I recently attended a fascinating lecture from a White Labs scientist. I was drinking, so I don’t remember everything she said. But I do remember one thing very clearly: sometimes under-pitching yeast makes sense. Under-pitching, she said, creates more yeast flavors and aromas. Really? Huh. The yeast get stressed out, they work harder, and they create more flavors and aromas. Which could be good or bad. For certain styles, commercial brewers do this on purpose. Over-pitching, on the other hand, does the opposite: less flavors from the yeast, less aromas from the yeast. But it gets the job done, and faster.
Okay. Who cares? Well, some beers are defined and dominated by the yeast flavors and aromas, like the banana flavored Hefeweizen. The White Labs scientist specifically referred to Hefeweizen several times, but I assume this would apply to my beloved Belgian Tripel just as well, which also tastes like banana. Needless to say, I will not be over-pitching my Belgian Tripel from now on (which is what I was doing).
For homebrewers that reuse yeast (like me), we usually over-pitch all our beers by throwing a jar full of yeast slurry or whatever. After all, we can’t count cells. And in fact, I think this is a good practice for most beers. In fact, brulsophy has shown that it doesn’t make a different in a wheat amber ale and in a Double IPA and in a lager.
Even if you think under-pitching is a good idea, it’s not for everyone. You could run the risk of a stuck, or stalled, fermentation. That’s bad; that means your beer stopped fermenting and you need to somehow revive it. I don’t worry too much about that because I have a temperature controlled fermentation chamber, which keep the yeast maximally happy. So I don’t think they will crap out. I’m not fermenting in a cold, 50 degree basement.