Disgusting, inexplicable DMS-like flavor in 3 batches!

My last two batches, tasted both during fermentation and after fermentation completed, were totally undrinkable. I had to even spit out the tasting samples, they were so nasty. They tasted like cooked, rotting corn chowder. So my intial thought was that somehow, despite extremely rigorous sanitation (both heat AND Star San sanitization of everything that the wort touched post-boil), the batches had become contaminated with a coliform infection, which does produce massive amounts of DMS. Both batches were all-grain Bohemian Pilsners, pitched with a starter using 4 Activator Packs of Wyeast Urquell Lager. So, not knowing where the infection could have possibly come from, I decided to brew an extract batch and to vary some aspects of the process. For instance, instead of using my whirlpool tank and immersion chiller, I used a plate chiller and whilrpooled in the boil kettle, to reduce the wort's exposure to outside equipment.

So this time I tasted the wort before even pitching the yeast. It already had this disgusting, strong corn-like character. In other words the wort has exactly the same off-flavor as the previous two batches (which used completely different ingredients, being all-grain). Now I know that the taste of wort doesn't mean anything, since it completely changes during fermentation; but now that it has finished fermenting, it still has the same character. So clearly the off-flavor was not due to biological contamination, since it was there immediately after cooling.

And I can't believe that this flavor is DMS. In the previous two batches, I boiled for 90 minutes with 15 - 17% evaporation per hour and cooled to 48 degrees F within 40 minutes. For the current batch, which was extract-based, I boiled for 60 minutes with 15% evaporation and cooled to 46 F in 45 minutes. The boil was NOT covered, of course. In all three batches I whirlpooled and left a large trub cake in the kettle or whirlpool tank. Yet, the wort was still extremely cloudy.

What the heck could be going on here? How is it possible to mess a beer up so completely before you even pitch the yeast? And how can three batches end up the same using such different ingredients (two all-grain vs. one extract)? And what in the world am I tasting if not DMS? If it is DMS, how could it possibly be so strong when I have done everything that you are supposed to do to minimize it.

Even if you have an off-the-wall guess, I would appreciate the input, because I am completely stumped. My best guess at this point is that somehow I am getting trub throughly mixed in with the wort, in such a way that it is not settling out, and that this is what is giving the beer such a horrendous flavor. But that doesn't make sense to me because, in a previous batch (an Imperial Stout) I intentionally pitched the entire contents of the BK (i.e. all the trub included) into the fermenter and it turned out fine (though it had a limited shelf life).

Two things I would check out:

1) Has your water supply changed recently? I know that some areas use seasonal water supplies (ie. River during the rainy season and water table during the dry.) It may be possible that one of the water sources for your region has way too much sulfate in the water to be used.

2) What kind of hops are you using, and how fresh are they? You may have gotten into a batch of Cascade with a distinct hint of incontinent cat.

Have you ever tasted the wort pre-boil? What is your kettle made of? Pilsner malt has a lot of DMS, are you chilling immediately when the boil is done?

First thing that came to my mind was water also. Is this the only brew that's turned out this way or have others you've brewed turned out fine? Sounds like you've been doing it awhile so I'd guess you'd checked to see if your hops have oxidized and causing cheese-like off flavors.

Thanks for the suggestions, guys. I really can't think of any problem with the ingredients.

As for water, I am using 7 parts distilled to 1 part spring water (straight out of the container). This is what I have always used. Total water was 7.5 gallons, pre-boil. Boiled down to 6.25 gallons in a little over an hour.

The only additions to the water were 13 grams 5.2 mash stabilizer pre-boil (to ensure that the pH would be correct, hoping to ensure good trub conglomeration), 1/2 tsp. Wyeast yeast nutrient at 15 minutes remaining, and 2 whirlfloc tabs at 10 minutes remaining in the boil.

The hops were Czech Saaz pellets (total of 4 oz: 2 oz. bittering, 1.5 oz. flavor, 0.5 at end of boil) ordered directly from Midwest Supplies two days before brewing, so very likely to be fresh. One 1 lb. bag of DME was maybe 6 months old, the rest of the extract (1 lb. DME and 6 lbs. Briess Pilsner LME) was just ordered from Midwest.

Prior to the last three batches, I have not had anything similar to this flavor in my beer, but my equipment does keep changing. Prior to the last 4 batches I was not using a March pump, used stainless steel kettles on my stove-stop, and mashed in an igloo cooler. In the last year I have built an entirely new system employing natural gas Hurricane burners, 15 gallon aluminum kettles, two march pumps, a plate chiller, temperature controllers, etc. But in this particular case, I was only using my BK and my march pump to recirculate through the plate chiller during cooling. In the previous two batches that had the same off-flavor, I chilled using a 50 foot 1/2" diameter immersion chiller and then drained into a whilrpool tank to clarify. If the last two batches hadn't also been bad, I would have suspected the pump of perhaps negatively affecting the wort (maybe creating shear forces on the trub that caused it to break up and cloud the wort). But Mr. Malty uses a variation of the same technique (recirculating wort using a pump and immersion chiller), so I don't think the concept is unsound.

Yeah, I am chilling as soon as the heat shuts off. Basically the wort is already recirculating during the last 20 minutes of the boil to ensure that the tubing/pump are heat sterilized, and then as soon as I shut off the heat, I turn on a pump that runs ice water through the plate chiller, thus cooling the wort below 150 within a few minutes. It only takes 40 - 45 minutes to get all the way down to 46 F with this method, so chilling is very efficient.

Yeah, I have tasted the pre-boil wort before and it definitely didn't have this very distinct corn-like taste to it. In the previous two all-grain batches, I only tasted the wort during and after fermentation. The flavor didn't go away even after a month of conditioning.

Well that rules out pretty much anything easy.

The only suggestion I can make now is to do a dry run with no ingredients, just water. Taste the water at every stage to see when the flavor arrives.

If it never shows up, then it's narrowed down to something in your ingredients.

13 grams of the 5.2 buffer. That seems about "right". I don't use it anymore, since I'm adjusting the water through other additions.

Instead of using distilled water and adding the buffer, why not try just one batch with 100% bottled spring water, and see if that fixes it?