Clearly this is a thing one wants to avoid. And to avoid it at home is pretty simple. Mostly this just entails: 1.) not introducing copious amounts of air while mashing and, 2.) not introducing air while collecting wort. These things are pretty easy to accomplish just by having good techniques.
While I was aware of HSA when I first started brewing all-grain, I wasn't really cognizant of how it works or whether or not I should be worrying about it too much. My first attempt at mashing was for a partial-mash APA. I mashed in my kettle on the stovetop. Holding temperatues constant wasn't very easy and at some point I think I even bumped the temp over 165F. Uh, less than professional. I then did the following to "sparge." I removed most of the grains from my kettle, put them in a strainer that was suspended at the lip of a plastic fermenter, then dumped the liquid content through the grains. As you might expect, it was fairly hazy (as I didn't yet even know the word vorlauf) and efficiency was super low. I was unaware of the importance of these things at the time, and continued on my way by adding in some DME and boiling hops, etc.
My technique was not very good. In fact, it was just...bad. But hey, I didn't know at the time and the beer tasted quite nice. No worries.
Fast forward to my first straight-up all grain brew. Still aware of HSA in the back of my head, but not enough to keep me from splashing some hot wort around, I ended up mashing in a rectangular cooler with an improvised manifold concocted by a housemate. I let my hot wort come running out out full speed into my brew kettle about 3 feet from the outlet on the cooler to the bottom of my kettle. At least this time I vorlaufed and had something closer to a legitimate sparge. Later I boiled, cooled, fermented, packaged and lo and behold, no off-flavors indicating that something had gone awry. Not too shabby.
I will readily admit that this is still my process. I batch sparge, I don't regulate flow, and I splash. As much as I'd like to be, I'm not a by-the-books brewer where these areas are concerned. But hey, I'm working on it and the beers are tasting just fine. Trust me, I'm working my way there.
But I couldn't leave well enough alone. I had to know what the deal was with HSA. If I'm breaking several rules, why am I not seeing the results in the finished product?
Enter more confusion. I recently acquired Dr. George Fix's Principles of Brewing Science, 2nd Edition and Greg Noonan's New Brewing Lager Beer. Woah. There is so much going on in beer that I'm willing to say there's just not enough time to fully understand it all without a lifelong dedication to it. (Also, it really helps if your chemistry background is solid.)
What I learned from these texts is that splashing your hot wort around can be bad for a number of reasons. One in particular has to do with an enzyme called lipoxygenase. According to John Palmer in a Basic Brewing Radio interview (start listening around the 7:46 mark), lipoxygenase binds oxygen to lipids and melanoidins to form complexes that will eventually impart stale off-flavors like those mentioned above. Good to know. However, he also mentions that lipoxygenase is denatured at 60C (140 F). If that's the case, where's the worry for brewers doing a single infusion at normal temperatures (i.e. 140F-158F)?
Tentatively it appears as though we can just Relax. Don't Worry. Have A Homebrew.
Of course, there is more to do with HSA than just one teensy tiny enzyme*. This will be discussed as I continue to learn more about the various mechanisms involved. Expect more on this topic soon - it's a good 'un!
*According to Fix, Trans-2-Nonenal, the compound principally responsible for the paper/cardboard-like flavors associated with staling, has a threshold of 0.1 parts per billion. Yes. Billion. That's some sort of intensely pungent compound, no?