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What is yeast starter?

What is yeast starter?
 
A yeast starter is nothing more than a very small batch of wort, usually between a 1,000 and 2,000 milliliters of fermentable sugars. The use of DME (dried malt extract) is one simple and effective way of creating wort for your starter. This starter provides an easy way to increase the number of healthy yeast cells.
 
The yeast cell count after creating a starter can only really be calculated by a laboratory with the proper testing equipment. However, according an article written by Jamil Zainasheff (05/24/2007), adding one tube of White Lab's yeast to a 2-liter starter will produce approximately 240 billion healthy yeast cells.
  
Many experienced brewers suggest about 1 million healthy yeast cells per milliliter of wort per degree Plato-Supplier Of Beer Yeast.
 
Most American home brewers measure their wort with a hydrometer and get specific gravity readings. For this discussion we can estimate our specific gravity being 1.004 for every degree Plato. So to get an equivalent degree Plato from our specific gravity reading, simply divide the number on the right of the decimal point by 4 and that should give you your degrees Plato. An example is a specific gravity measurement of our wort being 1.048 would convert to 12 degrees Plato-Supplier Of Beer Yeast.
 
A five-gallon batch of wort contains 18,927 milliliters so we will round up to 19,000 milliliters of wort when making the English to Metric conversion.
 
Using the above information, we can plug the numbers into the formula to calculate the number of needed healthy yeast cells to pitch into our wort-Supplier Of Beer Yeast.
 
1,000,000 X 19,000 X 12 = 228,000,000,000
 
That means we need 228 billion healthy yeast cells ready to convert our wort to beer. The most common liquid yeast used by home brewers comes from White Labs or Wyeast. The tubes and smack packs on average contain about 100 billion yeast cells. You can see we really need a little over 2 times that number for our beer.
 
Using two tubes of White Labs yeast or two smack packs of Wyeast will provide the approximate correct number of yeast, but at twice the cost. This is where a yeast starter comes into play in home brewing.