This section relates to the mashing and sparging process: extracting sugars from grain and collecting it as wort in the kettle.

The temperature of the mash is one of the most critical elements of brewing. A higher temperature leads to a less fermentable wort, resulting in a sweet final beer. A lower temperature leads to a more fermentable wort, resulting in a dryer beer. Too high or too low of a temperature can cause off flavors or poor efficiency.

A sour mash is when the wort temperature is allowed to fall into the 90F - 120F temperature range, where bacteria and wild yeast are active. This can either be during the mash or after collecting the wort. The infection causes the mash or wort to taste and smell sour. The wort can then be boiled, killing the infection to prevent further souring. This method is used to create sour style beers in a controlled and comparatively quick manner, thought the sourness is different from that of a sour fermented beer.

A common technique is extracting 20% of the wort after the mash, souring it, and adding it back to the normal wort after a few days to revitalize fermentation and mellow the sourness.