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Best practice

Although the yeast storage process follows cropping, ‘storage' effectively begins when yeast sediments (often) early in fermentation into the cone. Accordingly, best practice is to use effective cone cooling (2-4°C), remove the first cut of dead yeast plus trub and practice warm cropping to remove yeast to a more controlled and homogeneous environment in a storage vessel. The importance of early warm cropping increases with high (15°P) and very high (> 15°P) gravity fermentations. A similar argument applies to the use of higher temperature fermentations (to accelerate rate). Although cropping is often managed manually (visually or by time), there is value in the use of ‘radiofrequency permittivity' technology to better manage viable yeast solids.

Consequences of failure

Rapid yeast cropping minimizes the inevitable damage to yeast that occurs during this process. Arguably the conditions in the fermenter cone are the most damaging that yeast experience in the fermentation cycle. Prolonged residence in the cone results in metabolic ‘hot spots' where the local temperature can increase markedly. Here, the reserve polysaccharide glycogen is dissimilated to glucose, which through glycolysis is converted to ethanol, ATP and heat. As the local temperature rises, the rate of glycogen break down increases, resulting in the formation of more heat. The insulating effect of thick yeast slurries reduces heat dissipation and results in the hot spot getting hotter and bigger.

These events result in a spiral of decline encouraging glycogen breakdown, ethanol formation, heat generation and inevitably loss of yeast viability. If this was not bad enough, there are environmental graduations radiating from hotspot(s) where yeast cells retain viability but have reduced levels of glycogen, which may compromise performance in the upcoming fermentation.

 
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