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Boil-off Rate

November 27, 2010

I learned a little something recently.  I’ve always been a little confused by why brewing software represents boil-off as a percentage.  I’ve done very little investigation into it, but what I have done hasn’t turned up much.  Recently however, a guy responded to a question on the MHTG mailing list that gave me a good explanation.

Basically, he said that boil-off is expressed as a percentage not because it will change based on volume, but that a brewer should seek a consistent boil-off percentage in order to be consistent from batch to batch.  I.e. the evaporation rate expressed as a % is not to say that increasing the volume causes the volume boiled off to increase, but that maintaining a consistent (or minimum) boil off rate is a desirable goal.

To me, this seems to make a lot of sense.  Maintaining a constant evaporation rate indicates that you’re driving off DMS, etc. at a consistent rate; isomerizing alpha acids at a consistent rate; etc.  There is however, a tradeoff between achieving consistent rates and caramelization, darkening, etc. of the wort (e.g. if you increase the heat applied to a larger batch to maintain evaporation rate, you’ll have more caramelization and darkening of the wort from the higher temperature at the bottom of the kettle).

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