Fermentation - Sourdough


Any home baker will be familiar with "fast-acting" baker's yeast, but instant yeast has only been around for about 150 years, whereas natural leavening with sourdough has been the norm for all of human history.  Sourdough is a symbiotic culture of lactobacillus bacteria and wild yeasts, that ferment the sugars in wheat.  The bacteria produce tangy-tasting acids and the yeasts cause dough to rise by filling it with carbon dioxide.  Sourdough yeasts perform much more slowly than commercial yeasts.  As with other popular ferments (yogurt, kombucha, sauerkraut), sourdough carries a hefty list of health benefits: it's loaded with healthful enzymes and it increases the bioavailability of many nutrients.  Furthermore, sourdough breads are easier to digest than those made with unfermented wheat, making them a good choice for people with wheat insensitivity.

The Basics: Baking with sourdough is a lengthy, multi-stage process.  Once you have a stable starter culture, you prepare to bake by building up a "leaven" with regular heavy feedings.  When you have sufficient leaven, you make the dough, let it ferment, shape your loaves, and let them rise.  Finally, you're ready to bake!

  • All-purpose unbleached flour (or 50/50 blend of all-purpose and whole grain)
  • Water, preferably filtered  
  • 1-qt. mason jar or medium-sized glass or plastic bowl (not metal) 
  • fork or spoon for stirring
  • digital kitchen scale (recommended) 
  • elastic jar cover or cheesecloth and rubberband
  1. Fill bowl or jar with 1/2 cup (4 oz.) lukewarm water.  Use kitchen scale to weigh 4 oz. flour and stir this into water until you have a thick batter with no lumps.  If you don't have a scale, 4 oz. of flour is a little more than 3/4 cup.  A 1:1 ratio of flour to water--by weight, not volume--is recommended for most baking, although some recipes may vary.  (In that case, you can divide your starter in two.  Continue feeding one as usual and keep this as your main culture; feed the other with the recommended ratio and use it for the recipe.)    
  2. Cover with clean cheesecloth held in place by a rubber band or use a fabric jar cover and set in a cool spot for 48 hours.  Stir vigorously twice a day to aerate and distribute yeasts.
  3. After 2 days, check for air bubbles around the sides and on the surface.  Air bubbles indicate that fermentation activity has begun.  If you see nothing, give it a stir and wait another day.  It's not uncommon for the surface to dry out and form a crust; just skim it off and discard.  
  4. When bubbles begin to form, indicating active fermentation, start feeding your culture.  Each day, discard half the amount of culture in the jar and replace with equal parts flour and water.  Stir and cover with elastic jar cover or cheesecloth held in place with rubber band.
  5. Culture is sufficiently active and ready for use in baking when it reliably grows big and bubbly within a few hours of being fed.  A strong culture can easily double in size.
  6. Most recipes call for at least 1 cup starter culture to make dough for a single loaf.  When preparing to bake, you'll have to build up your culture so you have enough.  Instead of discarding some of the culture, add to it when you feed it and feed the same ratio as usual.  When you have enough for your recipe, go ahead and make a dough following the recipe instructions, but be sure to reserve a small amount of culture to feed and maintain for future baking.
Frequently Asked Questions

Is there a way to "jump start" the growth of my culture by seeding it with sourdough microbes?

 It really is not necessary to do because the microbes required to ferment wheat flour are all around--they're in your kitchen, on your hands, and on the flour you use to feed your culture.  Occasionally, you will see sourdough instructions that call for the addition of a small amount of yogurt or a few grapes or raisins as a way of upping the population of microbes in your sourdough mixture.  But unless they can feed on wheat, those microbes won't do any good.  A better bet would be to use a proven sourdough strain, which can either replicate a certain taste such as San Francisco sourdough, or is already acclimated to feeding on a grain other than wheat (if, for example, you want to bake with rye or spelt flours).  See below for available sourdough cultures.

Should I be concerned about a grayish liquid pooling on the surface of my culture?

Not at all.  It is usual for a gray or brownish liquid to collect on the surface of a starter culture that hasn't been fed or stirred in a while.  It's quite acid stuff and as long as it smells pleasantly sour (like sourdough), there's no cause for alarm; simply take it as an indication that your starter needs to be fed.  Some people pour the liquid layer off before feeding their culture again and others just stir it back in.  (See the questions and answers below for further explanation.)  If the liquid is tinged red or pink, has visible mold, and/or has an unpleasant "off" odor, it is best to discard this culture and start again. 

Is there something I can do to limit the amount of culture I'm discarding?

We agree that discarding culture seems wasteful and we don't like it, either.  It's necessary because microbe populations increase exponentially and if you didn't discard half each time, you'd quickly be drowning in it!  To reduce the frequency of feedings, you can refrigerate your culture between feedings.  If it's only for a day or two, the culture won't have time to go dormant.  Feed it right before putting it in the refrigerator and let it warm up to room temperature before doing anything to it when you take it out.  Another way to reduce the waste of discarding culture is to maintain only a very small amount at a time.  You can keep as little as a couple spoonfuls for each feeding, just keep the ratios constant.  Also, if you don't want to throw away culture at all, there are lots of ways to use it up: pancakes, crepes, and crackers all come highly recommended.  The discarded culture will be nutritionally exhausted and, therefore, incapable of leavening a dough, but if you just want a tangy sourdough flavor, feel free to try it in any baking recipe of your choice!  Check out these discarded sourdough culture recipes from Cultures For Health.

Can I make a sourdough bread that's not super sour?

Yes!  It's the bacteria in a sourdough culture, rather than the yeasts, that create sour-tasting acids.  Since the population of yeasts increases early on and bacteria come to dominate later in the fermentation process, you can limit acid production by shortening the time between feedings and shortening the overall time that your dough ferments.   Temperature also affects which microbes are most active in a culture: bacteria dominate at higher temperatures, whereas gas-producing yeasts dominate at lower temperatures.  Try adjusting your recipe to use a larger percentage of starter when making your dough.  This will allow a shorter rise (fermentation) that can be done at a cooler temperature, yielding a fluffy, mild-flavored loaf.   

What if I want a really tangy sourdough loaf?

Increase the length of time you allow your dough to ferment and do it in a warmer location.  Higher temperatures and longer fermentation times both help increase the population of bacteria over yeasts in your starter: it's the bacteria that produce the acids that create the complex, tangy flavor of a really sour sourdough.  Another way to favor bacterial dominance in your culture, is to increase the amount of time between feedings.  Often, when you wait a long time between feedings, you'll notice a brown or grayish liquid start to collect on the surface of your starter: that's really acidic stuff so stir it back in!    

How can I keep my culture if I want to take a break from baking with sourdough?

Maintaining a sourdough culture is a little like having a pet in that it requires daily attention and feeding.  But, unlike with pets, you can simply refrigerate your culture when you want to go out of town or take a break from baking for whatever reason.  At refrigerator-low temperatures, fermentation activity slows down to such an extent that the culture only needs to be fed once a week at most.  Don't fret if you're going to be away for more than a week.  Even a severely neglected culture can usually be revived.  

Additional Resources

http://www.thekitchn.com/how-to-make-your-own-sourdough-starter-cooking-lessons-from-the-kitchn-47337  Blog post on The Kitchn about creating and maintaining a sourdough culture.  The instructions differ from those presented here only slightly and chronological pictures show the process and growth of a new sourdough starter.

http://www.theguardian.com/lifeandstyle/2014/aug/12/rise-sourdough-bread-slow-fermented-health-benefits  A great article in the UK's leading newspaper about the history, culture, and health benefits of sourdough bread.

Robertson, Chad.  Tartine Bread.  (2010)  A thoroughly engrossing book of recipes, anecdotes, and mouth-watering food photography from the owner/head baker of the Tartine Bakery in San Francisco, where the bread sells out within an hour of emerging from the oven.

Sourdough Supplies