Why is my cold brew coffee sour?

The perception of 'sourness' in cold brew is a classic case of incomplete extraction. In coffee chemistry, flavor compounds are extracted in a predictable sequence: acids and fats emerge first, followed by sugars, and finally, bitter plant fibers. A sour cup indicates that the brewing process stopped prematurely, leaving the acidic compounds dominant without the necessary sweetness to provide balance. While cold brew is often marketed as 'low acid,' research from the Specialty Coffee Association and the Coffee Science Foundation clarifies that this is a sensory distinction rather than a chemical one; cold brew is often perceived as less sour simply because the lower temperature suppresses the extraction of certain acidic compounds. However, when that extraction is insufficient, the result is a sharp, unpleasant sourness. Producers and farmers often roast coffees specifically for cold brew to be slightly more developed, as light-roast, high-density beans require significant energy to release their sugars—energy that cold water simply cannot provide. If you are using a high-quality, light-roast single origin, you are fighting a losing battle against the physics of cold water. To correct this, do not simply blame the beans; adjust your variables. A finer grind increases the surface area for extraction, and extending your steep time—sometimes beyond 24 hours—allows the water more opportunity to pull out the sugars required to round out the profile. If the coffee remains sour, the roast level is likely incompatible with your chosen extraction method.