Dear Mr. Wizard,

I recently got an old refrigerator at a garage sale. Armed with a fancy thermostat, I’m ready to brew some lagers.

I normally use Wyeast smack packs that I grow up in an Erlenmeyer flask to make my starter. Should the smack pack and starter be "fermented" at the same temperature as the lager will be fermented? Should the carboy be allowed to start its fermentation at a warmer temperature before it is placed in the fridge to ferment at lager temperatures?

I’ve been using about 50° F for a fermenting temperature in the fridge and cooling to the upper 30s once fermentation is complete for lagering. What temperatures would you suggest?

Mike Oberbeck
Westminster, Colo.

Mr. Wizard replies:

There are two different schools of thought when it comes to lager yeast propagation and fermentation. The traditional approach is to propagate the yeast and begin fermentation at lager fermentation temperatures. This method results in a slower propagation rate and also tends to start the fermentation off a bit slower compared with warmer propagation and initial fermentation temperatures. Proponents of this method argue that the finished beer tastes better.

The other school of thought basically argues that warmer propagation gives better cell counts at pitching time and that a warm start to fermentation gets things off to a vigorous start. This reduces the chances of a bacterial problem. Although these are both strong arguments, opponents to these methods cite studies showing an increase in fruity aromas in lager beers produced using these warmer temperatures.

Some brewers use a blend of the two philosophies. They propagate at warm temperatures, usually around 68° F, and carry out the entire fermentation at cool temperatures, typically between 46° and 54° F.

When it comes to lagering, there are also several different practices used around the world. The world’s largest lager brewer, the one with the Eagle trademark, ferments its beer near 55° F and holds its flagship brand at the same temperature for its three-week kraeusen process. The idea behind this is that the yeast still have work to do and cooling the beer to very cold temperatures will drastically slow the metabolic activity of the yeast. Other brewers slowly cool the beer from around 50° F to 32° F over several days to allow the yeast to finish its work and to start the beer’s clarification process. Other brewers quickly cool their beer from 50° F to 32° F and have the "don’t waste time" philosophy.

One lagering method that is gaining in popularity, especially in Europe, is to warm the beer to 68° F after fermentation for an accelerated diacetyl rest. After about two days the beer is then chilled to 32° F for lagering. In a commercial brewery tanks are designed to cool beer, not heat it up, and this method requires special equipment. However, at home it is easy enough to carry a five-gallon carboy from one spot to another to change the beer’s temperature, so it’s easy to try this method at home.

All of these methods of propagation, fermentation, and lagering are used successfully by commercial brewers and homebrewers to make good beer. The method of choice really depends on which one seems to work best for the individual brewer.

My personal preference is to propagate lager yeast at 68° F and conduct the entire fermentation at 50° F. When the beer is about 2° Plato (eight to 10 specific gravity points) higher than the terminal gravity, I transfer it to a soda keg and allow the beer to naturally carbonate at 50° F for four days. Then I cool the beer to 39° F for 10 days and finally cool the beer to 32° F for two weeks.

Dear Mr. Wizard,

Is there anything I can do to increase the cell count of my starters without increasing the volume of the starter? I have read that magnetic stir plates increase the cell count, but is it a significant increase?

Jim Bordine
Frankfort, Mich.

Mr. Wizard replies:

Propagating yeast is like growing any other life form; you need to provide a good environment and a supply of nutrients. Growing yeast is relatively simple on a small scale and becomes more difficult as the size of the propagator increases.

The essentials of yeast propagation must include a pure strain of yeast. This means there should be only one strain of yeast present in the starter and no bacterial cells. If unwanted yeast strains or bacteria are present, they will be propagated along with the yeast you really want to grow. The only way to really know if you are starting with a pure culture is to have a laboratory. Since most homebrewers don’t have labs in their basements, it is common to rely on the skills of a reputable yeast supply lab and hope everything goes well. Fortunately for American homebrewers there are several excellent yeast suppliers around the country.

After acquiring a yeast strain to propagate, you need an appropriate nutrient media. Lucky for us brewers, wort is the perfect media for growing yeast. Wort contains amino acids for protein synthesis, nucleic acids needed to synthesize DNA and RNA, essential vitamins and minerals, essential fatty acids for membrane growth, and an abundant supply of fermentable sugars. The three most important points to note regarding propagation wort are specific gravity, hops, and sterility.

The healthiest yeast is produced from moderate wort strengths, usually in the 1.040 to 1.050 range. If the wort gravity is too high, then the ethanol concentration can reach levels that begin to damage the yeast. If the gravity is too low, the growing yeast soon run out of food. Hops should always be added to propagation worts to suppress the growth of certain beer spoilage organisms. Although hops do not suppress the growth of all bugs, they do prevent the growth of many that will grow in wort and beer. The third point to remember is sterility. Always boil the propagation wort for at least 30 minutes and store it in a clean and sanitized container. The best method is to sterilize the wort in the propagation container — canning jars and a pressure cooker work well for this method.

Once you have yeast strain and a sterile, hopped propagation wort in hand, it is time to start the propagation. If you’re starting with a concentrated liquid starter, use approximately one part starter to 100 parts of wort. After inoculating the wort with your starter, it is time to set the environmental conditions. Yeast grows fastest and produces higher cell counts at higher temperatures. If you are after high cell counts, conduct the propagation at 75° to 85° F for ale strains and 65° to 75° F for lager strains. Although these temperatures will stimulate rapid growth, they may lead to the production of funky flavors, so it is important to pick a temperature that produces a good flavor with your particular yeast strain.

The next important growth factor is oxygen. Although yeast grown in full-strength wort (1.040 to 1.050) always produce energy through fermentation (as opposed to respiration), they still require oxygen for the synthesis of sterols and fatty acids. These compounds are the foundation of new cell walls. When yeast are growing they must produce a lot of new cell walls.

Oxygen can be infused into the growing yeast with an aeration stone or can be absorbed from the atmosphere. On a large scale the only way to provide the yeast with adequate oxygen for maximum growth is through some sort of direct injection. However, small-scale propagations can absorb enough oxygen from the atmosphere for good growth. For this to occur, the propagator must not be equipped with an air lock. Sterilized cotton wads have been used for more than 100 years in brewing labs for yeast growth. The cotton wadding allows oxygen to enter the propagation container but traps airborne contaminants. Continuously shaking or stirring the propagator dramatically increases the absorption of oxygen into the wort and increases the yeast cell density. Magnetic stir plates and shaker tables are the two most common ways of incorporating atmospheric oxygen into growing cell cultures used in labs. Shaken or stirred propagation really does make a big difference in the cell count of the slurry.

The last key point to remember is to transfer the growing yeast into a larger container every couple of days to maintain adequate nutrient concentrations for continued growth. The rule of thumb used in many brewing labs is to increase the volume tenfold every two to three days until enough yeast has been produced for brewery use. In the event you produce enough yeast to use in your home brewery but are not ready to brew, simply store the yeast between 32° and 40° F. The yeast should stay alive and well for up to a week under these conditions and will still be good enough to brew with for two weeks.

Mr. Wizard, BYO's resident expert, is a leading authority in homebrewing whose identity, like the identity of all superheroes, must be kept confidential.