How to Develop Sour Flavor in Sourdough

I just posted this as a response to a query, but thought that it deserved to be posted as a separate topic.  I spent months trying to figure this out, and there was so much inaccurate information posted on the internet on the subject, that I thought this would be useful to lots of people trying to figure out the "secret" of developing sour flavor in sourdough.  It's a heck of a lot simpler than most people seem to think.

In short, there are three key factors in the development of flavor in sourdough:

1. Ash Content of Flour (which affects the Buffering Capacity of the dough)
2. Fermentation Time
3. Fermentation Temperature

Everything else is either secondary or, in some cases, simply wrong.

The ash content of the flour is a key issue for development of total acidity (TTA - Total Titratable Acids) and flavor.  The higher the ash content, the higher the buffering capacity.  The buffering capacity of the flour reduces the volatile acidity (pH) of the dough, allowing the bacteria to work longer before they over-acidify their environment and stop producing acids and flavor compounds.  In addition, ash content is critical for allowing the bacteria to develop amino acids and volatile flavor compounds that contribute to that signature sourdough flavor.  While sourness can be obtained using a low ash content flour, the bread will contain a lesser overall acidity, and will contain fewer amino acids and volatile flavor compounds that contribute to flavor.  All purpose flour normally does not have a high enough ash content to allow substantial flavor development.  High gluten flour (aka bread flour) usually does.  Whole wheat and rye breads have an even higher ash content, which is why people are often more successful in developing sourness and flavor in doughs containing these flours.

The fermentation time must be LONG, meaning 12 to 20 hours.  Acidity and flavor develops during fermentation of the dough, and it takes the bacteria a long time to do it.  If you ferment your bread for less than about 8 hours, you'll get a very tasty, but non-sour bread.

The fermentation temperature should be between 20C and 30C.  Any less, and you're simply slowing down the bacteria in their quest to eat food and develop flavor.  Any more, and you're overheating them and hindering their growth.  However, anything within the indicated range is just fine.  Bacteria do produce some different volatile flavor compounds below 25C than they do above 25C, so this is one way to fine-tune the flavor of your bread, if you so desire.

For more information on the above, here's an excellent, freely available paper on the subject:
[1]http://ethesis.helsinki.fi/julkaisut/maa/elint/vk/katina/ [1]
Katina, Kati, "Sourdough: a tool for the improved flavour, texture and shelf-life of wheat bread"
Academic Dissertation, August 2005.
University of Helsinki, Faculty of Agriculture and Forestry, Department of Food Technology.
ISBN 951-38-6650-5

Here's a VERY SIMPLE procedure for creating a bread with a fully-developed sour flavor with any starter (I've got a collection of three of them, including Carl's 1847 Oregon Trail Starter, and this technique works wonders with all of them).

Step 1 - Make a fairly stiff dough using 5% to 20% starter
Step 2 - Place in oiled bowl, cover with plastic wrap, and let sit at room temperature for 12-15 hours (no need to punch down, worry, fret, or whatever... just let it sit).
Step 3 - Gently remove from bowl* and bake.

Yep, that's it.  After months of trying to figure out how to get that sourdough flavor, trying various complicated methods involving overnight refrigeration, letting the starter go sour, multiple starter stages, chanting of mystical incantations, etc., I found out that you just make the dough and let it sit on your kitchen counter.  How's that for uncomplicating things?

Hope this clarifies things a bit.  Good luck with your sourdough baking!

SourdoughSam

* I usually scrape the dough from the bowl with a spatula directly onto an oiled baking sheet, slash the top, and bake.  No shaping and re-proofing necessary if you're simply making a round boule or carefully stretching the dough into baguettes.  If you want to shape the bread otherwise, you should do so 3 to 5 hours before baking.

11 Apr 2008 - Made some updates and changes based on the excellent comments posted in response.

13 Apr 2008 - Added note regarding shaping, clarifications regarding the effect of ash content on acidity, and a note that different flavor compounds are produced below and above 25C.