Hi I like the process your using. My question is are you using 10% Melanoiden Malt and doing a Hockhurz Mash? When I do a decoction I normally skip the Melanoiden and rely on the decoction for flavor.Here's my recipe. I've won a couple medals with it and I'm happy with it.
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The biggest tip I can offer you is that this beer is just as much about process as it is about ingredients. Because it is a malty beer, everyone focuses on selecting malts to get the flavors they want, but the single most important thing in a Marzen (or any German style really), is the balance. It needs to smell malty, initially taste like its going to be rich and filling, and then finish crisp and dry and leave you wanting more.
So to achieve that you need to have a very fermentable wort, and I like the hops to be right up against the limit for the style. I use a modified Hockhurz mash schedule, starting at 143, then I pull a decoction with a kitchen strainer (basically all grain and almost no liquid). Boil it for 10-20 minutes, and add it back to the main mash to hit 156. Then I ramp the temp up to 168 for a mashout and sparge. I have also baked the decoction in the oven for 30 minutes at 400 to add some really nice bready flavors.
Another thing is yeast. I like WLP833, WLP820, and WY2633. I also like a BIG starter, like 3L. I oxygenate my wort with pure oxygen, and use servomyces yeast nutrient. I ferment at 50 for about 7 days, and then ramp the temperature by 2 degrees per day for a diacetyl rest at 65, and then cold crash to 38 for a few days before kegging. Keep oxygen on the cold side at a minimum, it's the enemy of good malt flavor. So closed transfers, liquid purged kegs, etc.
Hope that helps!
Thanks
Tom