Multi-Rest Mashing: An Introduction

Most all-grain brewers practice single-rest mashing, i.e. using a single (saccrification) temperature.  Not only is it the easiest and fastest method, its usually good enough or even preferred for most styles.  But multi-rest mashing is a great tool to keep in your brewer’s tool belt.  Its something I have just started using, and plan to do so more frequently.  In this introduction, I’ll explain what mutli-rest mashing is, and why you might (or might not) want to use multiple rests.

various mash temperatures overlaid graphically

The various temperatures of the mash. The lighter colors represent active but non-ideal ranges. (Click to enlarge- this diagram got busy quick!)


Why use multiple rests?

Malting starts the breakdown of proteins, starches, and other materials inside of the grain, and mashing continues this process to make the sweet wort.  Maltsters have gotten so good at their job over the years that most malt is “well-modified,” and the breakdown is so advanced that the brewer need not employ a more complicated mash spending timeor “resting”at multiple temperatures.

Some malts, like European pilsner, are not quite as well modified (though these and most other base malts are still considered “well-modified”), and other ingredients can be used unmalted. These grains or adjuncts can benefit from additional breakdown during the mash, especially if you are brewing a style requiring a delicate, clean malt touch or higher fermentability.  I believe all Trappist breweries use multiple rests, as does Ommegang, an excellent American brewer of Belgian styles, for example.  Much of the German brewing tradition, too, uses multiple rests via decoction.

If you have ever used a significant amount of unmalted grains, you probably noticed a much gummier mash than usualperhaps it even stuck.  This is the result of β-glucans (“beta-glucans”), normally broken down during malting.  After malting, wheat, oats, and rye all  have higher levels of β-glucans than barley, about 1 to 2 %, 2 to 3 %, and 2 to 3 %, respectively, verses about 0.5 % for malted barley; corn and rice do not contain significant levels of β-glucans.  No malted grain will cause generally lautering problems on there own (though combined with the lack of husk on wheat it can), but it can carry through to the final beer if not dealt with.  James Spencer over at Basic Brewing Radio once made a 100 % rye pale ale he described as “hoppy cough syrup.”  These issues can be treated with a β-glucan rest.

Less modified and unmalted grains can also be deficient in free amino nitrogen (FAN), a critical yeast nutrient normally derived from the breakdown of protein during malting.  After malting, wheat, rye, and oats contain much more high molecular weight (i.e. large) protein than malted barley, which can cause haze issues if not broken down.  Unmalted grains tend to have even higher levels.  A protein rest will combat these problems, and can improve head retention.  If used to excess on well modified malts, however, a protein rest can hurt head retention and body in extreme cases by breaking down too many medium molecular weight proteins.

But, the enzymes responsible for β-glucan and protein breakdown are active to a significant degree only at temperatures outside of the saccrification range used with single-rest mashes, necessitating multiple rests.  Additionally, there are numerous opportunities for improving brewhouse efficiency, should you be so inclined.

The rests

Listed below are all of the major rests you might employ.  I personally use only one or two rests: one in the saccrification (sugar producing) range, and optionally a combined β-glucan and protein rest.

Acid Rest (86 to 126F/ 30 to 52C; several hours): The enzyme phytase breaks down a compound in the malt called phytin, producing a weak acid in the process.  Though not used anymore, the brewers of Pilsen employed an acid rest to acidify their mash of pale grains (which did not contain enough acid-forming compounds) in conjunction with their extremely soft water.  Today, mash pH is controlled by water treatment; Reinheitsgebot followers can use acid malts or simply more roasted grains (ever wonder why most classic south-German beers are dark?).

Dough-In (95 to 113F/ 35 to 45C; 20 minutes): A rest in this range helps wet the malt and distribute the enzymes evenly, which improves brewhouse efficiency a bit.  However, you should be especially careful to minimize oxygen exposure below 140F/ 60C, perhaps even boiling and cooling your strike water to an appropriate temperature before use to drive off dissolved oxygen.  Below this temperature, the enzyme lipoxygenase will begin oxidizing long-chain fatty acids leading to stale flavors later.

β-glucan Rest (ideally 95 to 113F/ 35 to 45C, up to 122F/ 50C; 30 minutes): Various β-glucanases and cytases (two families of enzymes) work to break up β-glucans, which are non-starch long-chain sugar molecules.  If left intact, these molecules can be quite gummy.  This rest is useful when unmalted grains account for more than about 10 to 20 % of the mash, or for extremely high levels of malted wheat or especially malted oats or rye.

Protein Rest (ideally 120 to 131F/ 49 to 55C, though still somewhat active up to 155F/ 67C; 15 to 30 minutes): Proteases break large insoluble proteins into smaller chunks, and peptidases break these down even further to FAN needed by the yeast, improving head retention and clarity in the process.  Useful only for less modified malts, when unmalted wheat, rye, or oats are used for more than 20 % of the grist, or when unmalted adjuncts are used.

A protein rest can be combined with a β-glucan rest at 113 to 122F/ 45 to 50C.

Saccrification Rest (140 to 158F/ 60 to 70C; 1 hour): This is the rest used when employing a single-rest mash, although some breweries use multiple rests in this region.  Using multiple rests in this region are usually done to ensure a highly fermentable wort (lower temperature rest) while getting the most yield (barley starch is not fully soluble until the higher end of the range, requiring a second high temperature rest to gain efficiency without significantly altering fermentability).

Mash-out (170F/ 77C): While not actually a rest, raising your mash to this temperature will (1) denature (disable) all enzymes, locking in your fermentable profile, (2) thin the mash for faster lautering, and (3) dissolve a bit more of the sugar, increasing brewhouse efficiency. Mashing-out is especially useful when fly sparging is used, as it takes some time to lauter with this method.  If you are batch sparging, especially on to a kettle on a burner, the enzymes are going to have less time to continue working.

Higher mash-out and sparging temperatures, up to 190F/ 88C, are sometimes employed for sour beer production to make unconverted starches soluble, ensuring that there is a food supply for non-S. cerevisiae microbes (i.e. microorganisms other than standard brewers yeast) after the main fermentation is complete.  Certain styles also depend on the extraction of astringent tannin compounds from grain husks that occurs at these higher mash-out temperaturesnormally not desired in beerfor balance in the absence of higher hop levels which might inhibit some “wild” microbes.

My next post will discuss how to mash with multiple rests.

[Most information comes from John Palmer’s How to Brew, 3rd ed., chapter 14.  If you don’t have this book, get it already!]

Life Fermented


About Dennis
Home brewer, home chef, garage tinkerer. Author of Life Fermented blog.

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