Mash Hopping is the addition of hops, typically whole hop flowers, during the mashing phase in the brewhouse process. It is used by a small number of craft brewers to add complexity to their beers. Although the expected hop utilization in the traditional sense is extremely low, it is recognized that mash hopping can impart unique hop flavors to the resulting beer. With this method, there is contact time between the mash and both the soluble and the nonsoluble hop compounds at mash temperatures ranging, depending on beer style and brewhouse process, from 122°F to 170°F, at a preboil wort pH of roughly 5.4 to 5.8. Because the hops remain behind in the spent grain after lautering, they are not exposed to boiling temperatures. Therefore, there is very little expected carryover and isomerization of hop bittering compounds (alpha acids). See isomerization. Nonisomerized alpha acids are fairly insoluble in wort, and brewers using this method expect little or no additional bitterness from these hops. See hop utilization. If mash hopping is done with pellets rather than whole leaf hops, some bitterness might result, but this would be attributed to hop powder carryover to the kettle during runoff or to a lesser extent, any isomerization that has occurred during the pelletizing process. Bittering acids aside, there will be some carryover of other flavor active compounds; however, similar to other early kettle hop additions, much of the lighter fruity or flowery volatile oils will be lost. Heavier oils, oxidized hop compounds, and other flavor active compounds, on the other hand, can be passed on to the kettle and therefore can make it into the final beer. There are some similarities between this method and first wort hopping in that the flavor active compounds that survive to the kettle are exposed to the wort for a greater period of time prior to the boil. See first wort hopping. The expected hop-related flavor characteristics therefore would be very similar. The hop material being in contact with the mash, wort, and sparge water for the duration of the process no doubt results in some flavor carryover.

There are differing opinions among brewers about what beer styles are best suited for mash hopping. The prominent view is that mash hopping is best suited for lighter styles with subtle hop flavors and textures. But mash hopping has also been used in highly hopped beers, such as “double India pale ales” and other “extreme beer” styles to pack in added hop notes. The subtle notes thus gained, however, are likely drowned out if large hop charges are used downstream. Although it may seem wasteful by some standards, mash hopping with whole flowers has a very positive effect on lauterablity, adding porosity to the grain bed, not unlike rice hulls or other lautering aids. There is some reference to this method of hopping in the traditional production of Berliner weisse, where low hopping rates and very low International Bitterness Units are important to reduce the antimicrobial effect of the hops, which in turn allows for the growth of lactobacillus bacteria in this intentionally sour beer style.

See berliner weisse.