Ask Mr. Wizard

Freezing hops to kill microbes


Mark Meadows • Enterprise, Alabama asks,

I’ve read the hops could be loaded with infectious microbes, and that the only way to avoid a hop-born infection is to employ a hopback, whereby the hops are sanitized by the hot wort. My question is this: if my hops (leaf or pellets) are kept in the freezer does the low temperature kill microbes that may be present?


There are few practices in brewing that transform beer as much as dry-hopping and I think every homebrewer who loves the aroma of fresh hops should try dry hopping. The great thing about dry hopping is that all of the stories you cite about it are totally bogus. Specifically, you can add hops whenever you want to your beer and there is NO risk of spoiling your beer with microorganisms from hops.

Most brewers do dry hop after fermentation is complete because the carbon dioxide produced during fermentation does strip hop volatiles and reduces the impact of the hops added. Dry hopping during fermentation can also plug up your airlock. That’s a bad thing, so dry hopping after fermentation is complete is practical and that’s why it is usually done at the end. As far as what vessel the hops are added to I beg to differ with those who make a big deal about it needing to be done in a secondary fermenter.

Many craft breweries who dry hop add hops through the top of the fermenter after primary fermentation is complete. Brewers using this practice usually use pellet hops and there is no reason to contain the pellets in any device since they settle to the bottom of the fermenter and the beer is easily racked off of the hops after aging. So if you want to add hops to your conical fermenter go ahead and throw them in!

Other brewers prefer using cone hops for dry hopping and it is fairly common to put the cone hops in some sort of container. Sierra Nevada, for example, uses what is akin to a giant teabag to dry hop some of their brews. The bag is sewn up after filling with hops and then hoisted into the tank through the bottom. Fermented beer is then brought into the fermenter where the hoppy goodness from this giant hop teabag infuses into the beer. The practical advantage of this method is that the hops are contained in the bag and much easier to remove from the beer versus whole hops simply floating around in the tank.

Some beers, notably cask ales, are dry hopped with whole hops that are not contained in any special device. You can do this at home if you wish, but whole hops can cause clogging problems, for example in the dip tube of a Cornelius keg. For this reason I recommend putting whole hops in some sort of container like your PVC and stainless steel mesh tube. At Springfield Brewing Company we use nylon mesh bags with nylon zippers to contain whole hops used for dry hopping. These bags are used to contain lacy things like women’s underwear during laundering, but they also work for dry hopping.

I can cite scientific studies explaining why dry hopping does not spoil beer, but a more convincing and practical argument can be made in favor of dry hopping. Breweries have been producing dry-hopped beers for hundreds of years and empirical evidence clearly shows that dry hopping does not lead to beer spoilage. Brewers as a rule do not continue to use a practice that results in failure. So whatever stories you have heard about hops causing spoilage in beer because of bacteria on the hops should be ignored.

And now for something other than anecdotal evidence: In 1990, Dr. Jean-Xavier Guinard conducted a nice study with students enrolled in FST 102B (that’s the Malting and Brewing Science lab class at UC-Davis) while working on his PhD. Fermenting wort samples were dry hopped and samples were taken over time to monitor the types of yeast and bacteria found. Hops do harbor bacteria and wild yeast, but these organisms don’t grow in beer. The results of this study showed that wild yeast and bacteria populations found in the hops diminished after being added to the fermenter; two days following the addition of hops to the fermenting wort there were no extraneous organisms detected by the microbiological plating methods used. This study was published in the Master Brewers Association of the Americas Technical Quarterly in 1990 (MBAA TQ, 1990, 27(3), 83-89. The microbiology of dry hopping.  Guinard, J.X., Woodmansee, R.D., Billovits, M.J., Hanson, L.G., Gutierrez, M.J., Snider, M.L., Miranda, M.G. and Lewis, M.J.)

One brief note on freezing as a method of killing microorganisms . . . it doesn’t work. Bacteria and yeast survive freezing quite well, especially if ice crystal formation is limited. This is the reason that raw chicken that has been frozen has to be treated with proper precautions in the kitchen. Since hops are dried, putting them in the freezer simply makes them cold and there is no ice crystal formation that could marginally reduce the population of microbes (ice crystals kill, but don’t render anything sterile).

Response by Ashton Lewis.