When is oxygen okay?

When is oxygen okay?

Q:  I am somewhat confused about when it is okay to aerate your beer. I’ve heard that oxygen is important for fermentation, but racking to clarify beer can introduce unwanted air and expose beer to infection. What gives?

 

A: These questions point out one of the biggest dilemmas that brewers face. On the one hand, anytime you expose cold wort or beer to air, you risk infection. On the other hand, you sometimes have to move the beer around to accomplish something important.

Racking: The main reason for racking beer after primary fermentation is not to clarify it but to separate the beer from the layer of sedimented yeast on the bottom of the fermentor. When fermentation is drawing to a close, the yeast cells sense that their sugar supply is dwindling. They quit fermenting sugar and instead concentrate on building up their reserves of glycogen, looking to survive the lean time ahead. Many of them also flocculate at this time and drop out of suspension. They lie dormant at the bottom of the vessel.

Even in this dormant phase, however, metabolic activity continues, albeit slowly. Eventually, the yeast cells use up their glycogen reserves and face starvation. At this point they resort to what can only be called cannibalism. They excrete enzymes that dissolve the walls of neighboring cells, thus making available the nutrients that those cells contain. This process is called autolysis. It is easy to identify autolysis by examining the yeast slurry. Autolyzed yeast darken and take on a red or orange tinge. Autolyzed yeast also give off a sulfury odor that is much stronger and less pleasant than the clean, yeasty smell of a healthy yeast crop.

How long does it take for autolysis to begin? That depends on temperature and the strain of yeast. The colder the yeast layer, the longer it will stay in good condition. At the Saint Louis Brewery, we crash cool our ales (drop the temperature from 70 °F (21 °C) to 34 °F (1 °C) over 24 hours) as soon as fermentation is over. The yeast drop to the bottom and are usually good lot repitching for about a week. It takes at least three weeks for autolyzed flavors to noticeably taint the beer. If we were unable to crash cool, however, autolyzed flavors would develop much more rapidly — most likely in less than a week.

Because most home brewers lack the facilities to crash cool their beer after fermentation, I generally recommend racking as soon as most of the yeast has settled to the bottom, three to four days after the end of primary fermentation. If you exercise caution and thoroughly clean and sanitize all equipment that touches the beer, the risk of picking up an airborne infection during transfer is minimal. Let experience be your guide. If your friends are having no problems with infected beer, then I would recommend they stick with racking.

Wort aeration: It is absolutely vital that the wort be saturated with air immediately before or after pitching. Yeast need oxygen to grow, and a strong growth of young, actively fermenting yeast cells will rapidly choke off most competing organisms (infections, from our point of view). They do this by creating conditions that make it difficult for their competitors to survive and by using available nutrients for their own growth.

Any method you can use to ensure complete aeration of the wort will almost certainly produce a cleaner flavored beer than you would get without it. The test here is your lag period. Assuming you pitch a large enough yeast slurry or starter, ale yeast should show vigorous fermentation in 8 hours or less. Lager yeast may take longer (as much as 16 hours) because of the cooler temperature.

Having established that aeration is vital, I would add that a system that is capable of forcing more air through the wort in a shorter period of time, and that can do so without requiring exposure to airborne dust, is preferable to the less certain and riskier method of pouring wort back and forth between two containers. In my own home brewing, I was never able to get as good aeration by your method (judging by lag times) as I was with my aerating stone setup (2,3). I don’t want to exaggerate the risk of infection that your method poses, but because the stone works better and is less dangerous, why not try it?

And so back to where I started this reply. In working out a brewing method — which includes putting together a set of equipment — a brewer faces a lot of choices. Often, no answer is perfect. To minimize one danger, you may have to incur another. But experience and understanding will enable you to weigh the risks that each choice poses, and good techniques and equipment can greatly increase your chances of success.

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