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I'm in the process of making a pretty big pumpkin beer, and just transferred to secondary. There was an incredible amount of trub -- probably 1.5 gallons out of a 5 gallon batch. I guess it was a combination of a lot of malt and a lot of pumpkin. The beer had been in primary for about 10 days and so it was pretty well settled, and I think I got all the usable beer out of it I could.

Aside from being a little sad that my yield is down to 3.5 gallons, I have a couple concerns:

  • Is the large amount of stuff that settled out going to affect my ABV calculations? I.e., was all that stuff in suspension originally, and artificially inflated my OG reading (1.072)?
  • Should I reduce the amount of priming sugar I would typically, to avoid over-carbonation?
  • Is it common to lose so much volume to trub, and is there anything I should do about it in the future?

EDIT: So there are two answers, one saying that suspended solids do not change specific gravity readings, one saying that they do. There must be a correct answer to this question, right?

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I think I will experiment a bit with this when my next beer is finished. Take a sample without trub, measure SG, add some trub from the bottom and measure again. Might even try different levels of trub. :) –  Max Oct 9 '11 at 11:09
    
Suspended particles do (what I wrote) and non-suspended particles don't (what Denny wrote). The key is the use of the word trub that normally refers to unsuspended particles. Adding matter to a volume of a liquid increases its density and therefore its specific gravity. So the issue as I thought you were asking is not if the trub effected your FG (what Denny is referring to) but would what becomes trub effect your SG. If this is what you were indeed asking, the answer is yes it can. –  drj Oct 13 '11 at 16:11

3 Answers 3

Large amounts of suspended particles will effect SG readings, but it is hard to say by how much. In the future, take a sample, clarify it with some gelatin, then take the reading. The more accurate readings come from the most clarified solutions. Chances are that you are correct in assuming that the OG is off. Using a refractometer to determine the ABV on the fermented brew will give you a more accurate reading at this point. I'd take a sample to a local homebrew supply that has these and see if they will do a reading for you with a refractometer.

But trub won't effect the readings, because it is not suspended as I mentioned above.

The priming sugar is probably not an issue if you can get an accurate FG reading as described above and prime towards the lower end of the CO2 volume range. If you are really worried about it, how about only priming a sample, bottling that sample, and leave it in a relatively warm place (70oF in a closed bucket - in case of bottle bombing) for a week or two, then uncapping the sample bottle to test the carbonation? Then you can adjust the sugar amount if needed by using the tastybrew priming calculator ( http://www.tastybrew.com/calculators/priming.html ).

Loss due to trub ... I'm thinking that clarifying before pitching for primary fermentation would certainly yield a more clear wort to ferment (and better OG readings), but you may lose some flavor. How much is hard to say. Was the trub dense or "fluffy"? If it was pretty dense, then I don't think that there is much you can do.

I do know that when I have done fruit/pumpkin brews, I got a lot more trub, but always assumed that this was normal. But I didn't get quite the volume loss as you describe here (mine tended to be less than a gallon, estimating from vision).

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Undissolved solids, like trub, will not affect your gravity readings. Hydrometers and refractometers only measure what's dissolved in the liquid. Imagine if you had a swimming pool full of poodles. Would the density of the water increase? No.

As to priming sugar, all you have to do is calculate the amount based on the amount of beer you have. For instance, if you normally use 5 oz. of sugar for 5 gal., you would want to use 3.5 oz. for the amount you have now.

The amount of loss to trub will vary depending on your recipe and the ingredients you use.

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The poodles analogy is amusing, but doesn't prove anything. What if the pool were packed full of poodles and you tried to jump in? Presumably all the poodles would buoy you up a bit, the same way a denser liquid would. –  Henry Jackson Oct 9 '11 at 1:14
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Try this...put some water in your hydrometer and take a reading. Put some sand in the hydrometer, fill it the rest of the way with water, then take another reading. It will be the same unless you put in so much sand that the hydrometer rests on it. But that just means you need a bigger sample flask, not that the density of the water has changed. That's basically the response that Patrick Rue, owner of The Bruery, gives in the AHA's "Ask the Experts" section..." –  Denny Conn Oct 9 '11 at 16:05
    
He says "The sediment shouldn't have any impact on a gravity reading, unless your hydrometer was sitting directly on top of the sediment, or perhaps if the sediment was floating around, it could have some impact if it was helping to lift the hydrometer. If you're concerned about sediment that is at the bottom of the sample tube, Try putting a half inch of sand or pebbles in a hydrometer tube and fill it up with distilled water. You should get the same reading (1.000 hopefully) as you get without that material in the tube." –  Denny Conn Oct 9 '11 at 16:06
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The poodle analogy is poor if you take to the extreme that they float and hold you up, but the concept is fine. If you put a bunch of pebbles in the pool they have no bearing on the density or water. The trub has the same effect on the hydrometer, the density of the wort is only increase when stuff dissolves in it. You do need to let it settle out before you take a reading. As it can create nucleation sites where CO2 bubbles from holding up the hydrometer. –  brewchez Oct 9 '11 at 20:52
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If you go back to Henry's original question, he expressed concern that the originally dissolved stuff that settled out as trub may have effected his OG. The clear answer is yes it may have, if it was indeed suspended during his original OG measurement. He didn't ask about the effect after it settled out (see his "I.e."). –  drj Oct 10 '11 at 2:08

Specific gravity is density standardized to the density of water. Density is the measure of weight per unit volume. If density increases, SG increases.

Since the dissolved solids settle, they are apparently higher density than the fluid. A unit of the fluid containing 30% solids will have a density of (0.3)D[pulp]*(0.7)D[fluid]. Since D[pulp] is greater than D[fluid], the reading will be higher.

This is the most likely case. It is possible that as the ferment occurred, D[fluid] decreased to less than D[pulp], causing the pulp to settle. In that case, you can't tell.

Either way, the difference is probably not great. Since it takes a long time for the solids to settle, they can be assumed to be close to the density of the fluid.

So the reading was probably effected, but not by much, and the beer will likely still taste good. Enjoy:)

See http://www.epa.gov/eogapti1/bces/module4/gravity/txt_answer/t_answer.htm#2tt, question 2

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