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I use an electric burner when I home brew. It doesn't have a ton of excess power but it can get a nice rolling boil going. Occasionally, when I add malt extract to my kettle the temperature drops several degrees below boiling. It sometimes takes 5 or 10 minutes to return to an actual boil. In this case, should I pause my timer, wait for the wort to return to boil and then resume the timer, or should I just keep the timer running? My guess is that it doesn't make much difference if the temp is 205°F or 212°F so I should just keep the timer running.

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up vote 8 down vote accepted

There are several things to consider here. Certainly slowing down your boil will change your rate of evaporation, but that's only a problem if you're having a hard time hitting your target volumes. The main consideration is your bitterness contribution from hops.

Alpha acid isomerization, like most chemical reactions, is temperature dependent. It happens at a faster rate in hotter temperatures and at a slower rate in cooler temperatures. In the period of time during which your temperature is below boiling, the alpha acids from your hops are converting at a slower rate than they would be at a full boil. The relevant question is how much slower.

As a very, very crude rule of thumb, chemical reaction rates tend to increase by a factor of 2 for every 10ºC increase in temperature. According to this article, the generalization roughly holds up for hops use as well. If you are losing 7ºF (~4ºC) and then bringing it back up over the course of 10 minutes, those 10 minutes are acting more like 8.5 minutes in your hop schedule.

As you can see, the difference is not huge. For a 60 minute addition, it's negligible. If, however, you're planning to get a significant amount of your bitterness from large late additions (i.e., skipping the bitterness additions and "hop bursting" with several ounces of hops with ten minutes left in the boil), that minute and a half could make a more significant difference.

In reality, this is just another property of your system, and every system will produce each recipe differently. Unless you're trying to calibrate your process to produce pitch-perfect clones, it's not worth worrying about this kind of stuff. If I were in your shoes, I'd just let the timer ride and then tweak recipes over time to suit my tastes. Good luck!

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That was extremely helpful. Thanks for taking the time to provide such a thorough answer. –  PICyourBrain Mar 30 '13 at 14:45
    
Glad it helped! –  MalFet Mar 30 '13 at 14:54
    
Top answer, and nice to discover that isomerization happens below boiling! I could only read the abstract - how did you get double reaction rate / 10C from the figures given? –  mdma Mar 30 '13 at 17:44
    
Yeah, unfortunately the data is behind the paywall. On page 4437 of the article itself, there's a table that calculates isomerization rates, and the 90ºC rate is nearly exactly half the 100ºC rate. They don't provide data below that, though, and I've long wondered what happens at, say, 70ºC. For many reactions, there's a threshold effect below which rates of reaction are essentially zero. I'm not sure where that happens for alpha isomerization. –  MalFet Mar 30 '13 at 20:10
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