Yeast are a fungi and a living organism and breweries treat them as such.
Those yeast produce a lot of different things, like ethanol, CO2, but they also create more yeast and propagation and growth of a yeast strain is a step many breweries do as they make steps in growth.
The goal of fermentation is to produce ethanol, while the goal of propagation is to produce more yeast.
Ordering enough yeast to pitch can be a building block for many of smaller breweries said Victor Pool, the QA/QC Manager at Columbus Brewing in Columbus, Ohio.
“This is a big area I believe small craft breweries are ordering what they think is enough yeast, but it’s not,” he said to members of the Ohio Craft Brewers Association at its yearly meeting. “Give your new pitch a fighting chance.”
Supplying oxygen to yeast helps speed that growth. Some ethanol is produced, but adding biomass is the key.
A yeast company may tell a brewery that a batch of yeast has 1 million cells per milliliter per Plato of wort and a 10-barrel pitch batch is pitchable for under 16 Plato, but Pool points out that may only be viable for a wort at 10 Plato.
“If you order that, you may get enough yeast to pitch into a 10-bbl wort at 10 Plato and most of us aren’t making that low,” he said, indicating that most craft brewers work at 12 Plato and above. “So you need to order appropriately. When I order … I may need to order a 17-bbl pitch to make 10-bbl of wort.”
Pool did point out in the talk that there are positives and negatives of over-pitching and under-pitching as well.
Under-pitching can lead to: under attenuation; increased ester production; increased cell growth;; slower fermentation and a potential stall; and slower diacetyl reduction. Over-pitching can also mean: over attenuation; decreased ester production; a decreased in cell growth ; faster fermentation and a faster diacetyl reduction. Both over and under-pitching can lead to low viability and the ability to re-pitch the yeast for another batch as well.
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