Lids on Kettles & Evaporation

Brewing processes and methods. How to brew using extract, partial or all-grain. Tips and tricks.

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Lids on Kettles & Evaporation

Postby Mesa Maltworks » Tue Oct 30, 2001 11:57 am

I've recently followed a bunch of threads on kettles and wanted to interject an oft neglected topic regarding boiling... evaporation.

In the probrew world, you are supposed to reach an evaporation rate of 8~10% of the pre-boil volume. There are many reasons for this, but most important among them is volatile compound scrubbing. The reduction of wort volume by this amount ensures that nasties like DMS (corn/vegetal like flavors) and diacetyl precursors are "boiled off" so they do not entrain in the resulting wort. Now... my point relating to kettles... too many home brewers either leave the lid on the pot of "cock" it partially open when boiling. The first scenario does not allow for any volatile scrubbing and the second allows the volatiles to re-condense and run back into the wort. The second scenario was how the characteristic flavor of Latrobe Brewing's Rolling Rock was created. Up until the mid 70's they had a defective drip ring in their kettle stack that let the condensate run back into the boil. It resulted in their now famous DMS flavored brew, most associate it with corn flavor. In the late 70's they replaced this kettle which eliminated the "flaw". The consumers noticed the difference and did not receive the change well... so, Latrobe re-engineered the beer to contain a detectible amount of DMS and retained their following ! Here is where a "flaw" is not actually a flaw !!! The same can be said of beers that are are allowed to contain detectable levels of diacetyl such as most ESBs and even Pilsner Urquell. So, give it a whirl. You may not notice the difference without comparing, but if you make sure your evaporation rate is in range and that you are not re-entraining volatiles in your wort, the result will be a cleaner brew. The difference, of course, is most dramatic in light beers, but remains detectable even in some darker styles.
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