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Old 01-29-2010, 07:39 PM   #31
jkarp
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Originally Posted by martinworswick View Post
yes it is but if you have a burner that can bring 10 gallons to a rolling boil at full power you would adjust its power for 3 gallons to try and get the same boil off ratio.
Nope. The heat of vaporization of water is 2260 J/g. Volume makes no difference, except for thermal loss of the system.

Think of it this way. 15% of a 10 gallon batch would be 1.5 gallons per hour. With your logic, I should be boiling off less than a half gallon per hour in my system. That's not even a simmer, let alone a boil!

% boil-off numbers are only useful for evaluating identical volume batches on your system and are not relevant for comparison to other brewers. That's why we use gal/hr instead. From memory, that Brew Strong 'cast discussed this as well.
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Old 01-29-2010, 07:48 PM   #32
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Didn't feel like reading all of the pages. Here is the definitive answer.

This is taken from the BJCP study guide Wort Production, by David Houseman and Scott Bickham

http://www.bjcp.org/study.php#exam

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Boiling

Boiling wort is normally required for the following reasons:

* Extracts, isomerizes and dissolves the hop α-acids
* Stops enzymatic activity
* Kills bacteria, fungi, and wild yeast
* Coagulates undesired proteins and polyphenols in the hot break
* Evaporates undesirable harsh hop oils, sulfur compounds, ketones, and esters.
* Promotes the formation of melanoidins and caramelizes some of the wort sugars (although this is not desirable in all styles)
* Evaporates water vapor, condensing the wort to the proper volume and gravity (this is not a primary reason, it's a side effect of the process)

A minimum of a one hour boil is usually recommended for making quality beer. When making all grain beer, a boil of 90 minutes is normal, with the bittering hops added for the last hour. One exception to boiling was historically used to brew the Berliner Weisse style. Here, the hops were added to the mash tun, and the wort is cooled after sparging and then fermented with a combination of lactobacillus from the malt and an ale yeast.

Boiling for less than one hour risks under-utilization of hop acids, so the bitterness level may be lower than expected. In addition, the head may not be as well formed due to improper extraction of isohumulones from the hops. A good rolling boil for one hour is necessary to bind hop compounds to polypeptides, forming colloids that remain in the beer and help form a good stable head. An open, rolling boil aids in the removal of undesired volatile compounds, such as some harsh hop compounds, esters, and sulfur compounds. It is important to boil wort uncovered so that these substances do not condense back into the wort.

Clarity will be also be affected by not using at least a full hour rolling boil, as there will not be a adequate hot break to remove the undesired proteins. This will also affect shelf life of the bottled beer, since the proteins will over time promote bacterial growth even in properly sanitized beer bottles. The preservative qualities of hops will also suffer greatly if the wort is not boiled for one hour, as the extraction of the needed compounds will be impaired.

Boiling wort will also lower the pH of the wort slightly. Having the proper pH to begin the boil is not normally a problem, but if it is below 5.2, protein precipitation will be retarded and carbonate salt should be used to increase the alkalinity. The pH will drop during the boil and at the conclusion should be 5.2-5.5 in order for proper cold break to form and fermentation to proceed normally. Incorrect wort pH during the boil may result in clarity or fermentation problems.

The effects of boiling on the wort should match the intended style. It is often desirable to form melanoidins which are compounds produced by heat acting on amino acids and sugars. These add a darker color and a maltier flavor to beer. When desired, an insufficient boil will not form enough melanoidins for the style. Boiling the initial runnings of high gravity wort will quickly caramelize the sugars in the wort. This is desired in Scottish ales, but would be inappropriate in light lagers.

Vigorously boiling wort uncovered will evaporate water from the wort at a rate of about one gallon per hour, depending the brewing setup. In order to create a beer with the appropriate target original gravity, changes in the wort volume must be taken into account. Longer boil times or additions of sterilized water may be required to hit the target gravity.
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Old 01-29-2010, 08:43 PM   #33
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Originally Posted by wilserbrewer View Post
Problem is the scale slides rapidly towards diminished returns for a longer boil. Kinda silly to boil away for half an hour for 75 cents worth if grain?
It doesn't seem to slide at all until well after 90.
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Old 01-29-2010, 09:41 PM   #34
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I boil off about 27% in a 5.5 gal. batch

I will boil off 15% in a 10.5 gallon batch

In 90 minutes

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