by Christopher White
Most brewery fermentations are carried out with reused yeast, but the question of how to store and maintain it frustrates even the most skilled brewers. It actually is not as difficult as some believe, and there are techniques that brewers can use to significantly lengthen the life span of their yeast.

The fact that we can take a byproduct of beer production - yeast - save it, and reuse it in successive fermentations is quite unique. We can do this because yeast is still alive and healthy after most beer fermentations. The low alcohol level in beer prevents the yeast from dying, as yeast does in wine production. The problem for most brewers, then, is not whether to reuse yeast but how to store it and keep it healthy for future brewing sessions.

Yeast is a living organism and is most happy and healthy when feeding on wort sugars. When fermentation is complete, yeast cells flocculate to the bottom of the fermenter. They then go into a resting state. Yeast under beer is fairly stable, and most brewers agree that the best place to store yeast is under beer. But two crucial factors are temperature and time.

The yeast cake at the bottom of a conical fermenter can rise in temperature. Yeast is an excellent insulator, and heat can build up in the middle of the slurry, 10° to 15° F above the beer temperature for very flocculent strains. When yeast heats up, its life span plummets. If the cone is not chilled, effects are even more significant. For this reason, brewers try to remove yeast slurry shortly after fermentation is complete and the beer is chilled.

Once yeast is removed, you ideally should use the yeast again immediately. This allows little time for yeast to deteriorate and die. But this is often not possible, as you might not brew another beer until the next week. The most common way to store yeast is to put it into five-gallon, stainless steel soda kegs. These work well, and the lid can be modified to your desire. But the two problems with these kegs are the many small parts and gaskets that can harbor bacteria and the fact that they do not vent pressure unless modified in some fashion.

Carbon dioxide can build up quickly in yeast slurry, and if kept under pressure will cross the cell walls and kill yeast cells. Pressures over 35 psi can be toxic to yeast, and soda kegs are rated over 100 psi. So if you use these kegs, shake and vent pressure regularly, at least once per day.

Other vessels can be used for yeast storage. Brewers often shun plastic, because it scratches easily and scratches can harbor bacteria and wild yeast. But plastic actually can be a good choice. Be sure to use a high-grade and food-grade plastic (polyethylene, polypropylene), and be sure the buckets are used exclusively for yeast storage. The advantage of plastic is the fact that the yeast slurry is visible, so you can evaluate the condition and quantity of yeast by sight.

With most containers if you pull off yeast slurry and it is very runny, you will be unsure of how much yeast to use in the next batch without counting under a microscope. By using a plastic bucket to store yeast, you can see how much yeast settles out and pitch accordingly.

Plastic buckets also need to be vented occasionally. But if you use plastic, be careful about air contact. Use a lid that screws on tight or at least seals well.

Storage Time

How long can yeast be stored? It's best to use the yeast in one to three days. Again, this is often not possible, especially if multiple strains are being used in the brewery. The magic number seems to be two weeks. If you reuse your yeast in less than two weeks, you usually will have no problem. At two or three weeks you may or may not have problems. After four weeks, the viability of yeast slurry is usually 50 percent or lower.

As yeast cells sit in storage, they consume their glycogen reserves. Glycogen deprivation weakens their cell walls and makes them more susceptible to rupture. Cold temperatures retard this process, but you should avoid freezing yeast, as ice crystals also will rupture cells. The ideal storage temperature is 33° to 38° F. When yeast cells rupture, they release their contents into the liquid phase. Bacteria can feed off the nitrogen released in this process and multiply rapidly. So the yeast slurry needs to be as free of contamination as possible when stored. Cold temperatures also will help retard bacterial growth.

To be confident, brewers should test their yeast before using it. The yeast slurry should be tested at least once after storage and before use. Check it for viability and for possible contamination. Ideally you should use yeast that is more than 95 percent viable, but most craft brewers just compensate for lower viability by using more slurry. This can be successful but also can lead to problem fermentations. The overall health of the yeast may be low, so the slurry may not produce the expected range of flavor and aroma compounds and may not attenuate correctly.

Do not use yeast that tests to less than 50 percent viability. To check for viability, you need a microscope. If you don't have one, you can add 10 mL of yeast slurry to 1 liter of wort. You should observe normal lag time to onset of fermentation (five to 15 hours). If the lag lasts longer than you see in the brewery, compensate by using more yeast (you need to conduct this test the day before brewing).

Always keep extra, unused yeast on hand in case you encounter a problem with the yeast you intend to use. If you do not have extra yeast, call your yeast supplier. Suppliers are used to these situations. It's best to test the yeast a few days before brewing, because if you wait until the day of brewing it can be very difficult or impossible to get a replacement batch.

Contamination

To test for contamination, plate the slurry onto specialized media three to five days before use. You should check the yeast slurry for aerobic bacteria, anaerobic bacteria, and wild yeast. Of the three, anaerobic bacteria are the most common bacteria found in brewers yeast slurry and they are also the hardest for a brewer to irradicate. The most common anaerobic bacteria are the lactic acid bacteria, Lactobacillus and Pediococcus.

Remove a 10 mL sample of yeast slurry, dilute it 1:100 with sterile water, and plate 0.1 mL to 1.0 mL of it on suitable media. The types and procedures for this are numerous, but if bacteria counts are over 1 per mL and wild yeast is over 1 per 0.1 mL, the yeast slurry should not be used.

The best thing to do for yeast after it has been stored for two weeks, if it tests clean, is to add some fresh wort before using it. This helps to restore yeast strength and ensures a successful fermentation. Simply pour off beer that has separated from flocculated yeast, add fresh wort at 9° to 12° Plato, and let it sit at room temperature for 10 to 20 hours. Assuming yeast activity was evident in this "starter" or "activator," pitch into fresh wort as usual. This is also useful for slurries that are 55 percent to 90 percent viable; this method can "nurse" the slurry back to health.

Storing yeast under water, as opposed to under beer, is becoming more popular. Sterile distilled water storage puts yeast in a resting state, and some reports suggest yeast can be stored in this manner for years, with no refrigeration. Storage under water is generally done with small quantities of yeast, which are then propagated in a lab. But it is possible that this can be applied to storage of yeast slurries. Some brewers are now trying this. The key is to use sterile distilled water and wash the yeast slurry several times in the sterile distilled water to remove any traces of beer. This is best done with a centrifuge, but that is impractical for most craft brewers. White Labs has had mixed success with sterile water storage, so time will tell if this procedure will work for craft breweries.

Brewers have always reused yeast in brewing, long before they knew yeast was responsible for beer production. In fact the continual reuse of yeast has led to the impressive genetic variety of brewing strains and to their suitability for brewing. For most of history yeast has been skimmed from the top of fermentations and reused. Today we usually use conical bottom fermenters that aid in cleaning and yeast collection.

While these vessels help in yeast collection, the quality of yeast that is collected is not as good as from top cropping. Top-cropped yeast rises at a particular time in the fermentation, has a high viability, and is relatively free from trub. When yeast is forced to the bottom of a conical fermenter, it mixes with dead yeast, trub, and bacteria. This means it's important to be careful when collecting yeast, to store it for short times, and to test it before reusing it. With careful attention to these parameters, you should get five to 10 generations of high-quality yeast.

Christopher White is president of White Labs and is a chemistry and biochemistry lecturer at the University of California, San Diego. He has a B.S. and Ph.D. in biochemistry. 

Published in the September 2000 issue of BrewPub Magazine.