Mash hopping is a confusing thing. By all logic, it shouldn't work. One would expect most of the hops to be left in the grain bed, and any hops that make it into the wort to be isomerized during the long boil that follows.
I'm not sure anyone knows why it works, and many people will tell you it *doesn't* work, but let your own experiments be the judge of that.
The following is according to Paddock Wood Brewing Co, quoted from their blog:
Based on the research of De Clerck and Fix, the theory is that the aromatic oils react in a special way with wort at a higher pH than occurs with wort during the boil (the pH falls during a boil, and reduces the utilization of the hop oils) and at a lower temperature (150F). The complex reaction between hops and the wort results in the formation of more permanent flavour and aroma chemicals remaining in the beer than is the case with traditional hopping methods such as late additions to the brew kettle. It may be important to use pelletized hops in this situation, however, as the release of aromatic oils from whole hop cones is greatly assisted by the action of the boiling wort. The pelletized hops are not transferred to the kettle but remain behind as the mash bed acts as a filter. The hop usage appears to be about 90% less than what you could expect from a start-of-boil addition, but we have not performed any technical data gathering or analysis.I've tried it once, since it is a standard part of Janet's Brown Ale. Russian River Brewing apparently also uses it in their Pliny the Elder. Until I'm convinced, I won't be mash hopping unless a great recipe calls for it. I haven't done a side by side comparison, but some day I'd like to brew two identical batches - one that has been mash hopped and one that hasn't. I encourage any readers to try it and let me know the results.
