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Mashing: infusion vs decoction and temperature rests

Before anything is made from beer, the malt has to give up its sugars. This key stage is called mashing, and it consists of mixing milled malt with hot water so that enzymes break down the starch into fermentable sugars. It sounds simple, but huge control hides in it: the mashing temperature alone decides whether a beer will be dry and light or full and substantial. There are two main schools: infusion mashing, simpler and common, and decoction, older and more laborious. The key to both is the enzymes and the temperature rests. Here is a guide to mashing: how it works, how the infusion method differs from the decoction one, what role the alpha and beta amylase enzymes play and how temperature shapes the body of beer.

What mashing is

Mashing is the stage in which milled malt is mixed with hot water, to release and use the enzymes that break down starch into sugars. It is these enzymes, woken earlier during malting, that now turn the hard starch of the grain into fermentable sugars, giving a sweet liquid called wort. It is this that the yeast will later ferment. Without mashing there would be no sugars, and without sugars no alcohol. Mashing is therefore the foundation on which the rest of beer production stands. The most important thing is that this process does not consist only of mixing malt with water, but of the precise control of temperature, which steers the work of the enzymes. We cover malt itself more in malt in beer. Understanding that mashing is the controlled use of the malt enzymes, and not a simple soaking, is the starting point for all the rest.

The role of enzymes: alpha and beta amylase

At the heart of mashing are two starch-breaking enzymes: alpha amylase and beta amylase. Each works differently and at a different optimal temperature. Beta amylase works best at lower temperatures, around sixty-two degrees, and cuts simple, well-fermentable sugars from the starch, like maltose. This gives a drier beer, because the yeast has more to turn into alcohol. Alpha amylase works best at higher temperatures, around seventy-one degrees, and cuts the starch into larger, less fermentable sugars, giving the beer body, fullness and stable foam. Crucially, the enzymes are sensitive to heat: beta amylase dies quickly at higher temperatures, and alpha after a longer time too. This is why the mashing temperature is so important. By steering it, the brewer decides which enzyme dominates, and so whether the beer will be dry or full.

Temperature rests

From the role of the enzymes comes the idea of temperature rests. A rest is holding the mash at a particular temperature for some time, so that a given enzyme can act. The maltose rest, around sixty-two degrees, favours beta amylase, giving high fermentability and a dry finish. The dextrinisation rest, around seventy-one degrees, favours alpha amylase, giving the beer body and foam stability. The brewer can do one rest at one temperature or several in succession, heating the mash gradually so that each enzyme works at its optimum. It is precisely the choice of rests that lets the profile of the beer be shaped precisely: more dry or more substantial. Temperature rests are a tool of control, by which the brewer plays the enzymes like an instrument. It is they, and not the mere fact of mixing malt with water, that decide the character of the wort, and thereby of the finished beer.

Infusion mashing

The first main method is infusion mashing, simpler and common. It consists of heating the malt in one vessel and holding it at one or several temperatures. The simplest variant, single-temperature infusion, turns starch into sugars by holding the mash at one temperature, usually between about sixty-six and seventy degrees, for around an hour. The chosen temperature decides the profile: lower gives a drier beer, higher a fuller one. A more elaborate variant, step infusion, heats the mash to successive temperatures, doing several rests. Infusion mashing is simple, cheap and sufficient for modern, well-modified malts. This is why it has become the standard in most breweries. It is a convenient method, requiring no complicated equipment, yet giving full control over the profile of the beer through the choice of temperature and rests alone.

Decoction mashing

The second method is decoction mashing, older and more laborious. It consists of taking part of the mash, usually about a third, boiling it separately and then returning it to the rest, thereby raising the temperature of the whole. Crucially, only part of the thick mash is boiled, and most stays unboiled, so it keeps its heat-sensitive enzymes. The boiling physically breaks down the cell walls of the starch and improves its availability to the enzymes. Decoction gives the brewer more control over the body, strength and character of the beer than a single infusion, and on top adds deeper, malty notes from the boiling. It is a traditional method, associated especially with classic German and Czech beers. It is time-consuming and requires more work, though, which is why today it is used mainly by breweries that care about tradition and a particular, malty character.

A table: infusion versus decoction

Let us gather the two methods in one place:

The table shows the heart of the choice: infusion is simplicity and convenience, decoction is work and an extra, malty depth. Both let the profile of the beer be steered.

Why temperature decides the body

The most important practical lesson from mashing is this: the mashing temperature translates directly into the body of beer. A lower temperature favours beta amylase, giving simple, well-fermentable sugars, so the yeast will turn more of them into alcohol, and the beer comes out drier, lighter and brisker. A higher temperature favours alpha amylase, giving larger, less fermentable sugars that stay in the beer, giving it a fuller body, a sweeter remainder and a more stable foam. This is why the same recipe, mashed at a different temperature, will give a different beer. The brewer, in choosing the temperature, consciously decides the texture and dryness of the beer, long before fermentation. This shows how enormous a control over the final character is given by the seemingly technical stage of mashing. Temperature is one of the main dials shaping beer.

Mashing and beer style

The choice of mashing method and temperature ties to the style of beer. Dry, brisk beers, like many lagers, use lower temperatures favouring fermentability. Fuller, sweeter, substantial beers, like some strong ales, use higher temperatures. Classic German and Czech beers are traditionally made by the decoction method, to reach their characteristic, deep maltiness. This shows that mashing is not one rigid step but a tool tuned to the intended style. We cover how malting affects colour and flavour more in malting and beer colour. Mashing is the moment at which the brewer translates the raw material into a particular profile of sugars, and thereby into the character of the beer. It is one of the foundations on which the whole style of the finished drink stands.

The essentials in brief

Let us gather it up. Mashing is the stage in which milled malt is mixed with hot water, so that enzymes break down starch into sugars. The two key enzymes are beta amylase, working at lower temperatures and giving simple, fermentable sugars and a dry beer, and alpha amylase, working higher and giving larger sugars and fullness and body. Temperature rests let the brewer steer which enzyme dominates. Infusion mashing, simple and common, holds the malt in one vessel at one or several temperatures. Decoction, older and laborious, boils part of the mash separately, giving deeper maltiness, as in German and Czech beers. The temperature directly decides the body of the beer. Now you know how sugars are drawn from malt and why mashing shapes beer so much.

Note every beer in GustoNote - the style, the body and dryness and the notes you sense. Over time you will start to link the fullness or briskness of a beer to decisions made already at mashing, and understand more deeply how its character is made. We cover the whole process more in how beer is made.