So what is BIAB? It stands for „Brew in a Bag“ and really, it’s not much more complicated than it sounds! During the brewing process, malt is steeped in hot water to activate the enzymes and convert starches into sugars. Once this process is finished, the wort needs to be separated from the spent malt grains, commonly known as lautering. Breweries usually drain the clear wort from the bottom of the mash tun to separate it. Home brewers can also buy pots with drains and filters in them but this can be quite pricy. Another alternative used by some brew kits, is to use 2 pots and a strainer. The grains and wort are poured from one pot to the other through the strainer, thereby catching all the spend grains. The is practical and easy, but requires 2 pots and can be problematic with larger amounts of wort. The BIAB method is easy and quick and it works just like a teabag. The grains are in the bag, which is in the pot. after the malt has steeped, the bag is simply lifted out of the wort and drained and your left with your finished wort!
All these methods work fine, but they do extract varying amounts of sugars from the grains. The amount of sugar that gets extracted in comparison to the total amount of sugar in the grains, is known as the mash efficiency. If we were able to get all the sugars out the grain we would have 100% mash efficiency, but that is impossible to do. More common for breweries is around 90%. For home brewers, the average is much lower, ranging from 65-80% depending on the exact methods used.