Dammit. Dammit, dammit, dammit.
Our most recent brew was a really aggressive IIPA using all New Zealand hops and racked onto kiwi fruit that we’d dubbed Kiwi Kaleidoscope. We did everything right on this beer. Mash temps, CO2 purges, oxygenating, finely controlled fermentation temperature, pasteurized kiwi, CO2 purge before racking onto pasteurized kiwi, and even made a super-awesome label. We nailed this. So why the hell does it taste like soy sauce? Well, I don’t know for sure. But I’ve done some research and there are a few possible culprits.
Kiwi Kaleidoscope, we had such high hopes for you
Oxidation due to hot side aeration. One expected possible cause of soy sauce taste in homebrew is oxidation due to hot side aeration (HSA). HSA can be caused from splashing wort during mashing, lautering or cooling post-boil. There are many people that believe HSA to be a myth, while others say it’s a very real issue that affects certain malts more than others and greatly decreases the shelf life on beer, especially heavily hopped beers – and ours was 126 IBU. I’ll include some additional links at the bottom of this post on discussions of HSA.
Yeast Autolysis. RIP yeast. Autolysis happens when dead yeast cells rupture and release off flavors. Whether or not this is a legitimate concern for homebrewers is debated as one cause on a commercial scale is the weight of a large volume of wort compacting the yeast cake on the bottom of the fermenter. When homebrewing, it is unlikely that the weight applied in a typical 5-10 gallon batch would be enough to cause the cells to rupture, even after a long period in the fermenter. It is believed that you can avoid autolysis in homebrew by using healthy yeast in a well-prepared wort. Some people advocate for racking to a secondary fermentation in order to minimize the risk, however the necessity or racking to secondary versus the risks of exposure when transferring are another debated topic in homebrewing today.
Although autolysis seems unlikely in most homebrew applications, our beer had another factor that may have contributed to possible autolysis and off flavors…
Protease enzymes in kiwi (Actinidain) breaking down proteins causing autolysis. Let me preface this section by saying I know very little about the chemistry of how this works. The info I’m sharing here is from talking with others, Googling, and a discussion of this issue I posted on reddit.com/r/homebrewing. FYI… r/homebrewing is a great forum for homebrewers of all levels, check it out.
Kiwi contains the protease enzyme Acitinidain, which works in the same way as the protease Bromelain, both of these are used in meat tenderizers and found in kiwi, banana, pineapple, mango and papaya. It is possible that these protease enzymes broke down the proteins in the wort/beer and caused autolysis and the unfortunate soy sauce off flavor in our beer. Protease enzymes should be denatured at pasteurization temperatures, but it would likely take longer to denature the enzymes than the shortest pasteurization time. I did find a few other recipes that used the kiwi towards the end of the boil or at flameout, perhaps these methods would denature the enzymes at avoid any issues during fermentation.
Unfortunately none of these items are 100% guaranteed to be the issue. However I’ll take the info I’ve learned here and try avoid making these mistakes in the future. We might even try Kiwi Kaleidoscope again and see if we can beat whatever it is that got us this time around.