Premature Yeast Floculation

Premature Yeast Floculation (PYF) is a little understood brewing problem that causes poor fermentations and makes me think of Kevin "Bloody" Wilson.

I heard Professor Alex Speers of Heriot-Watt University give a talk about the other day and I wanted to write up my notes so I though I'd share them with your all. It's fascinating stuff.

Premature Yeast Floculation

Fermentability is related to yeast flocculation: early flocculation causes poor fermentation leading to flavour and filtration problems.

Floculation is reversible and adding the chelating agent EDTA or some sugars can cause yeast to un-floculate.

A number of factors affect flocculation: FLO and other genes coding for proteins such as zymolectin, mannan, oxylipin and affecting the charge on the cell.

Environmental factors also have an effect such as:

• Sugar
• O2
• CO2/turbulence (keeps yeast in suspension)
• Cell age
• Fimbrae
• N
• Temperature
• pH
• CSH (hydrophobicity)
• Ethanol
• Ca2+

The negative effects of PYF on beer include:

• High sugar levels
• Irregular flavours
• Yeast autolysis
• More chance of infection
• Lower ABV
• Higher diacetyl
• Money losses!

PYF is believed to be caused by factor/s from barley or malt. Possibly due to fungal infection, pressure during steeping, malting CO2 levels, other barley stresses.

Malt made with low O2 water may be implicated.

In breweries malts are blended by variety, year and maltster so it makes problems hard to trace!

Laboratory methods have been adapted to use smaller scale fermentations than tall tubes using 15ml of congress wort (9°P) +4% glucose at 21°C (three repetitions).
Measure turbidity/density. Use SMA yeast (a German lager strain).
Describe the °P change and absorbance change on graphs. Statistics will need to be used to look at the curves.
Test tube fermentations are slightly poorer than actual fermentations due to early yeast setting as a higher proportion will have settled out before CO2 starts being produced.

Looked for an anti-microbial peptide in malt that may have been affecting the yeast but they didn’t find it so looked for larger compounds.

Tried different degrees of filtration of wort and found a 100 kDa fraction causes PYF. Arabinoxylan may be involved, possibly with a linked peptide.

During PYF yeast has less surface charge. PYF flocs are more tightly bound than control flocs.

Barley exposed to different fungal pathogens was malted and brewed with. Infected malt caused PYF, more disease resistant barley varieties caused less PYF.

PYF malt has higher FAN

Lab analysis of malt tends to stop at wort production so fermentation not included.

More studies needed on:

• Comparison of yeast strains
• Varietal tests and resistance to pathogens
• Malt house effects (especially CO2)
• Hull less barley
• Treatment and re-use of steep water
• Mode of action
• Relation to cell toxicity