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Question: My mash tun looses heat during the mash. Can I use this to get both types of starch enzyme to work?
Answer: The mashing process depends on the action of two different enzymes,
called a-amylase and ß-amylase. Together, these enzymes convert starch
into fermentable sugars. Their exact modes of operation are different, though,
and each has an optimum mashing temperature. To understand these differences,
we need to know what starch is in the first place.
Starch consists of long strings of sugar molecules. These strings can take one of two forms: straight chains, or branched chains. These long chains are not fermentable, nor for that matter, are they soluble in water. To become soluble, these chains must be broken down no more than about 6 units long. To be fermentable, they must be broken down into sugar units of one (glucose), two (maltose), and sometimes three (maltotriose).
a-amylase breaks the starch chains at random places. Given enough time, it will eventually bust the chains into small fragments. ß-amylase works in a more methodical manner. It snips off the ends of the straight chains two sugars at a time. If starch consisted only of straight chains, either enzyme would work equally well. However, the branching points throw a monkey wrench into the works. The ß-amylase can't get past a branch, no matter how much time it has. a-amylase can't break the branch either, but at least it can snip around it. That in turn opens up new free ends for the ß-amylase to work on.
Given the fact that a-amylase opens up sites for ß-amylase, it makes sense that most fermentable wort will be achieved if the a-amylase as a chance to work first.
Now, then, what temperatures do they work at? Both enzymes work faster at higher temperatures. But at the same time, they also decompose faster. Hence, mashing is a race to finish starch conversion before the enzymes disappear. As it works out, the decomposition temperature for a-amylase is a bit lower than for ß-amylase. If you're not careful and get the mash too hot, the a-amylase will be gone, and the ß-amylase won't have anything left to work on. Some of the starches will be left as four to six-unit soluble saccharides. These won't ferment, leaving the beer with a high final gravity and a sweet finish. If you're making a rich malty beer, this is good. But for a dry beer, it isn't. According to George Fix in Principles of Brewing Science, 158°F is too hot for the a-amylase. So as long as the mash stays below 155°F or so, the mash will ferment dry. Going lower is OK, but then the rate of enzyme action falls off and you'll need to do a two-hour mash. If you end up at 149°F, the action will be just about perfect.
Note that for the fastest and driest possible mash, you actually want to raise the mash temperature. Some dry styles call for a mash starting at 149°F and then bumping it up to 155°F to finish. If my mash starts out on the cold side, I'll do this. Usually, however, I aim for 155°F and have a homebrew while the temperature slides down to 149°F.
One final point to keep in mind is the uniformity of temperature. The outer edges of the mash tun will be cooler than the center. Make sure you stir the mash every once in a while to keep the action uniform.
So keep an eye on your mash temperature, and give it a stir every once in a while. As long as the temperature drop is 5 to 6 degrees or so, you're OK, and both enzymes will do their job. If it's more than that, you'll either need to reheat the mash, or get some better insulation.