Having mashed many beers over the course of many years religiously trying to keep the temperature at 67ºC for at least an hour, I got asked a question by a none homebrewer - thank you Mr B - which stumped me: how critical is the 67ºC mash temperature?
67ºC is the generally accepted temperature to hold to for a infusion mash - one using a single temperature throughout the process. This is sometimes difficult to maintain & the temperature varies a little to either side of 67ºC, but how crucial is holding it steady?
The tolerances appear to be from 60-70ºC with an absolute limit at 76ºC.
At 60-65ºC the enzyme beta-amylase works optimally to break up the long sugar chains found in malted barley into smaller units by randomly attacking links in the chain. At 68-70ºC the enzyme beta-amylase works optimally to break up long chain sugars by attacking the end units.
Yeast can metabolise single unit sugars (monosaccharides) readily, double unit sugars (disaccharides) less so & triple unit sugars (trisaccharides) less so again. Any chain of sugars larger than three units cannot be metabolised by yeast into alcohol & survives the fermentation process into the finished beer.
So to take a simpified model: one long chain of sugars can only be broken down from either end by beta-amylase & will take ages as it can only attach two end units at a time. Whereas alpha-amylase will break the long chain into two, exposing four end units. If the brew is at the cooler end of the mash temperature then the optimally working beta-amylase will expose many end units for the sub-optimal working alpha-amylase to work on. If the brew is at the top end of the mash temperature scale then the sub-optimal beta-amylase will not open up many end units at all for the optimal alpha-amylase to work on.
The 76ºC limit is where the enzymes are denatured & cannot work on the long chain sugars.
This information can be used to make a drier, thinner beer by mashing it closer to 60ºC or a sweeter, fuller bodied beer by mashing it towards the 70ºC end of the temperature range. To make the driest beer you might hold the temperature at 63ºC for an hour & then step it up to 69ºC for an hour - the combination of having both enzymes working efficiently for an extended length of time would convert all the long chain sugars to simple ones that the yeast can digest.