Give Your Yeast a Good Home

The flavor of your award-winning homebrew owes itself primarily to the yeast that was used to ferment it. But how can homebrewers with limited time and resources be sure that the yeast they are using is up to the job? A little knowledge about yeast and a few easy techniques can go a long way toward improving your brew. Here’s how.

The yeast that we desire must be

  1. in sufficient quantity
  2. healthy
  3. free of other microorganisms

It is up to the yeast supplier to provide a healthy and pure yeast culture, and it is usually up to the brewer to both grow and take care of this yeast. Yeast cultures may be purchased in one of three forms: dried, liquid, or on a slant.

Liquid Diet
Dried yeast is probably the easiest to use. The trick to working with dried yeast is in rehydrating the yeast and pitching enough of it for a vigorous fermentation.

The best way to add dry yeast is by hydrating it approximately one hour prior to use. There are several ways to do this. Here’s one:

Boil 400 to 700 ml of tap water in a 1,000-ml flask. Remove the flask from the heat. Place a cotton plug in the top. Spray the cotton with 70 percent ethanol (grain alcohol mixed with water), and place the flask into a pan of ice water. This produces bacteria-free, cool water in about 20 minutes. Next, shake the flask to aerate the water, then pour the yeast into this pre-boiled water.

If you believe that this boiling/cooling step is unnecessary or too much work (as many homebrewers do), you may just rinse a glass with 25- to 50-parts-per-million iodophor solution and use regular cool tapwater.

Liquid “smack packs” are a popular selection among homebrewers. These liquid-yeast packages contain yeast that is healthier than dry yeast, and a larger percentage of the yeast cells in a liquid pack are alive. According to Wyeast Laboratory, a liquid yeast package that is less than 30 days old contains approximately 100 million yeast cells per milliliter of solution (a total of 5 billion cells for a 50-ml pack). Viability of more than 90 percent is common. Viability decreases quickly, however, and after 30 days of storage may be as low as 50 percent (50 million cells per milliliter).

Additionally, the smack pack contains a small pouch filled with wort (yeast starter solution). When this pouch is ruptured, the yeast in the package will grow to approximately 500 million cells per milliliter (a tenfold increase). If one pack of yeast were pitched into a five-gallon batch of beer, the total concentration of yeast cells would be 1 million to 2 million cells per milliliter.

If the brewer is pitching into a low-gravity, well-oxygenated, cool wort, this might suffice. Otherwise it will be necessary to propagate more yeast. One very simple procedure that can be used to increase the pitching rate follows. To use this method you will need a 1,000-ml Erlenmeyer flask with drilled stopper (cost $7 to $9, available at homebrewing or lab
supply stores), some cotton balls, a fermentation lock, dry malt extract, a small scale, a funnel, a six-quart or larger pot, your kitchen stove, and a 70 percent ethanol solution (diluted grain alcohol).

First, measure three ounces of dry malt extract and pour it through the funnel into the flask. Fill the flask with 750 ml of very warm water, then stir until mostly dissolved. Heat this wort solution, stirring occasionally, until boiling. Boil for 15 to 25 minutes.

Place the stopper with the fermentation lock on the flask and move it from the stove into a pot filled with cool water. The steam from the wort should kill any bacteria on the stopper and in the fermentation lock.

Spray a cotton ball (or dab it) with the ethanol solution and put it in the top of the fermentation lock. As bacteria from the air is drawn into the cooling wort, it should be killed by the ethanol solution. After about five minutes remove the cooling water from the pot and add ice water.

When the solution has cooled to 65° to 70° F, shake it vigorously to aerate it. This is now a sanitary (though not quite sterile) starter solution and should be pitched immediately after aerating.

When adding the yeast to this starter, be sure to sanitize the outside of the yeast package. Iodophor may be used to do this, although the iodophor must remain in contact with the package for an extended time to do its job.

Here’s an option: Instead of rinsing the outside of the package with a sterilizing solution, use a lighter. Sterilize the scissors or knife with the flame, open the package, then sterilize the opening with the flame. Once the yeast is in the starter, give it a shake or two. It should take off fermenting in four to 12 hours.

Billions and Billions

This 700-ml starter will eventually contain 60 million to 80 million cells per milliliter at high kraeusen (peak fermentation). With this concentration of yeast, approximately four to six liters of beer could be pitched at the proper rate of 10 million to 12 million cells per milliliter.

The rule of thumb in the brewing industry is to use a tenfold increase in volume when propagating yeast. If you were to use this starter to pitch a five-gallon batch (19 liters), the beer would be underpitched by three to five times. This is better than just adding the smaller amount of yeast provided by the manufacturer, but if you want to do better, another starter of a larger quantity or several 700-ml starters can be made from the first starter.

Yeast may also be purchased in the form of a slant. Slants are small tubes half filled with a wort or “yeast nutrient” solution. This solution has been mixed with agar (a solidifying agent produced from Japanese seaweed), usually at a ratio of 12 grams of agar to one liter of wort.

Yeast is added to this nutrient solution as it cools. To use this form of yeast, the brewer needs an inoculation loop, a flame source, and a sterile wort starter. These items may be obtained from yeast culture suppliers or lab suppliers.

The first stage of yeast growth is begun by simply flaming the inoculation loop, allowing it to cool within an inch or two of the flame, rubbing some yeast on the loop, and quickly transferring it to the sterile starter.

The cap to the starter should be placed tightly on and loosened each day to allow carbon dioxide to escape. After two or three days, vigorously shake the starter to bring all the yeast into solution, flame the outside of the small starter bottle, and pour the solution into a 700-ml starter.

Sterile wort starters may be either purchased from a supplier or made by the brewer. If the brewer is going to make a starter, an all-malt wort of approximately 1.040 original gravity works well. This may be prepared by mixing two ounces of dry malt extract with 500 milliliters of water. After boiling and cooling, pour the clear liquid off the solid hot-break material. This clear liquid may then be poured into test tubes, capped loosely, and heated in a pressure cooker. After the pressure cooker cools, tighten the lids to the test tubes. This sterile wort starter should be kept cool until used.

Yeast Storage and Handling
The best place to obtain yeast for your next award-winning batch is from your previous batch. After a batch ferments, there are approximately four to eight times as many yeast cells as you need to pitch the next batch. Taking this healthy yeast from the bottom of your fermenter and using it again is both easy and free. Here is some advice on keeping this yeast in the best condition possible.

If you don’t remove the hot break (the brown, snowflake-looking stuff) from the bottom of your kettle, you will probably want to transfer the beer after about three days of fermentation. This will remove the solid matter, the proteinaceous trub, from the fermentation and keep it out of the finished beer. It will also remove any hops, as well as dead or early flocculating yeast cells.

When transferring the fermenting beer from one fermenter to the other, try not to splash it, which would introduce oxygen. The receiving fermenter may even be purged with carbon dioxide, if available. The result is a layer of pure, live yeast at the end of fermentation.

When fermentation ceases (indicated by no bubbling of the fermentation lock and a final gravity of roughly 1.005 to 1.015), it is time to transfer the beer to the bottle or keg. Siphon the beer off the yeast and have a sanitized food storage container (such as Tupperware) nearby.

Leave about one-quarter inch of beer on top of the yeast and swirl this beer around until it is saturated with yeast. Quickly pour it into the food storage container and cover with the sanitized lid. Do not put it in a glass jar. Some fermentation may still take place and the jar could explode! Place the container in the refrigerator.

Try using a plastic container with a small hole in the lid. Cover the hole with cotton and spray it with 70 percent ethanol. If stored at 32° F to 34°F, it should last for about two weeks. If you desire, you may make another starter and pitch it with just a small amount of the yeast slurry you have collected, or you may just dump the slurry into your next batch of beer.

When storing yeast, it may be necessary to occasionally shake the yeast solution. This will keep hot areas from forming and add oxygen to the slurry, thus slowing the rate of yeast death. Beware, shaking the yeast will cause an increase in pressure as the carbon dioxide within the solution is released. Liquid yeast slurry should be stored as close to freezing as possible, while dry yeast should be frozen for optimal storage. Dry yeast may last up to 18 months if frozen, although viability will decrease significantly.

Evaluating Yeast

Brewers have several ways to determine whether the yeast they are using is up to its fermenting task. The primary method is to stain it with methylene blue solution and observe the yeast under a microscope. Yeast cells that stain blue are dead.

The best way for a microscopeless homebrewer to determine the health of a sample of yeast is through observation. If a strain of yeast is used over and over again to ferment beer, it may eventually show signs of wearing out. The flavor of the beer it produces is the most obvious (and the most important) sign of the health of the yeast.

The percent change in gravity, called the apparent attenuation, is another indicator. If similar mashing procedures are used, the terminal gravity of successive batches of beer fermented with the same yeast should be approximately the same.

If the terminal gravity rises, it may be due to premature settling (flocculation) of the yeast. The clarity of the beer is mainly due to yeast flocculation and is an indicator of yeast performance. If beer is less clear than it used to be, it may be due to a change in the flocculation characteristics of the yeast.

Finally, the amount of time for fermentation to begin and to be completed is another indicator. If fermentation is slow to begin, it is probably due to underpitching but may also be a result of the loss of yeast viability.

Yeast is one of the most important flavor contributors to beer. Consistency can only be achieved by using a similar yeast strain under similar conditions. Knowledge, experimentation, and record keeping will help any brewer learn to use yeast over and over again to create consistent, award-winning beers.

Enough Is Enough
Dried yeast is easy to use, but it’s important to pitch enough. According to the suppliers of the dried yeast, a typical sample contains 20 billion  (2 x 1010) cells per gram. That’s four yeast cells for every man, woman, and child on earth in each gram of dried yeast!

The viability (percent of living   cells in this sample) is 60 to 80 percent. Using an average viability of 70 percent, this means there are 14 billion (1.4 x 1010 ) living cells per gram of dried yeast.

To determine how many grams of yeast to pitch per liter of wort, start with a target pitching rate. The rule of thumb is to use 1 million (1 x 106) cells per milliliter of wort for each degree Plato. Plato is another way to measure specific gravity.

Since most beers begin with a wort original gravity of 12° Plato (1.048), a good target pitching rate is 12 million cells (1.2 x 107) per milliliter of wort. This is the same as 12 billion  (1.2 x 1010) cells per liter.

With these numbers in mind, the grams of dry yeast to be added to a brew may be determined by the following:

Grams of dried yeast required / liter of wort   =

(1.2 x 1010 cells/liter of wort) / (1.4 x 1010 cells/gram dried yeast)

Thus, to add yeast at the concentration of 12 million cells per milliliter, 0.86 grams of dried yeast per liter of wort (0.11 ounces/gallon) must be used.

Issue: October 1996