Brew in a bag (BIAB)

Brew in a bag, usually shortened to BIAB, is an all-grain method where crushed malt goes directly into a mesh bag suspended in the brew kettle instead of a separate mash tun. You mash and often boil in the same vessel, then lift the bag out to drain, which combines mashing and lautering into one simpler step and needs far less dedicated equipment than a traditional three-vessel setup.

Because everything happens in one pot, BIAB is a popular way for extract brewers to try their first full-grain batch without buying a mash tun, sparge arm, or extra burner. The bag needs to be fine enough to hold back grain husks but porous enough to let wort drain freely, and lifting a heavy, saturated bag of wet grain out of a hot kettle is the main physical challenge of the method.

A common mistake is using a kettle that is too small once you account for the volume of both water and grain, which leaves no headroom and risks a boil-over. Squeezing the bag too hard to extract extra liquid can also push tannins out of the grain husks, adding an unwanted astringent edge, so most brewers let it drain naturally instead of wringing it out.