Controlling Beer Fermentation and Fermentability (3)

Yeast Requirements During Fermentation

Yeast needs certain things in order to do their job. They need sugars, amino acids, fatty chemicals called lipids, and minute amounts of minerals such as zinc and copper. Fortunately, these are already in most beer worts and it isn’t necessary to add extra nutrients for the yeast to perform well.

The other thing they need is oxygen. They use oxygen to form the cell walls when they multiply. These cell walls are composed of sterols and unsaturated fatty acids. If there isn’t enough oxygen in the beer, the yeast quit manufacturing sterols. The lack of sterols means they can’t synthesize their cell walls so they must stop multiplying.

The lack of oxygen (and sterols for cell growth) is what limits the amount of yeast cell growth in your beer. A lack of oxygen will produce an under-attenuated beer (not fully fermented, leaving unfermented sugars). That’s why it’s so important to oxygenate your wort. Especially in higher gravity beers.

Oxygen is driven off when the wort is boiled. A little is incorporated by transferring the wort to the fermenter in a fashion that allows the wort to slosh around. You can aerate your wort by transferring back and forth from the kettle to the fermenter. A better way is to use an aquarium pump with an air stone. Since you are adding atmospheric “air” to the wort, the oxygen level can never get past a certain level, usually around 8 ppm. This is the minimum amount of oxygen needed for good beer fermentation.

The best way to incorporate oxygen into your wort is through the use of pure oxygen and a diffuser stone (not actually a stone but stainless steel). You can purchase the regulator and tubing with filter at most homebrew supply stores.

The amount of pure oxygen needed will vary depending on the specific gravity or your wort. I usually add oxygen for about 45-60 seconds for worts up to about 1.070 and about 1.5 minutes (90 seconds) for my barleywines. You can over-oxygenate your beer which will cause off flavors like excessive esters. This is a result of “oxidative stress” of the yeast. There is no exact consensus on how much pure oxygen to add so a little research is in order before you use it.

As you start to utilize the tools at your disposal as a homebrewer, such as proper pitching rates, good wort aeration, proper yeast selection and good fermentation temperature control, your beers will make a quantum leap in quality. Do these things right and you will be able to control the attenuation of your beer.  Consistent use will yield consistent results. And that’s what we as homebrewers are shooting for, consistency. After all, what good does it do to make a really great beer just once? If you can’t repeat it then it was just an accident. Take detailed notes of what happens during the entire brewing process from water selection to carbonation results and you will be able to repeat that award-winning beer.