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> Yeast and Starters, contributed by sidney porter
post Aug 4 2007, 10:39 AM
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It is often said that brewers make wort, while yeast makes beer. But how does a homebrewer work with the yeast to ensure proper fermentation? It is helpful to first learn a little bit about what yeast is. Yeast is a single celled non-photosynthetic unicellular fungus. They grow by budding.

Yeast goes through 3 stages during their life cycle.
  • Aerobic (respiration) lag phase - Yeast absorb oxygen, vitamins, amino acids and peptides. Some CO2, Diacetyl, and esters are produced, but the yeast is not producing alcohol. 4-8 hours
  • Anaerobic (fermentation) - Early in this stage the yeast is going through rapid growth through budding. Sugar is converted into alcohol, CO2 is produced. Oxygen is pushed out by the CO2. Can last 3-14 days.
  • Sedimentation - Fermentation slows, yeast cells settle out (flocculate).
It is often published that you want to pitch 1 million yeast cells for every milliliter of wort of every degree plato. The math: yeast X wort X plato= total cells needed. Let do the math for a 5.25 gallon batch with an OG of 1.048. There is approx 20k milliliter of wort in 5.25 gallons, to convert OG to plato 48/4.

1,000,000 X 20,000 X 12= 240,000,000,000. That is 240 billion cells are needed!

Both White Labs and Wyeast publish that they have between 70 and 120 billion cells. They also both state that this amount is amount is acceptable to pitch into 5 gallons of wort. But looking at the math this is 1/3 to of the recommended pitching rates. The reality is that a lot of homebrewers pitch these directly into the wort and make quality beer. But if I am spending $6-8 on liquid yeast I am going to complete a couple of easy steps to get the cell count up. Which leads us to making a how to make a starter.

http://www.mrmalty.com is a great source for determining, how much yeast you need. Not only does the software complete the above calculation it factors in cell viability based up the age and storage conditions of the starting yeast.

A starter is nothing more than a small batch of wort that is created to allow the yeast to grow. The OG of the starter should be in the 1.035 range, 6oz (weight) of DME in 2 qts of water. Boil for 15 mins, cool and add the yeast.

As mentioned above yeast absorbs O2 during aerobic stage. If you have an O2 system you can add O2 now, if not shake the starter ever couple of hours. Another popular option is a stir plate, this allow for continuous additions of oxygen. Keep around 75 degrees for ales, and 70 degrees for lagers. It is not necessary, nor desirable to attach an air lock, remember that the purpose of the starter is aerobic stage and the early growth period of anaerobic. A piece of sanitized aluminum foil is all you need.

Starting with a White Labs tube a 1 liter starter will generate approximately 150 billion cells, a 2 liter starter approximately 240 billion.

There are two schools of thought as to when to pitch the starter into the wort. On option is to create the starter and pitch after 12-18 hours this is right around the time that the cells have reached their maximum density, but before fermentation is complete. Using this option you should pitch the entire starter, liquid and all. The 2nd option is to allow the starter to go 36 hours which should get you through fermentation and through sedimentation. Using this option you can decant the liquid off and pitch just the yeast.

Any container that can be sanitized could be used for a starter. A gallon glass jug (growler) makes a great vessel. My personal favorite is an Erlenmeyer flask; the benefit is that they are made out of borosilicate glass that allows for heating the wort in the vessel and then chilling it with an ice bath. The Erlenmeyer flask also has a flat bottom which allows it to be used with a stir plate. Most jugs will have a concave bottom which makes it difficult to keep the stir bar spinning on the center.

Once you have made a batch of beer you will notice that you have a lot of yeast at the bottom of the fermentor. This could be perfect yeast to use for another batch assuming that the yeast was not put under high stress levels or sanitation issues. Some brewers will use the entire yeast cake. They report quick vigorous fermentation, the problem is that the yeast doesn't go through the lag phase nor the growth phases, both of these are important for flavor development in the beer. A better option is to collect a portion of the yeast. Once again http://www.mrmalty.com can help you determine how much you need to save. Many brewers believe that depending on the strain they get the best flavor development on the 2nd-5th generation of the yeast.

Another option is to use dry yeast. Historically dry yeast was seen to be not as pure. Over the last 10 years there have been tremendous advances in dry yeast. The limitation of dry yeast is the variety of strains. There are some styles of beers that you cannot make because there is not a dry yeast available. There are 20 billion viable cells in a gram of dry yeast 10 grams (typical size of the package) will get you right in the ballpark for a 1.050 beer. It is not recommended to make a starter with dry yeast, dry yeast have already started the aerobic phase prior to the freezing process. A starter would actually deplete the reserves that were created in the lab. You should hydrate the dry yeast with water. What you are doing is allowing water to pass through the cell membrane. If you don't hydrate with water and add the dry yeast directly to the wort many of the cells will burst and die do to the sugar concentration.
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