Sourness in sourdough comes primarily from lactic acid, produced by Lactobacillus bacteria during long fermentation. But here's what most home bakers miss: lactic acid develops best at cooler temperatures over longer periods. A 12-hour room-temperature bulk fermentation produces mild flavor. A 16-hour cold fermentation at 38°F produces pronounced sourness because the bacteria work slowly and metabolize differently at lower temperatures. The Temperature-Flavor Relationship Dr. Sarah Owens, a food scientist who specializes in fermentation, explains that acetic acid—which contributes the sharpest tang—actually develops more efficiently in warmer environments, but takes time to fully manifest. "If you want real sourness, you need both: a long, cool bulk fermentation followed by an even longer cold final proof, ideally 48 hours," she notes. This allows both acid types to develop fully. The difference between a starter that's one year old versus three years old matters far less than your fermentation timeline. Feeding frequency also affects flavor. A starter fed once daily produces milder tang than one fed twice daily—the more frequent feeding increases yeast activity relative to bacterial activity. To maximize sourness, feed your starter once daily for three days before baking, then use it at peak activity (bubbly, slightly collapsed top). Combine this with extended cold fermentation and you'll develop the complexity that bakeries achieve.