Whether you are part of the Haze Craze or would prefer crystal clear, everybody has an opinion on the clarity of beer. The main causes of haziness are yeast and proteins, below we’ll take a look at the influence of yeast. How does yeast make a beer hazy and what can a brewer do to affect this?

The way in which yeast causes haze is relatively simple. Yeast is a single-celled microorganism with a size of about 3-4 micrometer, impossible to see with the naked eye individually. Beer with secondary fermentation on the bottle and unfiltered beers may contain a yeast deposit of billions of cells. All these cells together refract the light and therefore cause a haze in the beer or if you wait long enough will collect at the bottom of the bottle. Yet some yeasts sink more easily than others and that story is a bit more complicated.


The process in which yeast cells aggregate and sink is called flocculation. Coming from the Latin word “floccus” meaning a tuft of wool. A floc of aggregated yeast evidently looks like a tuft of wool. There are a lot of factors which may influence how well a yeast can flocculate, physical, chemical as well as genetic. Here we’ll stick to those relevant to a brewer. To understand those factors we need to know how flocculation works.

The cell wall of yeast in beer is negatively charged, this causes yeast cells to repel eachother. As is they don’t aggregate which means an active mechanism is required to achieve this. The outer layer of a yeast cell contains a lot of sugar chains consisting of Mannose. Some yeasts contain FLO-genes which, if activated, cause the cell to express so called flocculin proteins at the cell wall. Flocculin can attach to the Mannose on the outside of other yeast cells causing them to aggregate.

Each yeast strain contains different FLO-genes en therefore shows different flocculation behavior. To an extent, which yeast strain you choose to brew with determines how clear the beer will become. This is one of the reasons certain styles have more or less yeast haziness, as the strain used for the style tends to flocculate a certain way. However, there are several things a brewer can do to promote the sedimentation of yeast.

Calcium ions

Calcium ions (Ca2+) are essential for flocculation. Flocculin is functional only if it has a bound Ca2+. It’s therefore important to ensure there is enough calcium dissolved in the brewing water.

Re-pitching yeast

FLO-genes are apparently very unstable. Take this into account when re-pitching yeast from a previous batch as the flocculation behavior may have changed significantly. This makes it difficult to cultivate and select yeast based on flocculation behavior. There is a trick that can be used in a cylindroconic fermentor. The yeast at the bottom of the fermentor is not homogeneous, the more flocculant yeast can be found at the bottom.


Sedimentation of yeast can be promoted by the fermentation temperature. Yeast can stick to the CO2 bubbles produced during the fermentation. At low temperature yeast metabolism slows down resulting in less CO2 being produced and consequently reducing the turbulence in the beer. A lagering period at the end of the fermentation can therefore help clarify the beer.


If the inherent sedimentation of yeast isn’t enough, the use of a clarifying agent in the form of Isinglass is an option. This is derived from the swimming bladder of certain kinds of fish and is a positively charged particle that binds to the negatively charged outside of yeast cells thereby inducing aggregation. The use of this particular clarifying agent is the reason why many beers are not vegan. We do not use isinglass in any of our beers, so all our beers are suitable for vegans.

See the effect

All our beers are unfiltered and therefore contain yeast cells. Especially the yeast in our French Rye Saison doesn’t sediment all too well but does intensify the flavor. Try to pour out an entire bottle and compare it to one with the yeast deposit kept aside.


  • Verstrepen et al. – Appl Microbiol Biotechnol (2003) 61:197–205 DOI 10.1007/s00253-002-1200-8
  • Soares, E.V. – Journal of Applied Microbiology (2010) 110, 1–18 DOI:10.1111/j.1365-2672.2010.04897.x


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