Ending enzyme activity
I brewed a Belgian clone with an OG of 1.091. Not knowing Laaglander Extra Light DME had such a high percentage of unfermentables, the wort got down to 1.042 and stopped completely. Adding fresh yeast, keeping the temperatures right and rousing the yeast regularly did nothing, of course, since the problem was the high percentage of starches in the wort. I didn’t want to bottle at 1.042 so I dissolved one teaspoon of amylase enzyme (and three teaspoons of yeast nutrient) into about 3?4 cup of boiled and cooled-to-lukewarm water and dumped it into the carboy. Within 24 hours, activity and bubbling started back up and a bubble was escaping the airlock every ten seconds or less, so the enzyme must have been doing its work despite being added at fermentation temperatures instead of mashing temperatures. Is there any way to stop the enzymatic activity when the gravity gets down to a reasonable level? Or will it just keep going until all the starches are broken down? I really don’t want to turn this batch into Belgian rocket fuel! Have I created a monster?
I am not sure you have created a monster but you may have a runaway train on your hands! There are only two ways to stop an enzymatic reaction. You can destroy the enzyme or wait until there is no more substrate for the enzyme to act upon. Enzymes are easily destroyed when heated to the point where the enzyme denatures. If you had a way to pasteurize your beer you could easily halt this reaction. You probably don’t have a convenient way to denature the enzyme and by the time this answer is published the enzyme you added will have run its course.
When an enzyme runs out of substrate the action ends and in non-living systems where enzymes are added to perform a function this is often how the reaction ends. It is difficult to know if this is likely to be a happy or tragic ending without knowing what type of amylase you added. If you added a mixture of alpha and beta amylases, the result would most likely be a pretty dry beer with residual sugars that yeast cannot ferment. Adding alpha and beta amylase would be akin to extending your mash profile to produce a dry beer, but even with these beers there are some unfermentable sugars.
If you used the ultimate amylase enzyme in your brew and blasted it with a de-branching enzyme like amyloglucosidase (AMG) you may end up with an extremely dry and high alcohol beer. Some brewers use AMG to brew low calorie and low carbohydrate beers with a lesser alcohol content. A couple of years ago I wrote an article intended as a joke about using Beano® at home to brew light beers and some homebrewers began using it. Beano® contains alpha-galactosidase but achieves a similar outcome as AMG. If you added a tablet or two of this stuff, I predict that the end result may be pretty disappointing for the style of beer you brewed.
There is a solution to your problem that falls into the band-aid category. You could halt fermentation and greatly slow the action of the enzyme you added by cooling the beer down near freezing. This would give you enough time to rack into a keg, carbonate it and drink it before much happens. The worst thing that could happen is that the enzyme activity continues and you have an over-carbonated keg. Or the enzyme produces some sugar from the residuals in the beer but there is no yeast activity, resulting in some added sweetness. I do not suggest using this band-aid fix if you do not have a keg because if there is yeast and enzyme activity still going on you could have a bunch of bottle bombs sitting around.