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If you use at least 50% light colored malted barley in your grain mix, you will have enough amylase enzymes to convert all of the starch to fermentable sugars and limit dextrins (which are not fermentable.) You can convert the dextrins to fermentable sugars by adding amyloglucosidase to the fermenter. This will increase the total amount of fermentable sugar and increase your yield of alcohol per kg of grain.
Starch is made up of many glucose molecules bonded together in chains. There are two different structures for the chains: linear and branched. The linear chains are amylose, and the branched chains are amylopectin. In amylose the glucose units are bonded to each other with what are called "α(1-4)" bonds, which connect the glucose molecules "end-to-end." Amylopectin is mostly made up of α(1-4) bonds, but there are occasional α(1-6) bonds which connect the end of one glucose molecule to the side of another glucose molecule (which also has two other glucose molecules connected with α(1-4) bonds. The α(1-6) bonds create branches.
Amylase enzymes can only break (technically hydrolyze) the α(1-4) bonds in amylose and amylopectin. They cannot hydrolyze the α(1-6) bonds - they cannot convert the region around a branch point to fermentable sugar. These small chunks, consisting of a branch bond and three chains, each 3 to 4 glucose units long, emanating from the branch are called limit dextrins, and they are not fermentable. Dextrins with longer chains of glucose units are not limit dextrins, because amylase can further hydrolyze the free ends of the chains into fermentable sugars.
Amyloglucosidase can hydrolyze both α(1-4) and α(1-6) bonds. Therefore it can convert the limit dextrins to fermentable sugar, resulting in more total fermentable sugar from the same amount of starch compared to amylase alone. Amyloglucosidase can also convert starch completely to fermentable sugars all by itself.
You don't need to use amyloglucosidase with malted barley, but if you do you will end up with more fermentable sugar, and more alcohol for the same amount of grain.
Brew on

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