Hi Brendan,
There have already been several helpful comments, but I wanted to add a few more.
Glucoamylase (GA or AMG) can be added to the fermentor and it is very effective to reach high levels of fermentability. Typical practice is to add the enzyme at the start of fermentation. On a practical basis, this makes the timing of the addition easier, as you do not have to worry about missing the window for addition, and final attenuation can then be determined by dose rate. In addition, pH is higher at the start of fermentation and that may be more helpful for enzyme activity.
As mentioned in a previous post, the benefit of fermentor addition is the significantly longer time for enzyme contact and activity. This offsets any negative from pH or lower temperatures.
You do need to make sure that the enzyme is inactivated prior to packaging. This is normally done with flash pasteurization, where higher temperatures (and shorter times) can more easily facilitate inactivation. If active enzyme passes through to packaged beer, there is the risk of continued activity, where the beer may become sweeter over time. Or the beer may continue to ferment if there is viable yeast still present in addition to newly created sugar. Plan this carefully.
Reaching 84% fermentability with a mash addition of GA becomes more difficult when the barley crop varies so much, particularly with low starch and high protein of the past few years. Longer rests tend to bring more material into solution, but not all of it is fermentable. On top of this, gelatinization temperatures can increase with drought and heat stress, and in our work, we do see this happening more frequently.
If the gelatinization temperature for starch is above your rest temperature, then there is the possibility that not all starch will be converted to fermentable sugars, so a traditional 149°F/65°C rest, typically listed in references and texts, may no longer be sufficient.
If your mash temperature is lower, at 145°F/ 63°C, then I would definitely focus on starch gelatinization first. It is the critical first step to fermentability. You can increase mash temperatures to gelatinize the starch, but alpha amylase activity may decline due to the higher temperatures. In this case, you would certainly need to add alpha amylase in addition to glucoamylase.
As an aside, Glen Fox has spoken about starch and gelatinization temperatures and he has a podcast available here on the MBAA website as well as a presentation on the ASBC website, if you are interested in tracking them down.
Cheers!
Andrew
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Andrew Fratianni, Dipl. Brew.
Sr. Enzyme Application Specialist
Brewing & Distilling Enzymes
IFF Health & Biosciences
andrew.j.fratianni@iff.com------------------------------