A story for anyone who’s wondering if there’s really any truth to mash temperature affecting final gravity. I accidentally turned the wrong burner on when I was brewing last month and almost boiled the mash. It was sitting at 68C for maybe 10mins before I made the mistake, and by the time I caught on it was at 85C. I chilled it back down as fast as I could. A month later, today, I went to bottle and took a final gravity reading: 1.028 instead of the estimated 1.012! I was confused at first and then checked my notes (a month is a long time) and remembered the foul up. So I have a 4.5% Session Coffee Stout instead of 6.6% Strong Coffee Stout but hey, the sample tasted good and maybe I can have one with breakfast and not ruin the day.
Some next-day thoughts: I effectively performed a mash-out after 10-20mins of normal conversion. Is this an existing technique used to create low alcohol beers? I recall our presenter on session ales talking about aiming for high FG to keep them flavourful - perhaps this is a method for achieving that as well.
If you’re using “modern malts” such as Canada Malting 2-row, your conversion after 10-20 min is already pretty much done anyway.
One trick to making low alcohol beer is to aim for a low OG to start with, then heat up the final beer to approx 80C (175F) where alcohol will evaporate.
You might lose some hop flavour aroma as well but you could always dry hop after cooling it again.
I guess it depends on the malt used. Some malts are more “modified” than others. Those convert very quickly. Others need a little longer.
Also, proteins and starches take time to degrade and don’t ferment out. Finally, your yeast might have petered out before it got a chance to complete fermentation. Lots of variable here!
I’m getting 2 concepts mixed up: mash efficiency and wort fermentability. The mash efficiency was normal, so Denis is right that conversion happened fast (an early mashout would have lowered efficiency). The wort fermentability seems to have been low either because of the mash temp error, grain bill, or the yeast misbehaved.
Actually, the normal efficiency (assuming you measured it and confirm it was normal) is not necessarily proving that the conversion was completed after 10 min. As your temperature raises, the speed at which the conversion happens is actually increased, so the conversion continues for a while. The problem is that the enzymes that make the conversion happen degrade quickly at high temperatures, so the conversion ticks up while you are reaching temperatures around and above, say, 22C, but then it crashes down fast as the enzymes break down. So the conversion continued after 10 min., and quickly, but probably not for long. On a different note, I have read that mashing at higher temperatures (though not usually that high) is a good way to make lower alcohol beers precisely because some of the sugars won’t be converted into fermentable sugars (so less food to be turned into alcohol) and also because the residual sugars will compensate taste-wise for the loss of alcohol, which contributes a lot to the taste of beer. So yeah, you might have stumbled on a variation of your recipe that you might want to replicate once you have tasted a few bottles for when you want a lower-alcohol version of it.
