HAPPY YEAST, HAPPY BREWER, HAPPY DRINKER
For the purists, beer consists of four ingredients: water, malt, hops, and yeast. While many breweries these days have deviated from the strict measures of the 1516 Reinheitsgebot–some farther than others–one cannot technically have beer without each of these ingredients. Each has its nuances, but perhaps the most difficult for the brewer to control is yeast. The saying “brewers make wort, yeast makes beer” rings true. It is the brewer’s job to set up the yeast for success in making the final product that he or she desires. A brewer has to pay special attention to the yeast prior to pitching into wort, throughout fermentation, and after the beer is finished to maintain its viability and vitality, in order to produce the beer that they envision.
As a living organism, the main goal for a yeast cell is to grow and reproduce. However, the conditions in a fermenter are not the ideal conditions for yeast to meet their goal. In an ideal setting, yeast would be at atmospheric pressure, an ambient temperature in the realm of 80-90 °F, abundant oxygen, and a low alcohol concentration. Compare these conditions to those in a cylindroconical fermenter: higher pressure, lower temperature, limited oxygen, and unhealthy alcohol concentrations.
In our fermenters, the hydrostatic pressure (the pressure exerted by the weight of liquid) experienced by yeast at the bottom of the tank is 2-3 psi, and later in fermentation, the tanks are pressured in addition to that. This pressure reduces the yeast’s viability. Making beer with the desired flavor profile for each style requires controlling the temperature of the fermentation. The biochemical reactions taking place inside the yeast cells produce a lot of thermal energy, and, if left unchecked, will drive too quick of a fermentation, production of fusel alcohols and esters, and can be lethal to the yeast. To control the fermentation, brewers use lower temperatures, often tens of degrees lower than those for maximal yeast growth. Oxygen is critical for yeast reproduction, but is undesirable in finished beer. So, the brewer has to provide enough oxygen to the wort to help the yeast grow initially, but not so much as to promote excessive biomass, which reduces the final yield of a brew. Generally, oxygen in the range of 8-10 parts per million is sufficient for ales, with lagers and high-gravity wort requiring more. Finally, yeast in the wild are not generally swimming in their own excrement, so the concentration of alcohol that they experience is relatively low. With the trend of big, boisterous, high-ABV beers, like imperial stouts, barley wines, and triple IPAs, brewers often push yeast to the limit of what is survivable and what is toxic. Like with most things, the key is balance.
TIPS FOR SUCCESS
Consistent Pitching = Consistent Fermentation
On the fermentation side, the key to producing consistent beer is consistently pitching the same amount of viable yeast each time. What the right amount is is somewhat dependent on the particular yeast strain, but a good starting point for an ale is 750,000 viable cells/mL/°Plato, and for a lager, 1,500,000 viable cells/mL/°Plato. Determining cell number and viability requires a microscope and hemocytometer (cell counter), along with a viability stain, such as methylene blue. Once the number of cells/mL and the percent viability are known, the density of the slurry can be approximated, and a weight of slurry to pitch is calculated for a given volume of a given gravity of wort. To approximate the slurry density, you can use the density of water (1 g/mL), plus the mass of 1 yeast cell (8×10^-11 g/cell) times the counted number of total cells/mL in your slurry.
Prompt Removal and Proper Storage
Once fermentation is complete and the beer has been cold-crashed for conditioning, it is important to remove the yeast from the cone within 48 hours. Cold-crashing promotes flocculation and settling of the yeast, that was once in suspension throughout the fermenter, to the bottom of the cone where the pressure is greatest. As I mentioned above, high pressure is unfavorable to yeast cells, so removing the yeast from the cone sooner removes the pressure from them and prevents their viability from declining. The best yeast cells will be in the middle of the cone–highly flocculent and dead cells will have settled first and will be in the very bottom of the cone, while the least flocculent cells will be on the top of the yeast cake. After removal, store the yeast in a cold environment (33-38 °Fahrenheit is ideal), and allow the pressure to escape through an airlock or blow-off hose. Even under these storage conditions, viability decreases with time, so plan to use the yeast again within 2 weeks, ideally sooner, and if it won’t be reused within 1 week, feed the stored culture with fresh wort or a sterile malt extract solution of around 10-15 °Plato.
Keeping a Pure Culture
Each time a yeast culture is transferred from one vessel to another, there is a risk of contamination with other microorganisms. To keep the culture pure, make sure that everything used in the transfer is properly cleaned and sanitized, and wear gloves and a face mask to prevent transfer of microbes from your skin and breath into your yeast culture, especially if you don’t have a closed system. It also doesn’t hurt to spray any connections and your gloves with 70% isopropyl alcohol as a secondary sanitization step. A step beyond a clean transfer is to acid wash the yeast cells. This will help remove any bacteria that might be resident in the culture, as well as remove any bits of trub.
Following some simple steps allows a brewer to maintain a happy and healthy yeast culture that will return the favor by producing great beer. Another great practice is to collect data on every batch to become intimately familiar with how each yeast strain behaves for your applications. Measure the gravity, temperature, pH, and cell count daily, and make plots to visualize how your fermentation progresses. The plots from batch to batch of the same beer should, for the most part, overlay, and if they don’t you can look back to see what might have gone differently and make adjustments in the next brew.
This article only begins to cover the microcosm of brewer’s yeast. For a deeper look, I recommend Yeast: The Practical Guide to Beer Fermentation by Chris White and Jamil Zainasheff, the main reference for the topics discussed here. It is a great read for the home brewer, professional brewer, and brewery scientist alike.