Yeast starter

A yeast starter is a small volume of wort, often made from a bit of dried malt extract and water, that you pitch a liquid yeast package into a day or more before brew day. The goal is to let the yeast multiply in a low-stakes, easy-to-monitor batch first, so that by the time you pitch it into your actual beer, you are adding a much larger, more active yeast population than the original package alone contained.

Starters matter most with liquid yeast, since a single vial or pack often does not contain enough healthy cells for a full-size, average or high-gravity batch on its own. A stir plate keeps the starter in gentle constant motion, which speeds yeast growth compared to a jar sitting still, and many brewers judge a starter's readiness by watching activity settle down and a layer of yeast build up at the bottom.

A common mistake is skipping a starter for a big, high-gravity beer and pitching a single standard package anyway, leading to a slow, stressed fermentation that can leave off-flavors or a higher-than-expected final gravity. Another is pitching a starter into wort before checking that both are at a similar, safe temperature, which can shock cells that were otherwise healthy and ready to go.