Reading the second edition of Brewing Yeast Fermenation Performance I realised it was not an update of the first edition but contained completely different content. So I had to go back and read the first edition. I didn't enjoy it as much, perhaps I've been over doing it on textbook reading, but there's a gem right at the end:
Yeast Management and Fermentation Performance: a Brewer's perspective
byWarren Quilliam, Gavin Hulse and Anna Cameron-Clarke
The Twelve Point Plan
- Ensure that the propagation technique is such as to produce biomass rather than alcohol, and then minimise the contact between the propagated yeast and the alcohol.
- Top up the various propagation stages at the appropriate times. Each brewery would need to establish their specific exponential growth pattern to determine the ideal top up timing. This should occur towards the middle of the exponential growth curve to expose the yeast to the sugar spectrum it can expect in the main fermentation process.
- Provide the yeast with the ideal oxygenation regime during propagation to ensure optimal growth potential.
- Provide the fermentation with enough nutrients (especially FAN) from the raw materials (paying more for better quality malt invariably costs far less at the end of the day).
- Provide the yeast with stress-free conditions during the fermentation process. This relates to temperature control and rate of oxygenation in particular. The wort should be collected at a temperature suitable for the brand profile. Sufficient oxygen must be provided to enable yeast development but also accommodate the required brand profile characteristics. Starving the yeast of oxygen might give the product a higher level of ester but will also cause the development of stress products.
- Minimise temperature shocks. If a refrigerant temperature of close to zero degrees Celsius is capable of controlling the fermentation process, it would certainly be of benefit to the growing yeast cells, which would not be subjected to temperature shock if the refrigerant were at a lower temperature. Similarly the cone temperature, which is often controlled at -4 degrees Celsius, could be contributing significantly to yeast autolysis at the interface. Consideration should also be given to a two-phase water chiller for reducing the temperature of recovered yeast, rather than the sudden reduction of temperature using propylene glycol or compressed ammonia at very low temperatures. The rate of chillback of the fermentation should also be prolonged to ensure that the possibility of temperature shock is minimised.
- Recover the yeast on time. When the fermentation has reached its attenuation limit there is no need to keep the yeast in contact with the product. Yeast which is still in suspension will mop up any residual fermentable sugars that may remain in the beer. More harm than good will result from delaying yeast recovery. It is also essential that all the yeast is removed and that slurry consistencies and crop sizes are monitored as indicators of fermentation performance.
- Scrap the 'tired' yeast. Phenomenal improvement in yeast performance has been reported if the first 10 to 15% of the recovered yeast is scrapped. Improvements in protease levels, attenuation, pH and flavour were reported after removal of the early flocculating 'tired' yeast from the base of the fermenter prior to recovering the crop.
- Ensure homogeneity, gas stripping and temperature control in recovered yeast. Gentle but effective agitation is critical to minimising yeast stress through the achievement of homogeneity and the removal of entrapped carbon dioxide. It is also critical that the temperature increase in the recovered yeast is restricted to a few degrees Celsius prior to pitching, or the quality of the subsequent fermentations will be negatively affected.
- Minimise the recover to re-pitch time. It is not always practical to minimise the occupancy of the recovery and pitching yeast vessels, particularly if the brewery uses several strains of yeast. The longer the yeast spends away from the nutrient, despite the temperature, the more prominent the stress placed on that yeast.
- Minimise mechanical damage. Consideration has been given to the gentle handling of the product. Care needs to be given to the selection of pumps, the complexity of the pipework, the number of valves, the action of agitators and the design of chillers in order to protect the yeast from unnecessary mechanical stress. .
- Use the best yeast … scrap the rest. With every fermentation potentially different to the others, it is critical that the brewer responsible for selecting the yeast scrutinises every aspect of the parent fermentations, ensuring that the best yeast is selected for the subsequent fermentations.