My beer has a sour taste. What is wrong?
Sour tastes are invariably caused by bacterial infections. The most common bacterial infection starts when the wort is cooled and before the yeast has started, because once yeast starts, it produces bacteria-suffocating C02 gas. To reduce the danger of bacterial infections in the future, make sure all your equipment is sanitized, avoid airborne bacteria by brewing in an area that is not exposed to drafts or dust (furry or feathered pets are notorious for spreading airborne bacteria through dust), and try making a yeast starter, to reduce the lag time before fermentation starts.
