Yeast Harvesting / Re-Pitching

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Harvesting and re-pitching yeast is a common practice in most breweries. Brewers should be able to re-use yeast for at least 7 generations and often as many as 10 generations if good harvesting and storage practices are followed. Harvesting and re-pitching yeast is a great way to spread the cost of the culture over many brews. The particular method of harvest will depend upon the yeast strain used and brewery configuration, however the principles will remain consistent.

Consistency
The most important concept in harvesting is consistency. Consistent harvesting and re-pitching practices will promote consistent fermentations and yeast performance. Variations in the timing or temperature of the yeast harvest will result in inconsistent cell densities and will affect the subsequent pitch rates. Other variables such as wort composition, dissolved oxygen at run-in, and the fermentation temperature will also affect the quality of the harvested yeast. Yeast harvests will never be identical from brew to brew but variables can be controlled and inconsistencies minimized.

Selective Mutation
The profile of a yeast culture can change over time if harvesting practices are not consistent. Not all yeast cells in a culture are genetically identical. There are genetic variations within any population and these variations cause some yeast cells to exhibit different characteristics. Cells that flocculate early tend to be less attenuative and will settle to the bottom of the tank with trub. Cells that stay in suspension and flocculate later tend to be more attenuative and will end up at the top of the yeast bed. Repeated selection of either of these extremes will change the profile of the culture and alter fermentation characteristics. Yeast to be harvested should be selected from the middle portion of the yeast bed when harvesting from the bottom of a tank, or from the middle skim when harvesting from the top of a tank.

What Yeast Should Be Harvested
Not all yeast in the brew house can be successfully re-pitched. When choosing a tank to harvest from and what culture to re-use, the following guidelines should be adhered to:

1. Yeast Generation: Always select the youngest generation of yeast available for harvest. Using fewer generations will minimize opportunities for mutation or contamination.
2. Previous Fermentation: Always harvest from a low gravity and low hopped beer. High gravity and/or highly hopped beers can stress the yeast and have detrimental affects on future fermentations. Do not harvest yeast from beers with alcohol contents greater than 6.5% ABV.
3. Yeast Evaluation: Only harvest yeast from fermentations that have exhibited normal fermentation characteristics. Always evaluate the yeast slurry as it is harvested. The slurry should appear thick and creamy with very little trub and no “off” flavors and aromas. Strong sulfur or phenolic aromas indicate possible problems with either sanitation or stress. Yeast should be tested for purity if possible and checked for viability and cell density. If there are any concerns over the health or purity of a culture, DO NOT USE IT!

When Yeast Should be Harvested
The timing of cropping will have effects on the quality and density of the slurry. Consistent timing of harvest will help maintain the desired characteristics of the culture.

• Cylindroconical and Dish Bottom Vessels (Bottom Cropping): Yeast should be harvested once the temperature has dropped below 40°F (4°C) and trub has been discharged. This will insure a large yield of clean, homogenous slurry.
• Open Vessels (Top Cropping): Yeast can be harvested once the gravity has dropped below 50% of original gravity. First head will rise approximately 24-36 hours into fermentation. Discard 1st skim (“dirt skim”). A clean, 2nd head will rise which can be harvested with a 2nd skim.

How Much Yeast Should be Harvested
Quantity of yeast harvested will depend on many factors. A brewer needs to consider how many batches he/she will be need to inoculate and how much time there will be between brews. We recommend using an increased pitch rate when pitching harvested yeast from a previous fermentation versus laboratory grown culture (See

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section). Laboratory grown culture will arrive in optimum condition for a quick and healthy fermentation. Yeast harvested from a previous fermentation is coming from rather harsh conditions. The culture can be void of sterols, enzymes, and glycogen, as well having possible poor cell membrane health. When re-pitching harvested yeast, we recommend using the traditional rule of:

1-2 million cells per ml. per degree Plato
Accurate and consistent pitch rates can be calculated after performing a cell count and viability test. When a microscope and proper lab equipment are not available, achieving proper pitch rates involves some guesswork and trial and error. With some simple guidelines to follow, a brewery without a lab can get consistent results when pitching harvested yeast.

Estimates of cell counts can be made using percent yeast solids of the slurry. Percentage of yeast solids per volume of slurry can be estimated by allowing a sample to sediment under refrigeration and estimating the percent solids. Generally 40-60% yeast solids will correlate to 1.2 billion cells per ml. This will vary with the yeast strain. By using this method with every brew, a brewer can achieve consistent pitch rates batch to batch resulting in a more consistent product. The following picture relates sedimentation to cell count.


Once the brewer has determined the desired pitch rate and cell density of the slurry the brewer can collect the appropriate quantity of yeast. Slurry can be harvested based on volume or weight. 1L (1 quart) of yeast slurry (40% yeast solids) weighs approximately 1.1 Kg (2.4 lbs.). The following guidelines will deliver the appropriate pitch rates.

Re-Pitching By Weight or Volume:

• Ales with a specific gravity < 1.064 (16°P): pitch 1.0 Kg (2.2 lbs.) of thick slurry (40% yeast solids) per 1 bbl or 1 Liter (1 quart) of thick slurry per 1 bbl
• Lagers with a specific gravity < 1.064 (16°P): pitch 2.0 Kg (4.4 lbs.) of thick slurry (40% yeast solids) per 1 bbl or 2 Liters (2 quarts) of thick slurry per 1 bbl
• High lagers with a specific gravity > 1.064 (16°P): pitch 3.0 Kg (6.6 lbs.) of thick slurry (40% yeast solids) per 1 bbl or 3 Liters (3 quarts) of thick slurry per 1 bbl

Yeast Storage
Yeast should be used as soon as possible and not stored for long periods before re-use (longer than 2 weeks). Yeast should be maintained between 34°to 36°F (1-2°C) in an oxygen free dark environment. Warmer temperatures and oxygen exposure will increase the rate of culture degradation. A slight amount (<5 lbs) of head pressure can be applied with CO2 to maintain a positive pressure environment. Excessive pressure can be detrimental to the culture.

Yeast Collection/Storage Vessels:
When choosing a yeast storage vessel, factors to consider include: ease and effectiveness of cleaning and sanitation, size/volume, ease of use, access for testing/stirring if necessary, ability to easily purge CO2. Extreme care must be taken when cleaning and sanitizing the storage vessel and all associated parts. Fittings should be disassembled and soaked before every use. Vessels should be purged with CO2 prior to collection to minimize exposure to oxygen. The vessel will need ¼ to &frac13; volume of headspace to allow for slurry expansion during storage and feeding. Vessels should be able to maintain storage temperatures of 34°to 36°F (1-2°C).

Cornelius Kegs / Beer Kegs
Adapted beer kegs or Cornelius kegs make simple and economical yeast collection and storage vessels. Tanks can be stored on ice in a walk-in cooler to maintain an optimal storage temperature of 34°F (1°C). Cornelius kegs can be filled and discharged through the “out” fittings, allowing for closed collection and re-pitching procedures. A closed harvest and re-pitching procedure will minimize the opportunity for air-born contamination and is recommended if the malt mill is in the same room as the brewery or the mash is a dusty procedure.

Cylidroconical Fermenter
Storage of yeast for more than 48 hours in a glycol cooled cylindroconical fermenter is not recommended. Yeast is an excellent insulator and will typically be several degrees warmer in the center of the slurry than at the outside edge. Higher flocculating strains tend to pack more densely in the cone and therefore be more prone to increased temperatures in the center of the slurry.

Yeast Brink
Usually exclusive to larger breweries, a glycol cooled, agitated yeast brink has many advantages over other collection vessels. The agitation of glycol cooled slurry offers precision temperature control. A yeast brink allows for easy yeast washing and QC sampling as well as propagation. A yeast brink can either be mounted on a scale or fitted with a flow meter for delivering accurate and consistent re-pitching volumes.

Harvesting Procedures
Cylidroconical Fermenters:
Conical bottomed fermenters greatly improve the ease and efficiency of harvesting yeast. The conical bottom allows for easy trub discharge. Trub discharge should be performed twice prior to harvesting yeast. The tank should be thoroughly cooled with the yeast settled to a thick slurry before harvesting. When the trub has been discharged and the yeast appears clean, an additional small discharge of yeast should be performed to eliminate the most flocculent and dead yeast.

Yeast should be harvested from the same portion of the yeast bed each time. The harvested yeast should be light in color, creamy in texture, free of trub, and taste fresh with no off aromas. Harvesting from the middle of the yeast bed helps prevent selecting yeast that is either more or less flocculent than the previous generation. Pitching from cone to cone, although sanitary, is not recommended due to the lack of quality control. Cone to cone pitching does not allow for a consistent harvesting method and allows too much variability into the process.

Lager Tank / Shallow or Dish-bottom Fermenters:
Harvesting yeast from a flat bottom tank involves a more difficult and less efficient method for cropping yeast compared with cylindroconicals. Harvesting from this type of tank requires manual collection through a manway with a scoop. Yeast should not be harvested from the bottom of the tank because the yeast will be a mixture of trub and yeast of varying quality. Tanks must be fitted with stand pipes or racking arms that allow for beer removal, leaving the yeast bed intact. Once the beer has been racked, the manway can be opened for access to the yeast bed. The yeast sediment will have three distinct layers as mentioned earlier. The top layer should be scooped away and discarded, leaving the middle layer for harvesting. The bottom layer should be avoided. This method of harvesting will allow for fewer generations of use due to increased risk of infection and harvesting of poor quality yeast. Careful tracking of the fermentation profile and evaluation of yeast performance will help to determine the number of re-pitchings possible.

Krausening:
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. This technique avoids the hazards of manual bottom cropping and will help to maintain a consistent yeast population. The downside to krausening is the difficulty in brew scheduling, tank availability, and the reduction of finished product volume.

Open Top Fermenters / Top Cropping:
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.

Yeast should be harvested once fermentables have been reduced by 50% or more. The timing of the skim should be consistent from batch to batch to help maintain fermentation profile. A head will rise approximately 24-36 hours into fermentation. The first “dirt” skim containing trub should be discarded as should the final skim. The middle skim should be harvested and used.