"Hey, man," I said to a homebrewing buddy of mine recently. "This is a really great cherry stout." "Thanks," he said. "Added four pounds of fresh-picked black cherries to the secondary." "Huh," I said, having limited experience putting fruit in a secondary. "How do you prevent bacterial contamination?" "Oh, easy," he said. "I just microwaved them for two minutes. Kills all the bugs." Now, I'm not sure if this is effective for killing bugs, or if there are any potential downsides to fruit-nuking, but the beer was tasty so I didn't get into it with him. Is this a standard practice that I just never encountered? Are there any reasons to NOT do this? Does it even work?
Um no. There is some cell death, but microwaving ins't a magical sterilizer that many people make it out to be. There is a certain degree of sanitation but its not enough to create a flavor stable beer. The beer itself in secondary with the lowered pH post ferment, and the presence of alcohol is whats saving the beer. Inevitably that beer may start to become drier and foamier over time indicating that its not 100% clear of microbes.
Heating fruit causes pectin which causes cloudy beer. The alcohol in the beer should be able to keep the microbes at bay. You could always use those cans of fruit if you are worried about microbes.
See, that's what I was thinking. That and the pectin haze (h/t Danimal) are enough to deter me from zapping fruit. But I could hardly argue with the results. Thanks for the sanity check.
I doubt if microwaving 4 #s of blackberries for 2 minutes in most microwave ovens is going to even heat them enough to cause pectin haze or pasteurize. Hopefully he checked the temp periodically and stirred...judging from the taste results, it sounds like he had it under control...even if it was essentially a waste of time.
