Enzymatic denaturing at room temperature
Everything I've read on amylase (beta and alpha) says that the primary denaturing mechanisms are temperature and pH, and often the reference will include a note about denaturing time for certain conditions (e.g. at 153 F, alpha-amylase will become denatured after two hours, etc).
Is there a temperature at which amylase (or other enzymes in the process) won't denature regardless of time, or will the enzymes get 'worn out' and denature after a certain number of reactions regardless of conditions? I'm curious because I may need to add an amylase powder to a fermentation to bring down a (very high) FG to a decent level.
If even a minuscule amount of enzyme will eventually be able to convert the entire wort into a fermentable form, I'll just drink it as is. But if the enzymes denature at *some* point at room temperature, I don't mind adding small small amounts and titrating the gravity down to the desired point...
The enzymes will denature over time. It will vary from enzyme to enzyme. Some can be quite stable, others not so much. Also whether it is stored dry or dissolved makes a difference. Usually dissolved proteins are stored in a glycerol solution and stored in the freezer. Dry enzymes can be quite stable. Most enzymes are not that stable once removed from the cellular matrix they normally "live" in. The denaturing reaction happens faster the warmer it is.
Amylases in general are pretty stable enzymes. It can be hard to judge how much to add if you are aiming for a particular FG.
So are you in fermentation and your FG just hasn't been reached? If so you may just have a stuck fermentation. You could also have had a mash that solubilized large starch chains without fully breaking it down to a form the yeast can ferment. If this is the case, adding amylase might help. However, if you are mid-fermentation and your gravity is no longer dropping, it may simply be that you have a stuck fermentation.
Also, to answer the initial question, as pjj2ba said, dry enzymes are very stable, liquid not as much (this is why I recommend powdered laundry detergent btw). Once the enzyme is introduced into a complex environment e.g. wort there are all sorts of other side reactions that can denature/deactivate the enzyme over time. pH and temperature are the obvious methods of accomplishing this.
Your FG problems really come down to a matter of mash temperature, fermentation temperature, and yeast strain used:
If you mash on the high end (158-160 F) you will get less fermentable wort, resulting in a higher FG. If you mash at the low end (148-150 F), you wil get more fermentable wort and lower FG.
If you ferment on the high end for the yeast strain, you will likely get better attenuation and lower FG due to higher metabolic activity. If you ferment on the low end of the yeast strain, you will get less attenuation and higher OG owing to lower metabolic activity.
This one is self explanatory. High attenuators yield lower FG and low attenuators yield higher FG.
Have you looked into the mash temperature? The yeast used? The fermentation temperature? How do you monitor mash temperature? Are your thermometers calibrated correctly? What is your water profile? What is your mash pH? How do you monitor fermentation temperature? How do you monitor fermentation progression? Do you take gravity readings throughout, or are you a bubble watcher?
It sounds like you need to take a solid look at your process before looking into adding enzyme (which again is just silly for brewers to do).
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