Basically what happens when we make our starter is that we harness the lactic acid bacteria and wild yeasts from the flour and surrounding air and create a nice comfy environment for them in which to thrive. If we play our cards right, they will multiply into oblivion, while keep all the harmful bacteria etc. at bay.


The interactions between the lactic acid bacteria and the yeasts are examples of two fermentation processes: “alcoholic fermentation” (from which the alcohol that the bacteria eat is formed) and “lactic acid fermentation”, also known as the Cori Cycle. As it turns out, lactic acid fermentation is not important only in our bread making, but also in our bodies.


Lactic acid is used in our bodies to help us generate energy in the absence of oxygen. Usually, our body generates all its needed energy through a process known as the Krebs Cycle (which uses the oxygen we breath and sugars from our food to create energy). Sometimes, however, our bodies use energy faster than can be produced using oxygen, such as during hard exercise, and another process has to be used. This process is lactic acid fermentation, the products of which can be used by the body to create energy completely oxygen-free.


The discovery of lactic acid fermentation has proved an important contribution to the field of medicine, particularly in the field of diabetes-research, having provided us with a greater understanding of the mechanism behind insulin’s affect on blood-sugar levels.


As I mentioned above, lactic acid fermentation is also called the Cori Cycle. Are you asking yourself why? Well, you should! The Cori Cycle is so-named thanks to its discoverers, the two researchers Gerty and Carl Cori. In the following post, we’ll welcome Nobel-month by introducing Gerty Cori, the third woman to ever get a Nobel Prize.



  1. BBC Radio 4 program on sourdough from 16 September 2012.
  2. Wink, Debra. (2009). Lactic acid fermentation in sourdough. The Fresh Loaf. Retrieved 19 October 2013 from: First published in Bread Lines, a publication of The Bread Bakers Guild of America. Vol. 15, Issue 4, Dec. 2007.
  3. Vuyst, L.D., Neysens, P. (2005). The sourdough microflora: biodiversity and metabolic conditions. Trends in Food Science and Technology. 16 (2005) 43-56. Retrieved 19 October 2013 from

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