Written for my microbiology class:
A summary of:
Gadsby, P. (2003) The Biology of . . . Sourdough: San Fran's Mighty Microbes.
DISCOVER, Vol. 24 No. 9, online edition.
Microbiologists Frank Sugihara and Leo Kline, while working with bakers yeast (a strain of S. cerevisiae) for the U.S. Department of Agriculture, took on a side project studying the miniature ecosystem unique to sourdough bread in San Francisco. They discovered that the main difference between regular bread and sourdough bread is in the yeast and bacteria that inhabit the dough. Regular bread relies heavily on S. cerevisiae bread for rapid growth and quick production of carbon dioxide for leavening. Sourdough relies instead on wild yeast in the environment and a type of lactobacilli. In San Francisco sourdough the lactobacilli are Lactobacillus sanfranciscensis. The L. sanfranciscensis drop the pH of the mixture by producing lactic and acetic acid, killing off many competing yeast and bacteria as well as making the mixture sour tasting. The acid tolerant strain of yeast Candida milleri thrives in this low pH dough and produces the carbon dioxide responsible for leavening. The C. milleri and the lactobacilli have a fortuitous metabolic arrangement. The L. sanfranciscensis metabolize maltose, which is the sugar in the starch that the C. milleri dont metabolize.
Bakers save some of the dough and subsequently some of the microbe colony each time a batch of bread is made. This starter is how bakers for thousands of years have inoculated fresh batches of dough. With fresh flour to metabolize, the starter colonies have been kept alive for over 100 years. Scientists have discovered C. milleri and L. sanfranciscensis in starters all over Europe. The ratio of bacteria to yeast as well as the addition of a local strains change from region to region, apparently contributing to a different flavor depending on where the bread is baked.
As a huge fan of sourdough and as a cooking and baking enthusiast, I found this article to be very eye opening. I believe strongly that cooking is all chemistry and biology, and this article reinforces that belief. Preparing sourdough now has a direct correlation growing and transferring microbe colonies in the lab. Competitive inhibition as well as low pH in the dough explains why the dough doesnt grow all sorts of strange and potentially harmful bacteria and fungus. I think the article should have explored the possibility of the co-evolution of C. milleri and L. sanfranciscensis. I have often wondered how something so complicated as bread making was developed so long ago. In sourdough, the wild yeast and lactobacilli take care of themselves as long as they have food and moisture. The article mentions that yeasts and bacteria probably come from the bakers hands and the work bench. Perhaps a baker 5000 years ago was mixing up some dough for flat bread, got distracted, and came back to the dough a few days later to find it had puffed up.
I would guess that the majority of people in the United States prefer white bread, something I have never been able to understand. It is interesting that variety in nature always produces something better be it tastier bread or a fitter organism. White bread utilizes only S. cerevisiae and the dough is baked into bread before any microbes can contribute any sort of taste. Another example of this lack of variety would be chickens that are fed only 1 strain of corn their entire lives giving a rather bland product in the end. I also assume the same is true in the various generic domestic beers: one strain of yeast, one strain of grain, one bland end product. This is an important lesson to remember as single strains of commercially controlled crops are being grown exclusively on farms.
A summary of:
Gadsby, P. (2003) The Biology of . . . Sourdough: San Fran's Mighty Microbes.
DISCOVER, Vol. 24 No. 9, online edition.
Microbiologists Frank Sugihara and Leo Kline, while working with bakers yeast (a strain of S. cerevisiae) for the U.S. Department of Agriculture, took on a side project studying the miniature ecosystem unique to sourdough bread in San Francisco. They discovered that the main difference between regular bread and sourdough bread is in the yeast and bacteria that inhabit the dough. Regular bread relies heavily on S. cerevisiae bread for rapid growth and quick production of carbon dioxide for leavening. Sourdough relies instead on wild yeast in the environment and a type of lactobacilli. In San Francisco sourdough the lactobacilli are Lactobacillus sanfranciscensis. The L. sanfranciscensis drop the pH of the mixture by producing lactic and acetic acid, killing off many competing yeast and bacteria as well as making the mixture sour tasting. The acid tolerant strain of yeast Candida milleri thrives in this low pH dough and produces the carbon dioxide responsible for leavening. The C. milleri and the lactobacilli have a fortuitous metabolic arrangement. The L. sanfranciscensis metabolize maltose, which is the sugar in the starch that the C. milleri dont metabolize.
Bakers save some of the dough and subsequently some of the microbe colony each time a batch of bread is made. This starter is how bakers for thousands of years have inoculated fresh batches of dough. With fresh flour to metabolize, the starter colonies have been kept alive for over 100 years. Scientists have discovered C. milleri and L. sanfranciscensis in starters all over Europe. The ratio of bacteria to yeast as well as the addition of a local strains change from region to region, apparently contributing to a different flavor depending on where the bread is baked.
As a huge fan of sourdough and as a cooking and baking enthusiast, I found this article to be very eye opening. I believe strongly that cooking is all chemistry and biology, and this article reinforces that belief. Preparing sourdough now has a direct correlation growing and transferring microbe colonies in the lab. Competitive inhibition as well as low pH in the dough explains why the dough doesnt grow all sorts of strange and potentially harmful bacteria and fungus. I think the article should have explored the possibility of the co-evolution of C. milleri and L. sanfranciscensis. I have often wondered how something so complicated as bread making was developed so long ago. In sourdough, the wild yeast and lactobacilli take care of themselves as long as they have food and moisture. The article mentions that yeasts and bacteria probably come from the bakers hands and the work bench. Perhaps a baker 5000 years ago was mixing up some dough for flat bread, got distracted, and came back to the dough a few days later to find it had puffed up.
I would guess that the majority of people in the United States prefer white bread, something I have never been able to understand. It is interesting that variety in nature always produces something better be it tastier bread or a fitter organism. White bread utilizes only S. cerevisiae and the dough is baked into bread before any microbes can contribute any sort of taste. Another example of this lack of variety would be chickens that are fed only 1 strain of corn their entire lives giving a rather bland product in the end. I also assume the same is true in the various generic domestic beers: one strain of yeast, one strain of grain, one bland end product. This is an important lesson to remember as single strains of commercially controlled crops are being grown exclusively on farms.
littledeadkid:
white bread is gooood
