Harvesting and repitching yeast is a common practice in many breweries. Depending on the yeast strain, harvesting and storage practices, quality control monitoring, and brewery standards, yeast can be repitched for 5-10 generations. This brewing strategy can spread the cost of fresh pitching quantities of yeast, though the brewer must weigh the benefit of reduced cost with the risks of contamination, mishandling, incorrect pitch rate, or poor culture performance should an issue arise during the harvest or repitch process.
The most important concept in harvesting is consistency. Consistent harvesting and repitching 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 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.
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 repitched. When choosing a tank to harvest from and what culture to re-use, the following guidelines should be adhered to:
- Yeast Generation: Always select the youngest generation of yeast available for harvest. Using fewer generations will minimize opportunities for mutation or contamination.
- Number of Strains at Brewery: Many brewers utilize a wide range of yeast strains for their different characteristics. However, maintaining fewer strains or having a versatile house strain will ensure an active yeast is always on hand to harvest and repitch, especially if a stuck fermentation occurs or brewing plans are changed.
- 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 effects on future fermentations. Do not harvest yeast from beers with alcohol contents greater than 6.5% ABV.
- 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.
- Quality Control: Brewers must have protocols for evaluating yeast viability, purity, and culture health as well as pass/fail standards if harvesting and repitching methods are utilized. Microbiological testing provides valuable information on slurry quality and aids not only in the identification of fermentation issues, but also serves as record of consistency when everything goes according to plan. “Forced” or “fast” fermentation trials are a low-cost option for evaluating harvested yeast health.
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 will be needed 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 PITCH RATES 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 repitching 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.
DETERMINING CELL DENSITY OF HARVESTED SLURRY
Several methods can be selected to calculate cell density of harvested yeast and how much to repitch. The accuracy of each method varies, therefore the brewer must determine which method is best for meeting their brewery’s fermentation and finished product standards.
- Cell count with Hemocytometer
- Spin Down or Sediment evaluation
YEAST CELL COUNTING FOR MULTIPLE BATCHES:
Example Beer: Lager
Wort Density: 12.5 ° Plato
Pitching Rate: 1.5 million cells/mL/ Plato
Total Brews: 4
Number of Brews KO: 3
Number of yeast cells: 48
Squares counted: 5
12.5 °P x 1.5 yeast cells/ml/Plato = 18.75 million cells/mL
48 x 5 x 10 x (3 ÷ 4) = 18.0 x 106 million cells/mL
In our example for a 12.5 °P beer Lager, 18.7 million yeast cells are required and after knocking out the third batch the yeast cell target goal was achieved. The fourth batch can be added on top without pitching yeast and without aeration.
REPITCHING BY WEIGHT OR VOLUME:
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.
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. 1.0 L (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.
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 (1.17 hL) or 1 Liter (1 quart) of thick slurry per 1 BBL (1.17 hL)
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 (1.17 hL) or 2 Liters (2 quarts) of thick slurry per 1 BBL (1.17 hL)
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 (1.17 hL) or 3 Liters (3 quarts) of thick slurry per 1 BBL (1.17 hL)
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 34-36 °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
- 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 1⁄4 to 1⁄3 volume of headspace to allow for slurry expansion during storage and feeding. Vessels should be able to maintain storage temperatures of 34-36 °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.
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.
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 repitching volumes.
The particular method of harvest will depend upon the yeast strain used and brewery configuration, however the principles will remain consistent.
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 repitchings possible.
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.
Commercial breweries using yeast for several generations commonly acid wash the yeast with Food Grade Phosphoric acid 75% (H3PO4) or Chlorine Dioxide (ClO2) before pitching to remove any contaminating bacteria. The acid washing lowers the pH to between 2.1 – 2.5. Temperature should be kept at 34 – 36 F (2 – 4 C), and gentle constant agitation should be applied for 1 to 2 hours prior to pitching. It must be emphasized that Phosphoric Acid does not eliminate wild yeast and if not properly operated this method can cause stress effects to the yeast.
TROUBLESHOOTING ISSUES WITH REPITCHED YEAST
An insufficient evaluation of harvested yeast can cause detrimental issues to the subsequent fermentation and overall quality of the finished product. Common issues include:
- Stuck fermentation
- Low yeast viability
- Not enough yeast pitched
- Poor yeast health/stressed yeast
- Insufficient oxygen and/or nutrients
- Repitched too many generations
- Contamination in previous fermentation or during handling
Solving common issues:
- Thorough evaluation of brewing and harvesting process
- Quality control, viability testing, and microbiological testing points
- Data collection
- Strain performance evaluation specific to brewery environment
- Limit number of generations used per strain
- Know when to start with a fresh culture