Yeast Requirements During Fermentation
Yeast requires certain elements to perform its function. These include sugars, amino acids, lipids (fatty chemicals), and trace amounts of minerals like zinc and copper. Fortunately, most beer worts already contain these nutrients, so there is usually no need to add extra supplements for the yeast to perform optimally.
In addition to these essentials, yeast also requires oxygen. Oxygen is used by yeast to form their cell walls during multiplication. These cell walls consist of sterols and unsaturated fatty acids. If there is insufficient oxygen in the beer, yeast will stop producing sterols. Without enough sterols, yeast cannot synthesize their cell walls effectively, leading to a halt in multiplication.
The lack of oxygen (and sterols for cell growth) limits the growth of yeast cells in beer. Insufficient oxygen results in under-attenuated beer, meaning that fermentation is incomplete and some sugars remain unfermented. This is why it is crucial to oxygenate the wort, particularly in higher gravity beers.
Oxygen is driven off during boiling of the wort. A small amount of oxygen can be incorporated by transferring the wort to the fermenter in a way that allows it to slosh around. Another method is to transfer the wort back and forth between the kettle and the fermenter. A more effective approach is to use an aquarium pump with an air stone. By adding atmospheric “air” to the wort, the oxygen level never exceeds a certain threshold, typically around 8 ppm. This is the minimum amount of oxygen required for proper beer fermentation.
The best way to introduce oxygen into the wort is by using pure oxygen and a diffuser stone (which is actually made of stainless steel, not stone). You can purchase the necessary regulator, tubing, and filter at most homebrew supply stores.
The amount of pure oxygen required depends on the specific gravity of the wort. Typically, I add oxygen for about 45-60 seconds for worts up to approximately 1.070, and around 1.5 minutes (90 seconds) for barleywines. It is possible to over-oxygenate the beer, resulting in off-flavors such as excessive esters. This is caused by oxidative stress on the yeast. There is no exact consensus on the appropriate amount of pure oxygen to add, so it is advisable to do some research before using it.
As you utilize various techniques available to homebrewers, such as proper pitching rates, adequate wort aeration, suitable yeast selection, and precise fermentation temperature control, the quality of your beers will significantly improve. By getting these factors right, you can effectively control the attenuation of your beer. Consistently implementing these practices will yield consistent results, which is what homebrewers strive for. After all, there’s little point in making an exceptional beer just once. If you cannot replicate it, then it was merely a fortunate accident. Keep detailed notes throughout the brewing process, from water selection to carbonation results, and you will be able to reproduce that award-winning beer.