# Effect Of Suspended Yeast On Gravity Reading?



## eamonnfoley (22/1/10)

May seem like a silly question, but if you take a sample of beer that has a heavy concentration of yeast in it, will the hydrometer read substancially higher than a clear sample? The sort of thickness im talking about is not being able to see the hydrometer in a pale beer. 
My guess is that it should read a few points higher ? It doesnt seem to be documented anywhere that I can see.


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## zabond (22/1/10)

not a silly ? I belive a hydrometer works on the density of a liquid so I'd say yes it would add a couple of points,if you droped it in honey what would it read?just take a reading with the suspended yeast then let the samplle sit for a couple of days to let the yeast drop out then take another reading


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## Pennywise (22/1/10)

If you were to let it sit for a couple of days you'd need to have it in a sanitised container to stop any wild yeast/bacteria fermenting it even lower, it'd also be a good idea to store it in the fridge to help drop the suspended yeast out. Of course letting it came back up to room temp before you took the reading


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## cubbie (22/1/10)

How do suspended proteins/yeast affect a refractometer?


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## jonocarroll (22/1/10)

cubbie said:


> How do suspended proteins/yeast affect a refractometer?


I've been considering this myself - since a refrac works by the refractive index of the *solution*, surely suspended particles don't affect it... right? Suspended particles in a hydrometer will increase the density and affect the reading, but they shouldn't change the refractive index - only the transmission coefficient. Or have I got it all mixed up?

If you're not part of the solution, you're part of the precipitate.


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## A3k (22/1/10)

I read a topic on this once.
someone was saying that the yeast isn't dissolved, but suspended, so it's a colloidal solution, with yeast as the colloid. And apparently colloids don't affect hydrometer readings.

Unfortunately this doesn't seem heaps logical to me, and if yeast is denser than beer, I'd think it would increase gravity readings.

Not much of a helpful thread, but it could spark a thought for someone more knowledgeable in this stuff.

Cheers,
Al


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## eamonnfoley (22/1/10)

Apparently hydrometers only measure dissolved solids. Is yeast and protein actually dissolved or just suspended?


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## davewaldo (22/1/10)

A3k said:


> I read a topic on this once.
> someone was saying that the yeast isn't dissolved, but suspended, so it's a colloidal solution, with yeast as the colloid. And apparently colloids don't affect hydrometer readings.





foles said:


> Apparently hydrometers only measure dissolved solids. Is yeast and protein actually dissolved or just suspended?




This has always been my understanding also. The density of a liquid doesn't increase with suspended compounds. Unless there is so much of the suspended compound that the friction from the compound slows the settling of the hydrometer so much it doesn't read properly. In other words, if there is so much suspended compound that the hydrometer can't make its way through the liquid. However even in this case if you left the hydrometer long enough it should settle.

For the same reason yeast settles out of suspension, the hydrometer should settle in the liquid regardless of yeast.

This is my understanding anyways.... and I haven't explained it very well


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## davewaldo (22/1/10)

another way to think about it is like this. Stick with me... 

Water soaked sand (sand saturated water) is quite easy to stand on, its very hard. If you add agitation (earthquake), this same material will liquify and you'll sink like its water. The agitation simply removes the friction of the suspended solid. the density of the liquid was always the same as plain water.

Does that help?


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## Snow (22/1/10)

This should be easy to work out without knowing the science behind it. Just sanitise your hydrometer and put it in the wort at the top of the fermenter and take a reading, then get a sample with suspended yeast from the tap and compare gravities.

- Snow.


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## eamonnfoley (22/1/10)

davewaldo said:


> This has always been my understanding also. The density of a liquid doesn't increase with suspended compounds. Unless there is so much of the suspended compound that the friction from the compound slows the settling of the hydrometer so much it doesn't read properly. In other words, if there is so much suspended compound that the hydrometer can't make its way through the liquid. However even in this case if you left the hydrometer long enough it should settle.
> 
> For the same reason yeast settles out of suspension, the hydrometer should settle in the liquid regardless of yeast.
> 
> This is my understanding anyways.... and I haven't explained it very well



Its very well explained! But I wonder if there is a threshold where the hydrometer just cant settle to its true position. Take the extreme case of a near slurry. Surely the yeast cells are so close to one another that the true reading cannot be reached ?

And with the yeast settling out of suspension in the fermenter, there is a heap of space. In the hydrometer test jar, the space is limited and if you have picked up the yeasty portion ?


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## Tony (22/1/10)

I dont think suspended yeast affects the hydro reading. The hydro will only be effected by stuff that disolved into solution in the water. The yeast is not disolved into solution. its just suspended

If you have enough yeast in there it can hamper the flotation of the hydro so this is usually perceived as effecting the reading. It does and it doesnt i guess.

Just my thoughts on the issue

cheers


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## chappo1970 (22/1/10)

Snow said:


> This should be easy to work out without knowing the science behind it. Just sanitise your hydrometer and put it in the wort at the top of the fermenter and take a reading, then get a sample with suspended yeast from the tap and compare gravities.
> 
> - Snow.




Brilliant Snow! B) 

I guess all the eggheads will argue it out atoms this nano technology that but this would certainly work for me.


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## davewaldo (22/1/10)

Well I'm bottling next weekend so I'm happy to do the experiment.

1. Clean beer
2. A fair bit of yeast in suspension
3. Pure Yeast slurry mmmmm yeast slurry


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## The Mad Hopper (22/1/10)

foles said:


> Its very well explained! But I wonder if there is a threshold where the hydrometer just cant settle to its true position. Take the extreme case of a near slurry. Surely the yeast cells are so close to one another that the true reading cannot be reached ?



... or even trub, the hydrometer isn't going to settle in that, it'll just sit on top.

I would have thought the best way to test this would be to take a hydrometer reading of tapwater and then resuspend some trub in tapwater and repeat the reading. There should be enough viable yeast at the bottom of the fermenter to create a suspension. I might try this next time I take a batch out of primary.

As for a refractometer, do they take an optical density reading (I'm not really sure on the principles of how these work)? In a lab, optical density of a culture is how you monitor the growth of a bacterial/yeast culture. So you would expect yeast suspension to affect refractometer readings if that's the principle they work on.


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## Screwtop (23/1/10)

The Mad Hopper said:


> As for a refractometer, do they take an optical density reading (I'm not really sure on the principles of how these work)? In a lab, optical density of a culture is how you monitor the growth of a bacterial/yeast culture. So you would expect yeast suspension to affect refractometer readings if that's the principle they work on.



Light Refraction...............bending of light waves passing from air to liquid. Measures the refractive index of a substance, temperature, frequency and amount of light illuminating the sample influence the reading. Murky (yeasty) test sample makes reading the scale difficult, it can appear fuzzy. A clear sample will give a good sharp reading.

Any solution being tested with a hydrometer should be filtered to remove any undissolved solids. Otherwise the readings will be inaccurate.


Screwy


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## Pennywise (23/1/10)

davewaldo said:


> This has always been my understanding also. The density of a liquid doesn't increase with suspended compounds. Unless there is so much of the suspended compound that the friction from the compound slows the settling of the hydrometer so much it doesn't read properly. In other words, if there is so much suspended compound that the hydrometer can't make its way through the liquid. However even in this case if you left the hydrometer long enough it should settle.
> 
> For the same reason yeast settles out of suspension, the hydrometer should settle in the liquid regardless of yeast.
> 
> This is my understanding anyways.... and I haven't explained it very well



I think you've explained it perfectly, thanks :icon_cheers:


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## b_thomas (23/1/10)

I've seen this question explained before in the following way - "If you drop a rock in to the ocean, does it make it any more salty?" 

Test the theory by putting a pebble in to a glass of distilled water and test the hydrometer reading - it should read 1.000 

As stated above yeast isn't soluble, hence it will have no impact on your reading - unless of course you have a thick mat of it on the bottom of your flask which the hydrometer sits on top of


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## Nick JD (23/1/10)

Anything that you add to a liquid that doesn't change its volume will increase its density. This is called "dissolving". 

Take a liter volume of dry sugar and 2 liters of water and add them together. Do you have 3 liters? No, you have 2 liters because the sugar _dissolved._

Yeast are probably about 95% water anyway, so I'd hazard to guess that a very thick solution of them probably has a SG of about 1.000 anyway.

The SG of aluminium is 2.7; lead is 11.3. See, it even works for solids - although how you get your hydrometer into a block of lead beats the hell out of me. :lol:


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## syd_03 (23/1/10)

Homebrewer79 said:


> I think you've explained it perfectly, thanks :icon_cheers:



He has also explained it incorrectly.


b_thomas said:


> I've seen this question explained before in the following way - "If you drop a rock in to the ocean, does it make it any more salty?"
> 
> Test the theory by putting a pebble in to a glass of distilled water and test the hydrometer reading - it should read 1.000
> 
> As stated above yeast isn't soluble, hence it will have no impact on your reading - unless of course you have a thick mat of it on the bottom of your flask which the hydrometer sits on top of



The first statement is true because the rock or pebble settles to the bottom of the glass in a very short period and doesnt change the density of the whole sample.

The second statement is incorrect. The yeast forms a suspension in the liquid and increases the density of the liquid, which is what a hydrometer reads. It works on Archimedes principle of buoyancy. The hydrometer is pushed upwards by a force equal to the weight of the liquid it displaces. Since the yeast increases the weight of the liquid displaced the hydrometer will float higher. 

In the example of the rock or pebble it doesnt remain in suspension for any appreciable amount of time. The hydrometer only displaces the liquid not the rock or pebble. Therefore the hydrometer is buoyed up by the same displaced volume of pure water or seawater as the case may be and gives the same reading as before the rock was inserted to the sample.

This is the principle used together with Stokes law (settling velocity of spherical particle in viscous fluid) to allow hydrometer analysis of fine soil particle distribution. The hydrometer reading gives the density of the suspension and is measured at certain time intervals. This allow the distribution to be plotted as Stokes law can be used to determine the settling time for a certain sized particle in a viscous solution. The change in density over that time gives the mass of soil particles of that size and therefore the relative percentages of particle sizes can be determined from the original mass. 

Small amounts of yeast or trub or even bits of hops wont appreciably change the density of the solution. Larger amounts will have a visible impact on the density.



Nick JD said:


> Anything that you add to a liquid that doesn't change its volume will increase its density. This is called "dissolving".
> 
> Take a liter volume of dry sugar and 2 liters of water and add them together. Do you have 3 liters? No, you have 2 liters because the sugar _dissolved._
> 
> ...


Yeast isnt soluble in water, it doesn't disolve. It is suspended in the water and since heavier will increase the density of the solution.

Cheers 
Jason


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## Nick JD (23/1/10)

syd_03 said:


> Yeast isnt soluble in water, it doesn't disolve. It is suspended in the water and since heavier will increase the density of the solution.
> 
> Cheers
> Jason



Of course yeast doesn't dissolve. Where did I say that? 

Oh and please prove to me that yeast is denser than wort. The trick here might involve your use of the word, "suspended". Had you have used the word, "sunk" or "float" I might not ask this. At most I would guess yeast has roughly the same density of a fresh wort and is probably why it settles out during the end of fermentation. 

Which begs another question: what is the biomass of yeast during fermentation? What is the ratio of wort to yeast?

What is a human's specific gravity (I'm not talking about fat people - we all know they float!) who has just drank five liters of beer?


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## OzMick (23/1/10)

Nick JD said:


> Oh and please prove to me that yeast is denser than wort.



Centrifuged beer will have the yeast pulled to the bottom.

Edit: Ok, so that is still using beer as an example. From http://www.wyeastlab.com/com-yeast-harvest.cfm



> 1L (1 quart) of yeast slurry (40% yeast solids) weighs approximately 1.1 Kg (2.4 lbs.).



So, 1L of pure yeast solids by that would weigh 1.25kg. So is denser than wort.


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## Nick JD (23/1/10)

OzMick said:


> So, 1L of pure yeast solids by that would weigh 1.25kg. So is denser than wort.



Are you assuming the other 60% in a yeast slurry is water?

I've read yeast is around 1.075-1.080 SG. That a bit different than 1.250. These yeast cells are denser than me?! I've got bones!


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## drtomc (23/1/10)

Nick JD said:


> The SG of aluminium is 2.7; lead is 11.3. See, it even works for solids - although how you get your hydrometer into a block of lead beats the hell out of me. :lol:



I've attempted to get several into the floor tiles over the years, and have not succeeded yet, so I cannot report their SG. :lol: 

T.


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## bum (24/1/10)

Nick JD said:


> These yeast cells are denser than me?!



h34r:


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## Jazzafish (24/1/10)

To be simple, yeast/break/anything in the solution being measured will influence your reading.

To be as close as possible to a correct reading, pour samples through a paper filter. Or you could have a bit of fun with a home made centrefuge... jar on an end of some rope, swung around for a while! :lol: Pour the liquid off the compacted cake in the jar and your all set for a precise reading!!!


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