How the Mash Works
Manipulating the Starch Conversion Rest
There are two other factors besides temperature that affect the amylase enzyme activity. These are the grist/water ratio and pH. Beta amylase is favored by a low wort pH, about 5.0. Alpha is favored by a higher pH, about 5.7. However, a beta-optimum wort is not a very fermentable wort, leaving a lot of amylopectin starch unconverted; alpha amylase is needed to break up the larger chains so beta can work on them. Likewise, an alpha-optimum wort will not have a high percentage of maltose but instead will have a random distribution of sugars of varying complexity. Therefore, a compromise is made between the two enzyme optimums.
Brewing salts can be used to raise or lower the mash pH but these salts can only be used to a limited extent because they also affect the flavor. Water treatment is an involved topic and will be discussed in more detail in the next chapter. For the beginning masher, it is often better to let the pH do what it will and work the other variables around it, as long as your water is not extremely soft or hard. Malt selection can do as much or more to influence the pH as using salts in many situations. The pH of the mash or wort runnings can be checked with pH test papers sold at brewshops, and pool supply stores.
The grist/water ratio is another factor influencing the performance of the mash. A thinner mash of >2 quarts of water per pound of grain dilutes the relative concentration of the enzymes, slowing the conversion, but ultimately leads to a more fermentable mash because the enzymes are not inhibited by a high concentration of sugars. A stiff mash of <1.25 quarts of water per pound is better for protein breakdown, and results in a faster overall starch conversion, but the resultant sugars are less fermentable and will result in a sweeter, maltier beer. A thicker mash is more gentle to the enzymes because of the lower heat capacity of grain compared to water. A thick mash is better for multirest mashes because the enzymes are not denatured as quickly by a rise in temperature.
As always, time changes everything; it is the final factor in the mash. Starch conversion may be complete in only 30 minutes, so that during the remainder of a 60 minute mash, the brewer is working the mash conditions to produce the desired profile of wort sugars. Depending on the mash pH, water ratio and temperature, the time required to complete the mash can vary from under 30 minutes to over 90. At a higher temperature, a stiffer mash and a higher pH, the alpha amylase is favored and starch conversion will be complete in 30 minutes or less. Longer times at these conditions will allow the beta amylase time to breakdown more of the longer sugars into shorter ones, resulting in a more fermentable wort, but these alpha-favoring conditions are deactivating the beta; such a mash is self-limiting.
A compromise of all factors yields the standard mash conditions for most homebrewers: a mash ratio of about 1.5 quarts of water per pound grain, pH of 5.3, temperature of 150-155°F and a time of about one hour. These conditions yield a wort with a nice maltiness and good fermentability.
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