Yeast should be cropped from fermenters as soon as possible. The conditions which yeast is exposed to in fermenter, particularly during the later stages, are stressful. The combination of starvation due to lack of nutrients, high carbon dioxide and ethanol concentrations and possibly elevated pressures all have the potential to cause deterioration in the yeast crop. In extreme cases, the imposed stresses are sufficient to cause cell death and autolysis with consequent adverse effects on beer quality. Brewers employ a number of methods to crop their yeast that vary depending on whether one is dealing with a traditional ale top fermentation system, a traditional lager bottom fermentation system, a cylindroconical fermentation system where the yeast (ale or lager) is recovered from the cone, or a non-flocculent culture where the yeast, still in suspension, is cropped with a centrifuge.
Ale Top-Cropping Systems
Harvesting yeast from the top of open fermenters (top cropping) is still a widely used practice in the modern brewing industry. Specific yeast strains which rise to form a dense head must be used, making top cropping nearly exclusive to ale and wheat strains. As with other methods of harvesting yeast, harvesting a consistent population which avoids the extremes is important.
Lager Bottom-Cropping Systems
In traditional open fermenters used with bottom-cropping yeast, the beer is removed first, at the end of primary fermentation, leaving a yeast sediment in the base of the fermenter. This is then removed manually. The yeast sediment will have three distinct layers as mentioned earlier.
An effective alternative to bottom cropping is krausening wort with fermenting wort. If brewing schedule permits, pitch 1/5 volume actively fermenting beer at peak activity (approximately 48 hours into fermentation) into freshly brewed wort.
Conical bottomed fermenters greatly improve the ease and efficiency of harvesting yeast. Yeast is often cropped from the bottom of the conical fermenter cone after fermentation, i.e., beer fully attenuated, which is preceded by cooling to ensure that fermentation has come to completion and cells have flocculated out of the beer and sedimented at the bottom.
Methods in Cropping the Yeast
Yeast cropping practices should be standardized within the brewery to get yeast from the same level each time yeast is cropped. The brewer has the choice of either harvesting the yeast first before transferring the green beer or first drain off the beer from the top of the cake.
Method One. The tank should be thoroughly cooled with the yeast settled to a thick slurry before harvesting. First, ensure that the tank either has adequate carbon dioxide top pressure or has some other method to make up for the lost volume, such as venting. Sanitize the bottom valve, and make the appropriate connections to route the yeast to the collection vessel.
Method Two. Alternatively, if the fermenter is fitted with a racking arm that can be used to drain off the green beer from top of the yeast cake (Figure 12.12). The racking arm is generally able to rotate off of the yeast cake. This rotation results in clear beer and less sediment being transferred. Below are some best practices for using a racking arm.
Transferring the Cropped Yeast
Cone to Cone Pitching. In some breweries that use cylindroconical fermenting vessels the yeast is taken from the cone crop of one fermentation and transferred directly into the empty cone of a new fermentation. The yeast can be transferred using gravity and top pressure, or pumped. Some brewers use mass flow meters with some success which not only gives volumetric measurements, but also mass measurements.
Transfer to a Yeast Brink. A better approach is to transfer yeast from a fermenter to a yeast brink (Figure 12.13). A yeast brink is a storage vessel that prepare yeast for fermentation by holding it in a temperature-controlled suspension. Sanitary transfer from brink to fermenter is controlled by applying pressure to the top of the brink.
The use of centrifuges for the removal of yeast and the collection of pitching yeast is often used in breweries. There are two principal ways to use centrifuges for beer separation after fermentation. The first method is to use the centrifuge to separate the yeast crop from the entire fermenter. The second uses a centrifuge to clarify the beer after the yeast has been separated by decanting or skimming for repitching.
Click on the following topics for more information on beer fermentation.