A lot of the sourness has to do with the starter, and how it's maintained. The "sourness" has to do with the bacteria in starter, which produce both acetic and lactic acid. Acetic acid is what makes vinegar sour, and is more sharp and pungent; lactic acid is the type of sour found in milk products & yogurt, and is a softer, creamier, more mellow sour. (On a side note: lactic acid is a beneficial part of the winemaking process, as part of malo-lactic fermentation in white wines, which converts green-apple flavored malic acid to creamier flavored lactic acid).
In any case, to make a sourdough bread more sour, do any one or more of the following:
- Maintain a firm starter (50-60% hydration)
- Keep the starter cool, around 50-65F (in fridge)
- Use higher ash flour (like high gluten flour)
- Feed the starter regularly
- Spike it (dough and/or starter) with rye flour
- Long and slow bulk fermentation (1st rise) (somewhere between 8 and 24 hours)
- Cool bulk fermentation (1st rise) (such as in the fridge)
- Lower overall hydration for the dough
- Increase the amount of starter in the recipe (the working range is 15-40% of final dough weight, or 25-35% of the flour weight)
- Use a liquid starter (90-170% hydration)
- Keep the starter at warm temp (70-85F) (such as room temperature)
- Use lower ash flour (like all-purpose gluten flour)
- Don't feed it as regularly
- Don't use any rye flour
- Bulk fermentation (initial rise) for dough for a short time (less than 8 hrs)
- Bulk fermentation in a warmer place (such as room temp)
- Higher hydration for the dough
- Reduce the amount of starter in the recipe (between 5-15%)
I keep 2 starters right now:
1. My original LBB starter, built using Nancy Silverton's organic grape-based starter recipe in the "La Brea Bakery" cookbook. I keep it at about 178% hydration (which is very liquid); I feed it as follows: 1/4c starter, 1 cup flour & 1 cup water. It stays in the fridge most of the time. If I need to bake, I take it out and feed it 2x per day for about 4-5 days to get it back up to strength.
2. A firm starter, which is just the LBB starter but it's maintained at 60% hydration as follows: 1 tbsp starter + 1 tbsp + 2 tsp water, 1/3 cup flour. This one sits in a plastic covered container on top of my fridge. I feed it maybe 1x per week. I'll feed it 2x per day for 1-2 days before baking.
I've been leaning more towards the firm starter lately, mostly because I can work in smaller quantities for maintenance, and it last longer at room temp w/o feeding, so I can bake with it more quickly.
I had a couple other starters; one was a Carl Griffin one that my friend John gave me. The other was a rye-based version that I started a few years ago. I threw them both out, just because I like the flavor of the LBB one, it's easy to maintain and works well.