Does suspended yeast contribute to high SG?

Question

I've got a SMaSH beer (Baird Maris Otter and EKG) finishing in primary. It's been fermenting for just over a week. Starting Gravity was 1.046. I mashed at 154F for 60 minutes. Fermented with WY1335 British Ale II. I didn't make a starter, since the yeast was very fresh, ~95% viability. I oxygenated with pure O2 for 90 seconds. High krausen was observed at 36 hours, and fermentation was vigorous.

1335 is described as medium attenuation, high flocculation. I expected the beer to have started clearing by now, but it's still very cloudy.

I pulled a sample with a wine thief and measured the SG, and took a swig. It tastes very slightly sweet. If I had to guess, I would say 1.012. However, the hydrometer was reading over 1.020 even after de-gassing the sample.

I've drunk a lot of hydrometer samples, and I can't believe that this beer really has so much residual sugar -- it just tastes too dry. Is there anything that could be affecting the hydrometer reading? I wouldn't think that suspended yeast would affect SG, since it's not in solution and therefore not affecting the density of the liquid. Am I wrong? Is there something else that could contribute to an anomalously high gravity reading?

Answer 1

A non dissolvable solid like yeast in a liquid does not increase specific gravity. Its like dropping stones in to a water, the water still has the same density as it did before as the stones (and yeast in your example) are two separate phases. The solids simply displace the liquid but do not become a "part" of it, as in the example of salts or sugars or dissolved proteins. A dissolved substance becomes a homogenous part of the liquid system. The liquid is no longer just water its a % sugar solution... the two have become one. Even if the yeast are in suspension they are not dissolved in the liquid. They exist as two separate phases.

Dissolved CO2 in the beer may nucleate on the sides of the hydrometer and sort of suspended in higher than it should be. A couple quick spins in the testing tube usually helps. Be sure you are at the correct temp for the hydrometer (usually 60F). Lastly, your reading may be off due to dissolved protein or starch. These things will contribute to the gravity, but would not register on your palate as sweet.

Answer 2

Dissolved solids, such as sugars, increase the SG since they increase the mass of the solution without any significant volume increase.

Suspended solids, like yeast, may increase or decrease the SG depending upon the relative density of the solids compared to the density of the liquid.

In the case with the yeast, we know that yeast settles out eventually, so it's more dense than the beer, and so it increases the SG.

CO2 bubbles can also affect the hydrometer reading, causing it to float higher than usual, and reading a higher SG.

To get an accurate SG reading, it's best to start with a clear sample, or wait until the suspended solids have sunk to the bottom of the sample, and then spin the hydrometer so that any bubbles underneath are dissipated.

Answer 3

SG only measures the density of a solution relative to the density of pure water. It does not necessarily indicate dissolved sugars per se. So suspended particles as well as non-fermentable compounds (ie, maltodextrin, proteins, etc) contribute to SG without contributing to sweetness. This would include yeast, protein and anything else suspended within the liquid.

The haze in your brew suggests residual yeast and/or protein which are likely contributing to your gravity readings.

edit: Think of it this way, the simplest way to measure SG would be to take a very precise volume of the solution (say, 10 mL) and then measure it's mass very accurately. This will allow you to calculate density and compare to the density of pure water to determine SG. Anything within that sample--dissolved solids, particles in suspension and anything else--will contribute to the mass and increase the resultant SG.

Answer 4

The analogy used with stones in water is a poor one, the reason being that stones are large and follow stokes law for particle size. They most certainly sink immediately, however, if you crushed up almost any rock or clay to the size of a yeast cell they would be suspended in the liquid and would contribute buoyancy to the hydrometer. The density change here would actually be quite dramatic. In fact, I look at suspensions of nanosized silica in water and for a 30% by weight suspension the specific gravity is 1.020, silica is a large component of rocks, clays and quartz, at the nanosize it has density of 1.9-2.4 g.cm3.

See table 12.1 Colloidal Silica: Fundamentals and Applications

However, in our case, our yeast cells are of a density much more similar to water at around 1.080 g.cm3 (at a minimum) . So a 10% by volume yeast suspension in water (perhaps active wort) would certainly increase gravity by a few points - the gravity would be eight points higher!

Source: http://book.bionumbers.org/what-is-the-density-of-cells/

Just thought this would be worth pointing out, I thought the original post could be very misleading to others.

Answer 5

I believe that mdma's answer to this very similar question is the more correct one, and in conflict with the accepted answer here. Helpfully, there is a citation to support the conjecture that sediment can influence hydrometer readings. It seems that best practice is to let the sediment settle out.

The relevant info as unearthed by mdma from this source:

Hydrometer analysis begins after thoroughly mixing the sediment and water, after which particles settle out of the water column according to Stokes’s law. The density of a sediment-water suspension depends on the concentration and specific gravity of the sediments present in the mixture. If the suspension is allowed to stand, particles will settle out of the suspension and the density of the sediment-water suspension will decrease.

Answer 6

Why not calibrate your hydrometer and take another reading?. If you're worried about suspended yeast, chill it and decant off and take a reading?. I don't think yeast contribute to gravity so much that it would throw your hydrometer off - different temperatures or uncalibrated hydrometer maybe. I'd give it two weeks and have a homebrew.

Answer 7

Yes, I agree BrotherLogic. It never hurts to calibrate your hydrometer every now and then. Even glass tube thermometers can require calibration over the years. Get a sample of pure water and do whatever you have to, to have the sample as close to the calibrated temperature listed on your hydrometer (usually 15C). It should read 1.000 (0.009 - 1.001). Don't forget when your sampling your beer to use a correction factor if the temp is not 15C. I've seen hydrometers out by a couple points. If the hydrometer was off by a few points, and you mis-measured your samples temperature by a few degrees, the numbers can become much closer together then you expected. If nothing else, calibration will give you piece of mind that your hydrometer is working properly!