The symptoms: most of my beers developed distinct flavor notes of oxidative deterioration after a few months. The good news was that few of my batches lasted that long! But I definitely had a stability problem, although not a horrible one. My thinking was that, since the only time anything warm got splashed was between the end of the mash period and the beginning of the boil, the problem must be in there somewhere.
My previous procedure: After stovetop + oven mashing (described briefly a few days ago), I would simply dump the goods into my plastic lauter tun (retired fermenter; has slotted-copper manifold). Ker-splash! Then I would recirculate by collecting the initial runnings via a short length of tubing (no splashing there at all), and return to the grain bed by pouring from a height of a 30-40 cm (couple of feet) onto an inverted coffee-cup saucer. A bit of splashing there but not too much. Recirc in my setup typically requires only 4 or 5 litres (close enough for goverment work to 4 or 5 qts), after which runnings go into the boiler again through the short length of hose (no splashing). Sparge water added the same way, poured from the same height, but of course by the time one is adding water it doesn't matter if the water splashes (does it?).
Suggestions I got: The unanimous one was don't dump the goods but instead transfer gently with some sort of scooping apparatus; better still, change setups to mash in the same vessel as I sparge in. Minimize the amount of pouring of recirculated wort as well. Norm Pyle reckoned that water vapor dominates the makeup of the air immediately above the grain bed, since it is sitting there steaming, so that the splashing of the returned wort for recirculation should not be a very big source of oxidation.
Actions I took: Since getting the suggestions I have done 8 batches. Three of these are still in fermenters, so we have 5 batches to work with, and of those, three are > 2 months old, and two are about 6 weeks in the bottle. Recall that I was noticing the oxidation notes by the 2-month mark. The first batch I did was mashed in my plastic lauter tun, heaped with blankets for insulation, but it failed dismally. Temperatures stayed near 60 C (140 F) despite repeated infusions of boiling water, and the resulting beer (an alt) was thin (FG 1008) and astringent--but it did not show any of the oxidation problems as far as I could tell. The rest were all done like this: after the mash period, I very gently transferred the goods to the lauter tun with a pitcher. After the first couple scoops, the depth in the lauter tun was such that I could submerge the lip of the pitcher and literally lay the scoop o'mash down with virtually no motion at all (thanks to Ken Willing for that one). Recirc went pretty much as before, i.e. I am counting on the steaminess of the air above the grain bed. Everything else is the same--runnings go through the hose with no splashing.
Results: I am happy to report that this seems to have done the trick. The beers I have made thusly so far are the alt mentioned above, a Kolsch-like golden ale, another alt, a steam beer, and an oatmeal stout (in the bottles); and a US pale ale, a German pils, and a Scottish ale (still fermenting). Thus I have covered a good range of color, gravity, strength, and hop levels, and as far as I can tell my oxidation notes have been greatly diminished. I am pleased.
Bottom line: do whatever you can to not splash the mash, and if you can keep from splashing the recirculating wort, so much the better. I'll report again in another few months on these beers' longer-term stability.
Cheers,
Dave in Sydney,
david.draper@mq.edu.au