Questions and Answers

Q: What does the specific gravity of the wort measure, and how do I get an accurate reading?

A: The specific gravity measures the density of the wort. The density of the wort is proportional to the amount of disolved sugars in your wort and is indirectly proporional to the amount of water used. Their are, therefore, only two ways to adjust the gravity, adjust the amount of water used or adjust the sugar content of the wort.

To measure the gravity of your wort, you will need to use a hydrometer, an inexpensive insturment that you can get for about 10 dollars US. Take a reading by floating the hydrometer in a sample of wort; the graduations on the hydrometer will indicate the gravity. Hydrometers are normally accurate at 60F; if you take a gravity reading at any other temperture, you will need to add a correction factor to the reading. These correction factors should be provided with your hydrometer by the manufacturer. When reading the gravity, be sure to take a reading at eye level and take the number at the very top of the meniscus. You can read a bit more about hydrometers here.

Q: What can I do to hit my target gravity?

A: By knowing the appropriate amount of water to add to the boiled wort, you can hit your target gravity very closely. Ray Daniels dedicates an entire chapter of his book Designing Great Beers to this topic.

The first step in hitting your specific gravity is to take a hydrometer reading and volume reading as the boil comes to a finish. Taken at this time gives you option to make adjustments before it is too late. First off, take a sample of the wort and let it cool down. The closer to 60 degrees the wort temperature is, the more accurate your reading will be. Most hydrometers manufacturers only provide temperature correction values for a limited range. At a minimum, you need to let your wort cool until it falls within that range. Next, measure the volume of your boiled wort (use whatever method suits you).

After you have a volume and gravity reading, you can calculate the total gravity of your wort. This value tells you the total obtainable sugar content ( and gravity ) of your finished wort. Use equation (1) below to calculate the total gravity.

1) TG = Specific gravity of sample * V(boil) where TG is the total gravity and V(boil) is the volume of boiled wort.

At this point, you have added all of your extracts and the sugar content of the wort is constant. Under these conditions, the TG is constant and the follwing equation (2) always holds true.

2) TG * V(boil) = TG * V(ferm) where V(ferm): wort volume in the fermenter

To determine the expected original gravity of your wort in the fermenter, divide the total gravity by the desired volume of beer.

3a) OG = TG / V(ferm) where OG is the desired original gravity of the wort.

By adjusting the volume of the wort in the fermenter, you can increase or decrease your original gravity to hit your target. If your calculated original gravity falls below your target, you will need to use less water. For the extract brewer, adjusting the volume of the wort is most conviently accomplished when topping off your wort after transfering it into your primary fermenter. Rearranging equation (3a) gives you this equation which can be used to determine the final volume of wort that will hit the target gravity.

3b) V(ferm) = TG/ OG
Finally, the amount of water to add to the fermenter to hit your target gravity is the difference between the volume of wort in the boil and the required volume in the fermenter. This is shown in equation 4.

4) V(add) = V(ferm) – V(boil ) = TG / OG – V(boil) where V(add) is the volume of water in gal added to the wort to “top off ” the fermenter

Example:

If you are brewing up a batch of beer that has and indented original gravity of 1.042 and have boiled 3 gallon of wort ( with all extracts and extras added ) with a sample gravity reading of 1.278, how much water do you need to add to the wort in the fermenter to hit your target?
boiled wort volume: 3 gallons
boiled wort sample gravity: 1.278
target gravity: 1.042

TG = 1.278 * 3 gal = 3.834
V(ferm) = TG / OG = 3.834 / 1.042 = 3.679 gallons
V(add) = V(ferm) – V(boil) = 3.679 gal – 3 gal = .679 gal addition

This means that you need to top off your primary fermenter to 3.679 gallons or add .679 gallons to top off your wort. If you were brewing a 5 gallon batch, you might be pretty upset by the final volume of this beer. It is important to remember, however, that as homebrewers we seek to brew high quality beer, the volume produced comes only secondary. Hope this helps. Let me know you results.

Q: What does yeast flocculation measure?

A: Flocculation is the tendency for yeast to clump up and then settle out during fermentation. Yeast with a high degree of flocculation will settle out to a greater degree and leave a more clarified beer. Yeast with a low degree of flucculation do not settle out as redily and produce beer haze. For Belgium Wit Beers, you want a cloudy beer and should use a yeast with medium to low flocculation.

Q: I am seeing bubbling in the secondary fermenter. Is this Ok?

A: Yes, In all the brews that I have made, I have seen some activity in the secondary. Assuming that you waited until after primary fermenation had completed, this secondary activity is caused by yeast that have been reinvigorated while racking from your primary. If you are seeing more than a bubble every minute or two, it is likely that you should have kept your beer in your primary for a couple of days longer. Either way, this activity should subside in a day or two.

Q: How long should I boil the wort?

A: Check out this post.

Q: How many brews can I expect from a single propane tank?

A: From a standard propane tank ( one that you can exchange at the grocery store–1o lbs I think) you can expect to get about four or maybe five 90 minute brewing sessions. You will need to keep the gas flow closely regulated. Be sure to boil in a safe location that is free some a lot of wind; that will really rob you of a lot of heat. When brewing in the winter, set up shop in the garage with a least one door completely open. If you boil off more than about 15% an hour, your heat is way too high ( you don’t want to scorch your beer anyway)

Q:  How long do I have to stir the wort when boiling?

 A:  You need to activly stir your wort whenever you add anything to the brew pot.  This is especially true when adding extracts to the boil.   After the boil is established, there will be a natural convection in the pot that will do most of the stirring for you. It is still a good idea to stir it now and again though.  The more vigoursly you boil, the more you will need to stir.

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10 Comments

  1. panoramix said,

    April 10, 2007 at 3:16 pm

    hello, i’m a french home brewer, and i must reconize that you’ve a very and nice blog!
    i would like to know with what kind of water do you use to top up your batch to adjust the gravity? i think you must have a tap water preboil to take off the chlorine?

  2. brewbaron said,

    April 10, 2007 at 6:41 pm

    Depending on the quality of water in your area, boiling your tap water may or may not be necessary. I like the taste of the tap water in my area and have chosen to skip the boiling step; I just top it off directly from the tap.

    If your tap water is not pleasant tasting, I would suggest that you boil your tap water. Another option: You could also freeze bottled water and drop the ice into the wort directly after the boil. It will add volume and quickly cool your wort to pitching temperatures.

  3. madan said,

    November 6, 2007 at 8:14 am

    hello this is madhan…im doing a presentation on wort…i would like to know why the sugar content in wort is 5% instead of 1%

  4. madan said,

    November 6, 2007 at 8:15 am

    wht sugar content is present in wort

  5. madan said,

    November 6, 2007 at 8:16 am

    why sugar content is present in wort?

  6. brewbaron said,

    February 27, 2008 at 8:54 am

    panoramix, boiling the water that you top off your fermenter is always a safe way to go, but it is not required if you have a good local water supply. The chlorine content of your tap water is generally low enough as to not affect the flavor of your beer or significantly affect the yeast. I personally do not bother with the preboil step.

  7. Piexylixeld said,

    October 19, 2008 at 4:24 am

    How are you ?
    The interesting name of a site – brewbaron.wordpress.com
    I at night 2 hours
    has spent to the Internet So I have found your site 🙂
    The interesting site but does not suffice several sections!
    However this section is very necessary!
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  8. hydrometer said,

    July 10, 2009 at 8:23 am

    very informational!

  9. May 14, 2011 at 2:11 am

    Thank you for a great post.

  10. tim said,

    March 25, 2012 at 2:40 pm

    Thanks for the very specific information on the measurement and adjustment of the wort to hit the sweet spot of the desired starting s.g.

    I find that my extract beers do not have a very pronounced ‘mouth feel’ to them. I am referring to the way the beer stays a bit longer for the aftertaste.

    Here is the question: Does specific gravity affect the mouth feel of the beer?


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