A vast array of information is available to brewers today driven by the joy brewers have in sharing their experience. To avoid drowning in this sea of information, it's common to latch onto ideas that are easily within reach. All too often, what we grab onto is misconception instead of fact. As a brewer and engineer, and driven by my fascination with yeast, I have exposed some of these misconceptions through experimentation and research. Hopefully, this summary of yeast performance will be helpful to you as a brewer. More in depth discussion of these topics can be found in my book: Brewing Engineering, and on my blog: www.WoodlandBrew.com.
Misconception: Yeast growth and cell division only occur at the beginning of fermentation.
Fact: Cell division continues throughout fermentation until the wort sugars are depleted.
"[...T]he cells begin to grow and continue to do so until [...] the essential nutrient is exhausted." - "Brewing Microbiology," Edited by Fergus Priest Ph.D. and Iain Campbell Ph.D.
Even at the cellular level, yeast continues to propagate while there is sugar available. Yeast starts in the stationary phase (G0) until the correct nutrients are available. In brewing, this typically occurs when the yeast is pitched into wort. At this point, the yeast enters the growth phase (G1), when cell size increases. After the cell has grown sufficiently and following the S and M phases, a new daughter cell buds, at which point both the mother and daughter cells return to the G1 phase. Growth and cell propagation, therefore, generally continue until wort sugars are entirely converted to other compounds, including new yeast cells and alcohol. There are some exceptions that will be touched on later, but for the most part, while gravity of the wort is dropping the yeast is propagating. (For more information search my blog for "Balling observation")
Misconception: Yeast activity slows at the end of fermentation because the yeast is exhausted.
Fact: Depletion of nutrients slows yeast activity toward the end of fermentation.
You may have heard the analogy comparing a marathon runner to beer fermentation. While yeast activity does slow at the end of fermentation, and an inadequate pitch can result in a stuck fermentation, it's not because the yeast is tired. While a marathon exhausts a runners' resources, wort contains the resources yeast need. Rather than compare yeast growth running marathon it may be more accurate to compare it to being on a cruise with all expenses paid. No worries in the world and nothing to do but eat. How you feel at the end of this vacation might be affected by how much you indulge your taste buds.
Worts with increased glucose content can lead to a stuck fermentation. High glucose content will delay the point at which maltose consumption begins. A low pitch rate will deplete the nutrients before the switch to maltose fermentation making the transition slow or, in extreme cases, impossible. (See "Brewing Microbiology" 2.6.1)
Misconception: Water washing yeast removes contamination and dead yeast.
Fact: Only large debris is removed using water washing.
Water washing of yeast is performed by suspending yeast slurry in water and allowing it to settle into layers. In theory, the heaviest partials will precipitate first, creating a layer on the bottom, and the lightest partials will settle last, forming a layer on the top. The layer on the top is considered to be clean yeast. In practice, dead yeast, live yeast, and bacteria do not separate clearly like oil and water. During active fermentation, some of the living yeast will be suspended in the beer by bubbles of carbon dioxide that they have created. At fermentation completion there will be no further carbon dioxide production. When the yeast are homogenized and allowed to settle the live and dead yeast will all settle out together. Bacteria are much smaller than yeast, so when settling by partial size it will tend to reside toward the top. (See "Yeast Washing Exposed" and "Yeast
Washing Revisited" on my blog for more details)
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Resources:
Brewing Engineering Steven Deeds
Woodland Brewing Research - Blog
The Practical Brewer Professional text on brewing.
Brewing Microbiology Excellent collection of scientific papers relating to yeast.
Principles of Brewing Science Best summary of the science of brewing I have ever found.
Yeast Easy to read book about yeast in brewing.
