There are multiple factors that can affect the overall acidity of a bread dough. Acids are sour. There are two dominant acids in SD breads – lactic and acetic acid. Both are sour but with slightly different taste profiles. Lactic acid – think yogurt. Acetic acid is vinegar.

The temperature bands you want to target for sour are 40-68f or 80-85f. Between 40-68f, it will produce more acetic acid and over 80f – more lactic acid. General guidance is to avoid temps over 85f all together for breads. The different temperature bands have very different implications in how fast the dough will ferment.

A starter has both yeast and lactic acid bacteria colonies (LAB). The yeasts function quite the same as commercial yeast – though often not as vigorously. SD bread fermentation is typically longer. More yeasts will speed fermentation. You control this by the % of starter you add to the dough and by how you feed and maintain your starter and when you use it relative to it’s last feeding. For sour breads, you ideally want the starter itself to be acidic and LAB favorable. It is worthy to note that as you change the % of starter you add, along with the yeast, you also change the amount of bacteria you are adding. More bacteria = more acid production.

Thus becomes the baker’s challenge for SD – how to balance all of these to produce the desired outcome. Just as it takes time for the yeasts to produce enough CO2 to inflate a dough, it takes time to produce enough acids to sour a dough. Time is a key component with a general premise that longer ferments produce more sour loaves.

There are multiple approaches to achieving either non-sour or sour breads. For sour, the most common practice is to use a mature starter, reduce the % of starter added to the dough and have a long extended fermentation. This would translate best to a lower hydration starter which was fed 12-24 hours previously. The starter add would be low – maybe 5%. The dough would be fermented around 50-55f for as long as possible.

The general idea is that the starter will be acidic, somewhat LAB favorable and the low % add coupled with the cooler temps will create a long slow fermentation.

For most home bakers, achieving 50-55f is difficult, and more commonly the dough is simply refrigerated. The even colder temps of the fridge (typically 35-40f) will slow fermentation even more. Times will need to be extended further. A temp of 40f will be better than 35f. Even a regular yeasted dough left in the fridge long enough will acidify.

The other schools of thought would be to leverage temps between 80-85f or add a large % of acidic starter – helping to acidify the dough. The thing to consider when adding larger %s of starter is the state of the starter. Remember, the starter is going through the same fermentation process and is subject to all of the same exact considerations. The additional factor when using a mature starter is that the starter has effectively fully over proofed and the gluten structure has fully deteriorated/collapsed. A large add introduces a significant amount of “flour” which may impact overall gluten development.

I have not spent much time with high temperature fermentation as a means to achieve more sour. This method could use a high temp bulk ferment followed by a cold final proof as well.

As to starters, I have become convinced that outside of starter management, different starters can inherently favor yeast while others favor LAB activity. All are not created equal. I have one starter which favors non-sour and it is an effort to coax sour. Another favors sour and is an all out effort to produce non-sour breads. For sour, I would clearly choose to use the latter.

Relative to starter management. Lower hydrations favor acetic acid while higher hydrations favor lactic acid. Logically the same for bread doughs. Smaller %s of old starter (10-20%), frequent feedings (every 4-8 hours), and temps between 68-78f will favor yeast activity (non-sour). Retaining a larger % of old starter (50%), less frequent feedings (once per day) and cooler temps (40-68f) will favor LAB (sour). Using a starter at or near it’s peak will favor yeast while using at 12+ hours will favor LAB.

Some other recent personal experiences seem to contradict some of this conventional thought as low % starter adds coupled with relatively long ferments can still produce non-sour breads. I am using the starter which favors non-sour. I have not tried this yet with the other starter.

There are so many things one can control in SD – there is no magic right or wrong answer. Generally – for the home baker in the quest for sour, some time is spent in the fridge.