All-grain brewing opens up a huge realm of possibilities for the malt profile of your beer. Of course it comes with the cost of an extra step of complexity with the "mash". Mashing is a process that is described by John Palmer in "How to Brew" as "the hot water steeping process that hydrates the malt, gelatinizes its starches, releases its natural enzymes, and converts the starches into fermentable sugars" (p.141). More simply put, it's like making barley-sugar-water through steeping - just like tea. In this post I will discuss the choice of the mash temperature and how it will affect the wort that you produce.
If you want to use Palmer's rule of thumb for mash conditions, go with 3-4 litres of water per kilogram of malt, at a temperature between 65-68 C, and a pH of 5.4-5.8, for about an hour (p.151). That range will cover most brews. If you want a sweeter, more dextrinous wort then you should raise the temperature (not above 70) and if fermentability (thus attenuation) is the goal then go with a lower temperature (not below 60 C).
There are several enzymes that are at work during the mash to break down complex sugars and make them available for the yeast to eat during fermentation. While there is some overlap, each enzyme has its own range of temperatures and pH for it to be most effective. Each enzyme has a different function so your resulting wort will have a different profile of sugars depending on the conditions (temperature and pH) of your mash. The conditions above land right in the middle to allow most of the enzymes to work but not necessarily at their peak. Here are some of the major enzymes that we care about (Table 17, p.143, Palmer):
Alpha-amylase - 60 - 75 C (60-70 C preferred); produces sugars & dextrins (i.e. maltose)
Beta-amylase - 60 - 65 C (60 C preferred); produces maltose
Limit-dextrinase - 60 - 67 C (60-65 C preferred); cleaves limit dextrins (breaks up big sugars for alpha & beta amylase to work)
Beta-glucanase - 20 - 50 C (35-45 C preferred); breaks up "gummy" adjuncts such as unmalted barley, rye, oats and wheat for easier lautering
Proteases - 20-65 C (45-55 C preferred); solubilises proteins
Peptidases - 20 - 67 C (45-55 C preferred); produces free amino nitrogen (FAN) which yeast need for metabolism
All of these enzymes work together in the mash to break down and unlock sugars for the yeast to eat. Remember that as brewers our goal is to make fermentable wort, not beer. If you are thinking about your yeast and how to make the best wort for them to process then your head is in the right place. If you are loose about your mash temp and don't care about it too much then it's like not caring what your kids eat. I mean you can give them a can of Pringles and there are calories there and it will keep them alive, but you can also put together a nice plate of food and know that you are helping them to be at their best. Did I just make a parenting-brewing metaphor?
Looking at these temperature ranges it can be seen that there may be a need for a multi-step mash due to the lower range for proteases, peptidases and beta glucanases in comparison with the starch conversion temperatures for alpha- and beta-amylase.
"Protein Rest" - You may have heard this term before but it involves a 20-30 minute hold at 35-45 C. You typically only need to do this if you are using moderately-modified malt (i.e. some European malts) or if your mash contains 20% or more of the adjuncts that contain beta glucans. Additionally, this rest can allow for the enzyme "phytase" to acidify the mash to lower the pH which is not typically needed but was historically was used when using only pale malts (however, this rest takes much longer than 30 minutes to significantly lower the pH).
"Saccharification Rest" - The more important part is to have a rest in the range of the alpha and beta amylase so that we can have wort filled with digestible sugars. As you can see, however, this is a compromise between those too enzymes in the temperature range of 60-70 C. If you stay on the lower end of that range then beta amylase and alpha amylase can both work to chop up the larger sugars (amylopectin). With a peak performance at 65 C you will have a wort that is the lighter in body and most fermentable, thus with a higher attenuation (depending also on the yeast). So if you want a drier beer without a lot of residual sugars/dextrins then go with the lower mash temperature. Past 65 C the beta amylase will degrade rapidly so that it won't work but you still have alpha amylase. The result will be a wort that has more residual sugar which can leave the beer with a sweeter finish or fuller body.
To point out the obvious, if you are mashing under 60 C then you aren't allowing the main enzymes to break down sugars and you aren't going to have much of a fermentation - don't starve the yeast! If you are mashing well over 70 C then you are degrading those enzymes almost instantly to where they aren't going to do any work either. This also isn't great for the yeast. In both cases the yeast will be able o ferment some of the wort, but most of the sugars are way too complex for the yeast to break down. Going back to my parenting metaphor, it's like giving a child a lobster for dinner. There's some really great stuff in there, but that kid isn't going to be able to get to much of it. He/she might be able to tear open a part and get some meat, but most of it will be left behind.
The other point to note is that you can either go for the 67-68 C single-infusion mash to fall right in the middle of those values and have perfectly fine wort. Beta amylase won't be maximised but no big deal. Most brewers choose to go with a multi-step mash though which allows different rests to help to optimise the usage of all of the enzymes. This may include a protein/beta glucan rest at 45 C, followed by a 60 C hold for beta amylase to shine, followed by a 68 C hold for amylase to bat clean-up.
In this post I've largely neglected the discussion of pH. In general the pH range is similar for all of the enzymes and you should shoot for 5.4-5.8. It is also very important to monitor this, but I shall save that for another post. In short.....mash temperature matters!
-- Mr. Jackson
*Palmer, J. "How to Brew". Brewers Publications, Boulder. 2006.