Ask Mr. Wizard

Too Much Yeast? Is over-pitching a problem?

TroubleShooting

Jon K. Rodrigo — Manila, Arkansas asks,
Q

It seems like I often hear about the “dire” effects from under-pitching yeast, but can’t say I’ve heard much about the consequences of over-pitching, besides the advice of: “Don’t do it.” Would love to hear what problems are associated with over-pitching yeast, notably over-pitching with yeast slurry?

A

Averbelen et al. (Impact of Pitching Rate on Yeast Fermentation Performance and Beer Flavour, Applied Microbiology Biotechnology (2009) 82:155–167) demonstrated that increasing pitching rates of lager yeast from 10 to 120 million cells/mL resulted in an increase in fermentation rate, decreased net biomass production when pitching exceeded 80 million cells/mL, higher post-fermentation levels of diacetyl, and minimal changes in overall beer flavor and yeast viability after fermentation. This group suggested that reduced cell production with higher pitching rates may present viability and vitality problems in subsequent fermentations because of a higher percentage of older cells in these yeast crops. A key takeaway from this study is that fermenter residency time for lager beers can be reduced by increasing pitching rate above 10 million cells/mL with minimal flavor differences in the finished beer.

Performance and Beer Flavour, Applied Microbiology Biotechnology (2009) 82:155–167) demonstrated that increasing pitching rates of lager yeast from 10 to 120 million cells/mL resulted in an increase in fermentation rate, decreased net biomass production when pitching exceeded 80 million cells/mL, higher post-fermentation levels of diacetyl, and minimal changes in overall beer flavor and yeast viability after fermentation. This group suggested that reduced cell production with higher pitching rates may present viability and vitality problems in subsequent fermentations because of a higher percentage of older cells in these yeast crops. A key takeaway from this study is that fermenter residency time for lager beers can be reduced by increasing pitching rate above 10 million cells/mL with minimal flavor differences in the finished beer.

Empirical observations with pitching rates less than 10 million cells/mL suggest that ester and higher alcohol levels increase as pitching rates decrease and total cell growth increases. Restricting pitching rates when brewing certain beer styles, such as hefeweizen, is one method used to increase fermentation-related aroma compounds. One downside of increasing yeast density for certain styles of beers may be reduced ester production.

In practice, high pitching rates can be difficult to achieve when growing liquid yeast for first generation use because high pitching rates require a relatively high percentage of yeast pitch. Many brewers, especially homebrewers, grow yeast in wort that is different from the beer being brewed. And most brewers pitch the entire slurry of propagated yeast. This means that higher pitching rates will likely dilute the wort brewed for a particular beer with propagation wort. This dilemma is easily addressed by homebrewers and smaller commercial breweries by using dry yeast.

Dried yeast use is not as common in larger commercial breweries because of the cost of dried yeast in comparison to yeast produced in propagation system. There are also general concerns about re-pitching yeast crops from fermentations started from dried yeast, making liquid starts the most common in larger commercial operations. These brewers can concentrate cell density by allowing propagated yeast to sediment or they can increase cell density using a centrifuge. I am not aware that either of these methods is commonly used by commercial breweries to concentrate cell density following propagation. However, pitching high-density yeast slurry harvested from a fermenter is common and allows brewers to increase pitching rates without excessive wort dilution (harvested slurry typically contains approximately 1 billion cells/mL compared to slurry from a propagation system with approximately 100-150 million cells/mL).

Aside from better equipment utilization, another practical benefit to increasing yeast pitching rates is faster net uptake of wort nutrients by yeast, faster pH reduction, and a faster increase in ethanol concentration. All of these changes reduce the chances of beer spoilage related to wild yeast and bacteria.

In answer to your basic question, there are no major, stop-the-press, problems associated with increasing yeast pitching rates. Most practical brewers are conservative when it comes to changing brewing methods. If you want to experiment with upping your pitching rate, consider moderate increases over time to determine how these changes affect finished beer quality and harvested yeast health. In my book, slow and steady wins the race!

Response by Ashton Lewis.