Clarification / Flocculation
Clarification of beer involves many factors from biochemical to mechanical. A bright beer is the result of good brewing techniques, an understanding of the fundamentals of clarification, a good filtration system, or a combination of all three.
A hazy beer is commonly blamed on yeast that will not settle, but that is one of several possibilities. Yeast does not stay in suspension without help.
To get an idea of the complexities involved in clarification, we will look at several factors that affect yeast flocculation, but first we will define flocculation.
Flocculation refers to the ability of yeast to aggregate and form large flocs and then drop out of suspension. The definition of flocculation is, “reversible, asexual and calcium dependent process by which cells adhere to form flocs.”
It is very important to understand the basics of flocculation and what affects it because the flocculation and sedimentation process is the easiest and least expensive way to get bright beer. Flocculation also effects fermentation performance and beer flavor. Ideally, yeast will stay non-flocculent and in suspension until the desired final gravity is reached and then become flocculent and drop out of solution. As any brewer knows, yeast do not always cooperate with this concept.
Yeast strains have different levels of flocculation characteristics from non-flocculent (1007 German Ale) to highly flocculent (1968 London ESB).
Non-flocculent yeast have cells that appear smooth under a scanning electron microscope and that have a negative surface charge. When these cells are close and moving slowly, they repel each other. If these cells are moving toward each other fast enough, they will overcome the repulsion and collide but will not stick together.
Flocculent cells are cells that appear to be covered in hairs or spines under a scanning electron microscope. These cells also have a negative surface charge that causes repulsion between two cells. However, when these cells collide they overcome the repulsion and stick together.
The Lectin hypothesis is the current hypothesis that describes how yeast flocculate. This hypothesis explains flocculation as controlled by cell-wall to cell-wall interactions, specifically the binding of zymolectins to mannose residues of mannan in the cell wall of yeast cells.
Zymolectins are proteins produced in the cell and then secreted into the cell wall. Zymolectins bind to sugar molecules and require calcium ions to maintain proper configuration for binding these sugars. Zymolectins also bind to cell wall mannose residues of mannan.
Mannan consists of long and branched mannose sugar chains that are present in the cell wall. Mannan is present in the cell walls of all yeast cells. Mannan is attached to long peptide chains anchored in the cell wall.
The zymolectins and cell wall mannans basically work like Velcro. What causes the production and activation of zymolectins is not well understood. It is believed that zymolectins become active at the end of exponential growth and during the stationary phase. Most likely depletion of nutrients and increase of fermentation byproducts (ethanol and pH changes) trigger production and activation of zymolectins.
There are two phenotypes in brewing strains that are defined by the type of zymolectin they produce.
In the Flo1 phenotype, the zymolectins produced bind to only mannose residues and the zymolectins are inhibited only by mannose. In this yeast type, flocculation is not affected by the growth stage of the yeast. Many ale strains fall into this category.
In the NewFlo phenotype, the zymolectins produced bind to mannose and glucose residues and are inhibited by mannose, glucose, maltose, and sucrose. Flocculation is typically expressed late in the exponential phase and into the early stationary phase. This group contains most lager strains and some ale strains.
Co-flocculation can occur when a flocculent and a non-flocculent strain are used together. The combination of the two flocculation types can cause both strains to flocculate because the zymolectins of the flocculent strain bind to mannans of the non-flocculent strain.
It is difficult to predict whether two strains will exhibit co-flocculation so it is always important to run small scale fermentation trials before using two strains together.
Factors that Promote Cell Aggregation and Attraction (Flocculation):
The Lectin Hypothesis describes the mechanism that makes yeast cells stick together, but what factors promote this mechanism?
- Genetic background of the strain: For flocculation to occur, the strain must carry the FLO genes responsible for encoding and regulating the production of FLO proteins. FLO genes are very unstable and have extremely high frequencies of mutation. This instability leads to deletion of FLO genes and loss of flocculation characteristics.
- Zymolectin concentration: The increase of zymolectin concentration in the cell wall causes flocculation to occur. Depletion of nutrients, an increase in fermentation by-products, and temperature increases all cause an increase in zymolectin concentration in the cell wall.
- Mechanical factors that increase collisions between cells: Turbulence caused by CO₂ production, temperature gradients, or other factors will cause more collisions between cells. A higher cell concentration will also result in more collisions and increase cell aggregation.
- Factors that decrease repulsive electrostatic charge: Ethanol concentration, pH, and changes in the cell wall composition.
- Factors that increase Cell Surface Hydrophobicity or CSH (CSH is a measurement or description of how much the cell surface repels water molecules): Increases in surface protein concentration, increases in zymolectin density due to the hydrophobic regions of this protein, change in the ratio of phosphorus rich to nitrogen rich polypeptides in the cell wall, and an increase in the production and accumulation of oxylipids, sterols, and fatty acids in the cell wall.
- Reduction of zymolectin inhibiting sugars. Over the course of fermentation, sugars that competitively bind to zymolectins will be consumed by yeast; this will make these sites available to cell wall mannans.
- Cell Age: Older yeast cells tend to have rougher and more wrinkled cell walls than virgin cells which tends to increase the binding ability of the older cells. Older cells tend to have a more filamentous growth. Some research shows that older cells have a higher density of zymolectins in the cell wall.
What does all of this mean to the brewer?
Obviously flocculation and clarification are complicated issues and are affected by a number of factors. Some of these factors are beyond the brewer’s control while others are well within reach. Manipulating factors that affect flocculation will have a direct impact on flavor and aroma of the finished product. A brewer must balance the benefits derived in flocculation and clarification with the affect on flavor and aroma in the finished product.
- Poor or low wort aeration can result in early and incomplete flocculation
- Adequate aeration can result in delayed and more intense flocculation
- Affects sterol and fatty acid synthesis and presumably cell surface hydrophobicity (CSH)
- Lab trials have found that brewing strains have different optimum flocculation temperatures
- Flocculation for one lager strain increased from 41 °F (5 °C) to 77 °F (25 °C)
- One trial showed flocculation of lager strains was optimal at 50°F (10 °C) and decreased significantly below 41 °F (5 °C)
- In other strains tested, flocculation was repressed at 77 °F (25 °C) and optimal at 41 °F (5 °C)
This is obviously very strain dependent. Good record keeping will help to determine the optimum temperature range for your strain.
- Optimum range is 3.5 to 4.8
- Optimum is very strain dependent
- Flocculation can occur in the range of 1.5 to 9.0
- Lab trials have shown that both increases and decreases in ethanol levels enhances flocculation
- Very strain dependent
- One study of a NewFlo strain showed flocculation increased from 58% to 71% when the pitch rate was gradually increased from 1 million to 15 million cells/mL
- Higher pitch rates can yield populations with higher percentages of older cells
- Not a lot of information to date on effect of trub and/or protein on flocculation
- Observations at Wyeast Laboratories shows that changes in trub levels: increases flocculence of some strains, decreases flocculence of some strains, and has not effect on some strains.
- Obviously very strain dependent
Yeast Handling and Flocculation/Clarification:
Standardize pitch rates
- This will remove one factor from the equation to determine sources for changes in flocculation
- Cropping of yeast for subsequent repitching is very important for maintaining proper flocculation characteristics
- Cropping from different layers in the fermenter cone can be used to adjust and maintain flocculation characteristics. Cropping from the middle layer of yeast in the yeast bed will select for the highest flocculation
- Standardize storage times and temperatures
- Changes in storage temperatures can influence flocculation characteristics of some strains
- Some studies have shown that intensive acid washing leads to a decrease in flocculence in some strains. This is most likely due to changes in the cell wall and subsequent changes in cell surface hydrophobicity (CSH)
- These changes in flocculation can carry into subsequent repitches
- The flocculence of a yeast strain will change with serial repitching. This is due to changes in the cell wall composition and genetic variation.
- Very strain dependent, some strains are much more stable than others.
- Reculturing from the original strain stock or getting a new slurry from your supplier must be done when changes are observed.
Flocculation is one of the most complex and least understood mechanisms that yeast have. It is very difficult to determine exactly why a yeast strain has had a change in flocculation characteristics. Good and consistent record keeping combined with good and consistent yeast handling and brewing techniques will minimize the number of unknown factors affecting yeast.