Lots of trub affecting specific gravity readings?

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?

Topic trub original-gravity priming homebrew

Category Mac

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answered at

I did the experiment suggested in the comment by Denny Conn, above, using cocoa powder (as I'm developing a recipe for a chocolate mead).

I used 3/4 c tap water and measured the SG right out of the tap and with 0.15 oz cocoa powder mixed in (same ratio as using 16 oz cocoa for a 5 gallon batch).

The readings were taken within a minute of each other, so is probably not a factor between the two readings.

Plain Tap Water: 1.002

Cocoa Slurry: 1.006

Note that the opaque nature of the cocoa slurry made an exact reading difficult, so I'd put that at ±0.002.

So, it looks like suspended particles DO impact the SG. For my purposes, I'll be adjusting my SG reading -0.004 to account for them in the initial reading.

I'll have to wait and see what the mead looks like when I take a final reading and decide if it needs adjustment as well (as most of the suspended particles will have fallen out).

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answered at

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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answered at

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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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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