How the Mash Works
The Starch Conversion/Saccharification Rest
Finally we come to the main event: making sugar from the starch reserves. In this regime the diastatic enzymes start acting on the starches, breaking them up into sugars (hence the term saccharification). The amylases are enzymes that work by hydrolyzing the straight chain bonds between the individual glucose molecules that make up the starch chain. A single straight chain starch is called an amylose. A branched starch chain (which can be considered as being built from amylose chains) is called an amylopectin. These starches are polar molecules and have different ends. (Think of a line of batteries.) An amylopectin differs from an amylose (besides being branched) by having a different type of molecular bond at the branch point, which is not affected by the diastatic enzymes. (Or, theoretically, feebly at best.)
Let's go back to our yardwork allegory. You have two tools to make sugars with: a pair of clippers (alpha amylase) and a hedge trimmer (beta amylase). While beta is pre-existing, alpha is created via protein modification in the aleurone layer during malting. In other words, the hedge trimmer is in the garage, but the clippers are out in the grass somewhere. Neither amylase will become soluble and useable until the mash reaches protein rest temperatures, and in the case of moderately-modified malts, alpha amylase may have a bit of genesis to complete.
Beta amylase works by hydrolyzing the straight chain bonds, but it can only work on "twig" ends of the chain, not the "root" end. It can only remove one (maltose) sugar unit at a time, so on amylose, it works sequentially. (A maltose unit is composed of two glucose units, by the way.) On an amylopectin, there are many ends available, and it can remove a lot of maltose very efficaciously (like a hedge trimmer). However, probably due to its size/structure, beta cannot get close to the branch joints. It will stop working about 3 glucoses away from a branch joint, leaving behind a "beta amylase limit dextrin."
Alpha amylase also works by hydrolyzing the straight chain bonds, but it can attack them randomly, much as you can with a pair of clippers. Alpha amylase is instrumental in breaking up large amylopectins into smaller amylopectins and amyloses, creating more ends for beta amylase to work on. Alpha is able to get within one glucose unit of a amylopectin branch and it leaves behind an "alpha amylase limit dextrin."
The temperature most often quoted for mashing is about 153°F. This is a compromise between the two temperatures that the two enzymes favor. Alpha works best at 154-162°F, while beta is denatured (the molecule falls apart) at that temperature, working best between 131-150°F.
The brewer can use iodine (or iodophor) to check a sample of the wort to see whether the starches have been completely converted to sugars. As you may remember from high school chemistry, iodine causes starch to turn black. The mash enzymes should convert all of the starches, resulting in no color change when a couple drops of iodine are added to a sample of the wort. (The wort sample should not have any grain particles in it.) The iodine will only add a slight tan or reddish color as opposed to the flash of heavy black color if starch is present. Worts high in dextrins will yield a strong reddish color when iodine is added.
What do these two enzymes and temperatures mean to the brewer? The practical application of this knowledge allows the brewer to customize the wort in terms of its fermentability. A lower mash temperature, less than or equal to 150°F, yields a thinner bodied, drier beer. A higher mash temperature, greater than or equal to 156°F, yields a less fermentable, sweeter beer. This is where a brewer can really fine tune a wort to best produce a particular style of beer.