Can yeast in suspension affect gravity reading?

[5B-brewing]

Tried searching the forums for an answer to this, but no luck.

Been fermenting a wit for the past two weeks @ 68*. Went to put it in a keg tonight and took a reading. The final gravity should be somewhere around 1.010. My reading came out at 1.020, a full 10 points higher than it should be. Lots of yeast and trub in suspension in the sample. Can this affect the gravity reading?

It is drinking really well and doesn't taste too sweet at all. My wife loved it, even flat and at fermentation temperatures, so I kegged it anyways. It's for a local competition, entries are due by Thursday, so I'm really scrambling at this point. Didn't leave myself a lot of wiggle room. Hoping that is just a faulty reading and not my flaked wheat and oats not converting.

 

[DSorenson]

Nope! Only dissolved solids effect it.

 

[5B-brewing]

That's funny, because I have found a few threads since I posted this saying both yes and no.... The wit tastes good so, even though I may not do well in the comp, I'm (trying) not too worried about it.

 

[Beernik]

I disagree. Floaties can affect your hydrometer.

My most recent experience with this was pulling a sample off a freshly made peach hydromel. I puréed 13lbs of fruit for it. There were so many fruit particles in the sample, the hydrometer read 1.140. I knew it was way off. After filtering it through cheese cloth, it read 1.059, which is what I was expecting.

A little yeast won't affect the reading very much. A really yeasty/trubby sample could have a significant affect

 

[Hello]

I tend to believe trub can affect the reading as well. Not sure about yeast.

 

[DSorenson]

Well let me amend my statement. My assumption was that there was significantly more fluid than solids. I took a sample today, for example, from the trub and hop debris left over in my boil from my transfer to my fermenter. My reading was right on with my pre-boil reading and expected results. I used 14 ounces of hops and whirfloc, so that was one trub-y, hop-debris-y sample.

What beernic experienced is a case of too many solids and not enough fluid.

Think of it this way: You can put a rock in the bottom of your sample and still take a correct reading. Fill the sample tube with rocks and you cannot take a reading period, even though there is fluid in there. You will know when you have too many rocks- it will effect the behavior of objects in the fluid.

You should be able to push down on your hydrometer, let it go, and it will float up to a number. Then you also should be able to pick up the hydrometer, release it, and it will float to the same number. Otherwise you have too many solids to take an accurate reading.

I hope this will end all confusion of the subject. You would need collect your sample from the bottom of your fermenter to have enough yeast to impede the motion of the hydrometer. I assume you did not.

 

[GuldTuborg]

It can, but there would have to be an awful lot in there for it to have much of an effect. I highly doubt extra stuff in suspension like that is going to raise your gravity 10 points. How does it taste?

Any solution, colloid, or liquid with suspended particles can have its density measured with a hydrometer with the appropriate range.

 

[5B-brewing]

Not carbed yet, but sample was tasty.

I'll pull a new sample tomorrow, let it go flat and see if there is a difference.

 

[DSorenson]

There may be a difference, and in fact probably will be a difference, if you naturally carbonated.

If you force carb'ed though, you will be fine. Also keep in mind temperature is important. around 60 F is ideal for a reading.

 

[Homesteader82]

I just took a reading of 1.029 with Yeast suspended and 1.021 without Yeast suspended from the same batch at the same temperature...

 

[KiltedYaksman]

All particles that interact with a hydrometer, both dissolved and suspended, influence the reading.

For example, soil texture can be characterized using a hydrometer. After suspending a soil sample (usually about 50 g) in a volume of water (usually 1 liter) and pouring into a large, graduated cylinder, it begins to settle into three fractions: sand, silt, and clay.

In a fully suspended soil sample, the hydrometer would read 50 g suspended soil, but the sand settles too quickly to actually get this reading. After 40 seconds, all the sand has settled and the hydrometer will read something like 30 g (silt + clay) which means 20 grams of sand already settled. Over the next two hours, the silt will settle and the hydrometer will read something like 12 g of clay is still suspended which means 18 g of silt settled out.

Once a suspended particle settles and is below the hydrometer in the column of fluid, it no longer influences how high or low the hydrometer rests in the column.

Today I needed to determine if a fermentation is stuck. I drew a sample off the bottom of the conical and got 8.1 brix (1.033 SG) from a yeasty sample.



I wondered if this was a true reading because of the yeast and took a second sample by siphoning from the top. The second sample was 6.7 brix (1.027 SG); a significant drop but still a pretty decisive sign that fermentation is stuck. I'm adding a dose of fresh yeast in the hopes of dropping below 1.020 SG.

 

[dmtaylor]

Suspended solids should not impact specific gravity. If they do... it's a viscosity thing more than an actual specific gravity thing. For example, if I drop a hydrometer into grape jelly... it's probably not going to read accurately. But any reasonable liquid, even with yeast, then yes it should be accurate.

EDIT: @KiltedYaksman, I wonder if in the first photo your hydrometer might have been resting on the bottom of the cylinder? Is it possible? This could falsely read high if the hydrometer was not floating. Another likely possible contributor is that in the second reading siphoned from the top, alcohol *could* rise IF there are different layers in your fermenter, much in the same way as your example about soil samples, so I think it is possible that there could be different layers in your fermenter with different gravities, I am forgetting the word for this at the moment but it is a true physical phenomenon. Also... Welcome to the forum!

EDIT #2: Stratification! That was the word I was looking for. Might be possible, but I think it could be more likely your first reading wasn't floating? Otherwise, as a scientist, frankly I'm surprised that yeast could have this kind of impact. Maybe it really does, but... my intuition believes otherwise.