By Sam Tierney, Firestone Walker QC Brewer
I could open with some cliché about yeast making beer and brewers making wort—it always feels so appropriate when I cover yeast in this column, but you’ve heard that one already. Put another way, yeast is the most complex and important ingredient/partner in the brewing process. Most of the flavor of beer is derived from the activity of yeast, and utilizing yeast to the greatest effect is one of the most crucial parts of good brewing. The holy grail of fermentation is adding the proper amount of healthy, pure yeast to properly oxygenated wort at the proper temperature. Easier said than done.
Part of the incredible growth of the brewing industry in San Diego has been due to the presence of White Labs, one of the industry’s leading yeast laboratories. White Labs maintains and sells an incredible array of yeast strains for brewing, winemaking, and distilling, as well as providing brewers with analytical laboratory services and products. At the Craft Brewers Conference in April, White Labs announced a newly revised system of yeast propagation and packaging, called FlexCell and PurePitch, which is the result of five years of research into a more streamlined and effective system of growing and packaging yeast.
A single-celled fungus, yeast reproduces by asexual budding. Daughter cells bud off of mother cells in a continuous process while exposed to food and nutrients. This exponential growth leads to a very fast rate of genetic drift in populations. Breweries are all too familiar with this drift. Depending on how they harvest and store yeast for use in subsequent batches of beer, the fermentation characteristics of the yeast strain continue to evolve until beer character noticeably changes, or yeast harvesting becomes too difficult to continue.
To counter this drift, breweries tend to only reuse their yeast a certain number of times. Some strains can only be used several times, while others can be used a dozen or more times. Some breweries that practice open fermentation and harvest from the top of the actively fermenting beer are able to continue to use their yeast almost indefinitely, as this method tends to place little stress on the yeast. These days, most brewers harvest yeast at the end of fermentation by taking it from the bottom of closed cylindroconical tanks. At the end of fermentation, most yeast strains flocculate (clump together) and fall to the bottom of the tank. The resulting yeast slurry is like a creamy milkshake that can be collected in a smaller vessel and stored until it is needed in the next batch of beer. The problem is that this bed of yeast is stressed by its location under all of that beer and starts to die in a short amount of time. Yeast must be harvested and reused in a short amount of time to ensure the best health and fermentation of subsequent batches.
When a brewery decides to start back at a new yeast culture, they have several options. Many larger breweries maintain mother cultures in-house and can propagate a new pitch themselves. Others acquire their yeast from other breweries. The most common method these days is to purchase a yeast pitch from a lab like White Labs, Wyeast, or GigaYeast. Breweries can buy a yeast slurry directly from these labs that can then be pitched into a batch of beer.
No matter how a pitchable quantity of yeast is acquired, the method of propagation is roughly the same. Yeast cultures are stored frozen, which halts genetic drift in the mother cells. The lab will select a colony of cells from the original culture and then add this to a small amount of sterilized wort. This process is done under sterile lab conditions as any contamination at this point in the process will lead to an unusable yeast culture. For healthy growth, oxygen is added to the fermenting starter beer, which the yeast utilizes to build new cell membranes. After several increasingly large growth steps in the lab, you have enough yeast to ferment a small batch of beer. Yeast labs will then continue to grow their strains to package for breweries, and breweries might step the yeast up further with small starter batches of wort in the brewery, eventually growing enough for a full-scale batch of beer.
White Labs’ new FlexCell process changes this system in that the yeast propagation now takes place in a single vessel instead of multiple lab vessels and brewery tanks. This allows them to save large amounts of water, energy, plastics and chemicals in cleaning and sanitizing multiple vessels, leading to a more environmentally friendly process. It also reduces possible points of contamination during transfer to subsequent vessels. Once propagation is finished, the containers are divided into smaller amounts that then become the PurePitch packaging that is sent to breweries. This results in yeast cultures that have been in one closed, sterile environment from start to finish. This process eliminates the riskiest part of growing yeast, which is the transfer to new vessels. The new PurePitch packaging also differs from the old plastic bottle packaging in that it is a flexible, breathable package, eliminating carbon dioxide buildup, which is detrimental to yeast health. Breweries will see a rollout of the new packaging this month, but White Labs is still looking into utilizing it for its homebrew packaging, which remains the same for now.