Infusion Mash, also called the “single infusion mash” or “British infusion mash,” is a mashing technique where hot liquor (hot brewing water) is blended with malts to create a mash that only has one rest, at saccharification temperature. In most modern breweries, a temperature-programmed mash takes place in the mash mixer and the wort is separated in a separate vessel, the lauter tun. The infusion mash, a simpler affair, is carried out in a mash tun, a vessel that might be better described as a “mash/lauter tun.” This vessel has a false bottom with screens, and both the mash and the runoff (lautering) take place there. The mash/lauter tun is part of a two-vessel system, alongside the wort kettle. The mash tun is insulated, but unheated; therefore, it is not possible to perform a series of temperature steps. When the mash-in is complete, the correct temperature must have been achieved. This temperature usually ranges between 63.3°C and 68.8°C (146°F and 156°F).

Although the infusion mash seems the most simple of mashing techniques, in professional practice it is virtually an art form. As the crushed grist emerges into the mash tun, it normally goes through some type of hydrator that will mix the grain with the hot liquor before it hits the “foundation liquor” that barely covers the false bottom screens. The texture of the infusion mash is starkly different from that of European-style mashes. Whereas European mashes are somewhat soupy, often with water-to-grist weight ratios at about 3 to 1, the infusion mash is notably thicker. The European mash is thoroughly mixed and contains little or no air, but the traditional infusion mash is fluffy with tiny air pockets, and as the mash-in proceeds, the mash begins to float on a pool of heavy wort that forms below it. Because there are no protein rests or temperature rises, saccharification begins immediately and the starches in the malt begin to liquefy quickly.

If the brewer wishes to achieve a final mash temperature of 65°C (149°F), he or she will need to take account of the temperature of the malt (which may vary depending on the time of year) and the temperature of the mash vessel and then pay close attention to the texture of the mash. If the mash is too wet, it will be too hot; conversely, a mash that is too dry will be too cool. Infusion mashes usually use saccharification rests of at least 1 h, although 90 min is not uncommon. British brewers sometimes claim that the proper conversion of starches takes this long, although European brewers have been heard to joke that the long conversion rest is used just to allow the British brewer a leisurely breakfast.

Sparging of an infusion mash is performed gently, with the surface of the mash kept free of standing water and the mash continuing to float on its wort until the end of the runoff. At very small breweries, the brewer may occasionally cut through the grain bed with a wooden or plastic oar to loosen it and allow the wort to flow more easily. Only at the finish of the runoff is the mash allowed to descend and contact the plates as the last runnings drain off. If the mash is allowed to set down to the plates earlier, a stuck mash can easily result.

Infusion mashes, properly used, can yield excellent results, and they are still widely used at smaller British breweries and many brewpubs in the United States and around the world. However, because temperature steps are not employed, some types of beers, particularly wheat beers (which normally need a protein rest at a lower temperature), can be more difficult to brew using this technique.

See also protein rest, saccharification, stuck mash, and temperature programmed mash.