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).
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.