This has long guided my mash "temperature program."
As others have aptly noted ... it all depends on what you're trying to brew.
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GETTING THE MOST FROM AMERICAN MALT
Notes from a talk by Dr. Michael Lewis (BIO:
http://www.brewerspublications.com/auth ... ael-lewis/)
Home Brew U - March 27, 1993
For American pale malt, optimum fermentability is obtained at temperatures of 55° - 60°C (131° - 140°F).
At these temperatures, the beta amylase enzymes produce maltose most efficiently.
This happens early in the mash in a fairly short time, approximately 20 minutes.
The alpha amylase enzymes, on the other hand, produce the dextrins that give us the total extract we desire at temperatures between 70° - 75°C. (158° - 167°F).
It is possible to mash American pale malt with a single temperature infusion.
While this can be a reasonable compromise approach, it inevitably results in a loss of
either fermentability or extract, since the temperature is not optimum for either.
The best plan for mashing American pale malt is a "temperature program" in order to obtain the optimum balance of extract and fermentability.
A sample two temperature program, utilizing the popular "camp cooler" mashing method, would be something like this:
1. Stir in enough hot water at around 70°C. (approximately 158° - 160°F) to make a thick mash, so the temperature settles in between 55° - 60°C (131° - 140°F).
Initial mash temperatures as low as 50°C (122°F) are acceptable.
Hold for 20 - 30 minutes at this temperature.
2. After 20 - 30 minutes, add enough hot water just off the boil to raise the temperature to 70° - 75°C (158° - 167°F) for the remainder of the mash period.
What many American home brewers don't realize is just how low a temperature American pale malt needs for
optimum fermentability and how high a temperature it needs for
optimum extract.
Dextrins do not, as far as experiments disclose, contribute "body" as is frequently stated, but rather contribute a desirable aftertaste. The so-called "protein rest" usually advocated for American pale malt does not seem to have any real basis.
Everything that needs to happen in the mash will happen with a proper temperature program that addresses fermentability and extract. [Dr. Lewis's comment was actually that the protein rest was "bullshit"!]
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REF: HOMEBREW Digest #1660 Fri 17 February 1995
http://hbd.org/hbd/archive/1660.html