How to Make Sourdough Bread More Sour

Growing up, my dad would bake a loaf of sourdough every weekend. I loved that bread. It was nice and tart.  When we’d take trips to San Fransisco we’d get some local bread and compare to my dad’s.  The sourness level was comparable. Of course I was biased, but I thought my dad’s bread was just as good. About 12 years ago I started trying to recreate my dad’s recipe in a gluten free version.

I’ve done a lot of research on the topic of sourness in sourdough in order to reverse-engineer my dad's San Fransisco style sourdough recipe to make it gluten free. There is more info out there about wheaten/rye baking so that became the basis of most of my research. There are a lot of factors in the flavor of your home-baked sourdough product: the starter, the method, and the ingredients.

Starter

You don’t necessarily have to create a special starter for sour bread.  However, there are a few things that might help.

1. Use the starter after peak, even well after its optimal window for yeast growth.  I feed every 24 hours, it peaks at the 12 hour mark, and I typically use the starter at the 24-30 hour mark for a sour loaf.
2. Feeding with whole grains.  Whole grains can absorb some of the acid to buffer the dough so the acid doesn’t break it down so quickly.  Some people like to feed with all or part rye for a more sour flavor.
3. Low inoculation. Use a small amount of starter for a longer ferment, depending on your method.

Method

Not all recipes will result in a sour bread.  In fact, I’d say that most SD recipes will not result in a sour bread.  Sour bread is not polular universally. If your recipe comes with a photo of a wide-open crumb and a big ear, then it’s probably not a sour bread.  Open crumb and sour flavor are two different goals that are often at odds with each other. Most San Francisco style loaves are not a high hydration formula.  

There are three methods that I know of for createing a sour profile in your final loaf: Short ferment at high temp, 12-24 hour ferment at cool temps, and Long cold retard in the fridge.

1. High heat method. I’ve read that the old San Francisco bakeries used this method for sourdough, as it’s more efficient with time, storage space and production schedules than the other methods.  The concept is that yeast flourishes at temps between 72-82f (22-28c).  Lactobaccilis on the other hand flourishes at both lower and higher temps.  This method targets the higher temps for at least one stage of the process. The target temp for fermentation is around 90f-100f (32-38c).  The temp control has to be pretty spot on and you have to watch it like a hawk, so I’ve had a difficult time replicating this method at home without a proofer.  But I have been able to create some tinned bread following an old Estonian tradition of doing a long ferment overnight (12 hours at room temp) and a short hot ferment in a warm oven (100f/38c) for 30 minutes.  This was for a buckwehat bread, but I understand that the method is very similar for rye.  
2. 12-24 hour ferment at cool temps.  Again we’re looking at the temps where yeast are less active but the lactobaccilis flourish, but this time we’re going with the cooler temps.  I have the best success at the 50-60f (10-15c) range.  Using about 5-10% starter by flour weight, I mix the dough and let it rise in a cool place for 12-24 hours.  Then I either bake it as a single rise (shaped before rising in a banneton), or if I want to do a double rise I have held some flour back, I punch the dough down, add the additional flour, mix, shape, rise again at room temp for ~2 hours, and bake.  This will produce a loaf with a fairly close, even crumb like San Fransisco style bread.
3. The cold retard.  I have never tried this but it’s mentioned often on the bread forums I follow.  This sounds like the only way to get a somewhat open crumb with a sour profile. I’ve heard the flavor of this method described as more vinegar-like, which dissuades me from trying. But it does sound relatively easy.  It seems like most people agree that for a really sour loaf it needs 72 hours in the fridge after bulk ferment. Exact fridge temp will alter results, with ferementation being much more robust at 40f (4c) or above, while most fridges are below that temp.
4. I’m sure this list isn’t comprehensive.  Look up traditional injera recipes if you want to see another method that makes really, really sour flatbread that’s traditionally gluten-free.

Ingredients

There are various ways to accumulate and accommodate sourness in your loaf.  Some of them are just necessary to buffer the acids, and others are hacks.

1. Ash content and gluten content of flour.  If you are using white wheat flour only, and you want to keep the hydration high, make sure the ash content and gluten percentage of your flour is high.  High ash content/gluten indicates that the flour can buffer the acids produced so the dough doesn’t break down so easily.
2. Typical Gluten free bread flour blends are particularly prone to the effects of acid which break down starches.  White rice flour and pure starches are found in many GF bread blends, and high acid contents just eat away at them unchecked.  For a sour gluten-free bread, you must use a high percentage of wholemeal flour so the proteins and fiber can buffer the acids.
3. Whole grains have better buffering capacity, ash content, and in general they promote more flavor.  Rye in particular is conducive to sour bread baking. For gluten free, I have gotten very sour loaves using teff or buckwheat. In fact the acidity is a dough conditioner that softens the texture of whole grains and makes the bread more pliable.
4. Hack: apple cider vinegar.  I often add about a tablespoon of ACV to bread as a dough conditioner.  This doesn’t affect the flavor much if at all. There are some recipes that use larger amount of ACV as a hack to create a sourdough flavor without the fermentation.  In fact many storebought sourdough breads list vinegar as an ingredient and may be just yeasted bread with acidity added.
5. Hack: Citric acid, ascorbic acid, and others.  Like ACV, adding powdered acids can simply add sour flavor to bread.  I’m not a proponent of these methods but if you want to try them look up the specific ratio you will need as some of these acids are very concentrated and you only need a very small amount.
6. Milk. I’m not sure if this is a hack or not, but subbing half of your water for milk will make your sourdough bread more sour, if you’re already using one of the methods noted above.  I have an old family recipe that suggests feeding the starter with half milk, and I think that’s what my dad used to do for a sour starter.  I once tried a long ferment loaf with all milk, and finally got a bread that was too sour for me.

After a lot of research and a lot of trial and error, I was able to create a loaf of bread surprisingly close to my dad's original recipe. I call it the 24-hour Sourdough. What are your sourdough goals?  Are there any recipes that you're trying to recreate as gluten free?