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How long to boil?

  1. Jan 7, 2009 #1
    Hello. I am not quite sure this is the correct place for this question so I will apologize now if it isn't. As a home brewer I mash the malt with hot water and collect the runoff (called wort) which contains the sugars needed for fermentation. Most recipes call for a boil length of either 60 or 90 minutes with an anticipated specific gravity at the end of the boil. My question is this, if I know:

    The volume of wort collected in gallons
    The specific gravity of the wort before the boil
    The anticipated specific gravity post boil
    The temperature at which the wort boils, 212° F
    An evaporation rate of 20% per hour

    How long must the boil be to reach the anticipated specific gravity? I'm not too worried about humidity, ambient temperature, elevation, or other external factors, but knowing this could make planning a brew day much easier.

    Thank You,

    Tom Dible
  2. jcsd
  3. Jan 7, 2009 #2


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    Staff: Mentor

    Generally speaking... if you convert densities (specific gravities) to concentrations (that require density tables, like those built in my ), and if you know amount of wort, you can calculate how much water have to be removed - that combined with the evaporation rate should make estimation of time possible. But evaporation rate depends on many factors, so the result of calculations won't be too precise.
  4. Jan 7, 2009 #3
    Thanks for the information, and I will give the program a try as well as let some of the brewers I talk to know about it. I am aware that evaporation is dependent on many factors, but I'm just looking for an estimate anyway. Over the weekend I brewed and missed my ending gravity by a ton and knowing some of this information will better enable me to plan accordingly for future brews.
  5. Jan 7, 2009 #4


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    Staff: Mentor

    I wonder. Making a device that wil start an allarm once specific gravity gets close to the target value doesn't sound complicated.


    Remember to list me on the patent application.
  6. Jan 7, 2009 #5
  7. Jan 8, 2009 #6


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    Science Advisor
    Homework Helper
    Gold Member

    Go to your homebrew store and buy some dry malt extract. Make up what you want as far as concentration according to the directions (~1.1 lb per gallon of water). The reconstituted wort should weigh about 9.3 lbs and be 1.086 gallons. Add water to it in portions (record what you do!) and measure the effect on the SG. You will have a SG to concentration lookup table. Do you need help with that part?

    Assuming the loss rate is measured by volume, every 30 minutes, you lose 10% of the total volume due to water and you lose nothing of the sugars. Measure the initial SG and from your lookup table determine your water to malt ratio. Calculate how much water needs to be removed to get to the target SG and convert that into a percent loss. Do you need help with that part?

    Have fun with it. Keep in mind that as you concentrate the wort it will lose water at a slightly slower rate.
  8. Feb 13, 2009 #7
    As a brewer myself, I'd recommend getting the book Designing Great Beers, it does a great job at explaining the mashing and sparging processes including hitting the correct specific gravity.

    Don't forget water becomes denser as it cools, it reduces in volume by about 4% from 212F to 70F. So if you want a 5 gallon batch, reduce the volume by boiling to ~5.2 gallons, and as it cools it will end up close to 5 gallons.

    I believe Designing Great Beers uses 5%/hr lost due to evaporation (don't have the book with me), but I've thought that would vary based on surface area of the boil, wider brew kettles lose more faster versus narrow kettles, but have no data to support that.

    Not all boil sizes are 5 gallons or even of the same volume. I do not have a very large brew kettle so I usually do boil sizes of 2.5 to 3 gallons. But using the methods provided in that book, you can calculate how much DME or grains needed, and during the boil in my case the SG is considerably higher due to the smaller volume. Once cooled, the wort is transferred into my carboy in which I bring the volume up to 5 gallons, bringing the SG to the expected value, or close to it.

    The last two times I have brewed this way I have come quite close to getting the SG I wanted. One batch the target was 1.035 I got 1.043, less then 1% off I think. Next one target was 1.080, got 1.093, again ~1.3% off. I think why I was even off is due to extracting more sugar from the grains then expected. The book gives approximate range for each type of grain, and as I get higher gravities, the grains I was using were probably a bit on the higher end and thus extracted more sugars. Not going to complain about the SG being higher, just means higher potential alc%. The highest SG I've seen in an ale recipe is 1.123, and I've done meads that high too.

    Welcome to the world of homebrewing, its both an art and a science. If you have other questions, you can PM me. Don't get too wrap up in hitting an exact specific gravity. Relax, have a homebrew...
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