Monday, September 20, 2010

Stokes' Law and Order - Irish Moss Unit

This was originally going to be thrown in with my Pumpkin Pie Ale post because that was the first time I had used Irish moss but as I wrote it I decided that it deserved an entire post.

What is Irish Moss?
Chondrus crispus
or Irish moss as it's more commonly known is a species of red algae that grows along the coasts of the Atlantic Ocean.

Irish Moss: Why use seaweed in beer?
Irish moss in brewing is used as a fining agent which means it removes suspended particles in the beer that cause it to be cloudy. Sometimes the style of beer requires that you have suspended particles in the beer, hefeweizen for example, so Irish moss should not used.

There are two main causes of cloudy beer, proteins and yeast. When the yeast is at the end of its life cycle and most of the sugars in the beer have been consumed the cells will bond to each other, becoming heavier and dropping to the bottom. This process is called flocculation, and occurs naturally when the yeast go dormant. Not all yeast behave in this manner though, some will stay suspended indefinitely. A method of clearing beer further is to cool it down so that the yeast will go into hibernation mode and sink to the bottom. This is why white labs yeast are refrigerated to keep the cells alive but dormant until they are ready to be used.

There are two popular styles of Irish moss commonly used in brewing applications. Dried moss is just that, the seaweed is dried out, thrown in a jar and sold at brew stores everywhere. The other type is tabs, Whirflocis a brand of Irish moss that is concentrated and put in tab form. Whatever form that is chosen to add to the wort the implementation is the same, during the last 10-20 minutes of the boil throw in a tablet or about a teaspoon or the dried variety.

Stokes' Me, Stokes' Me
The principal used in determining the rate that suspended particles will fall out is called Stokes' Law. The equation is as follows:
V_s = \frac{2}{9}\frac{\left(\rho_p - \rho_f\right)}{\mu} g\, R^2

  • Vs is the particles' settling velocity (m/s)
  • g is 9.8 (m/s2) Assuming your not making space beer
  • ρp is the mass density of the particle (kg/m3)
  • ρf is the mass density of the wort (kg/m3)
  • μ is the wort viscosity (kg/m s)

Irish Moss can aide in this process by making the proteins and yeast stick together creating a heavier particle. Making ρp bigger will make the numerator bigger increasing the velocity that the particles settle.


  1. Where the hell was this yesterday? You referenced it but it was no where on your site until today.


  2. I didn't mean to hurt your feelings I was just messing with you and I'm sorry. You make really good posts and they are informative. Thank you for you wonderful blog I would have a darker and unhappy life without it.

  3. Hey wait, our verbal abuse got deleted? you can't have a blog without trolls! Sorry to hurt your feelings-- I love me some brewing blog, and even though I use Irish moss, I had no idea about any of the crazy Stokes law stuff. consider me edumacated.

  4. ah, they're still there under the pumpkin ale... I feel better. Going to go find some more snarky comments to make now...

  5. He wasn't even happy with my nice comment. I don't know what to do. So I'm going back to making snarky comments also.

    Starting with what are you thoughts on his formatting? I'm not really a fan of some of the stuff he italicizes. I really don't see a point most of the time for it.

    Also while most people know it I think you should have explained that seaweed is a general name for algae. In you post you switch between the terms.


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