From the Mr. Malty link:
-The starter’s purpose is to create enough clean, healthy yeast to ferment your batch under optimum conditions. The primary focus of a starter should always be yeast health first and increased cell growth second. Many brewers mistakenly focus on cell growth at the expense of yeast health. It is much better to have a smaller number of very healthy, young cells than it is to have a large number of weak, cells.
-Do I always need to make a starter? No. However, in many cases, making a starter will provide better fermentation. You should always make a starter if you suspect the viability (overall health) of your yeast might be low. If you have an old vial or pack of yeast or the yeast has been left out warm for an extended period of time (for example yeast in shipping for several days), you should make a starter.
-According to both White Labs and Wyeast, a White Labs Pitchable Yeast vial and a Wyeast ACTIVATOR™ 125 XL Smack Pack both contain an average of 100 billion cells and are enough to pitch directly into 5 US gallons (18.9 liters) of an ale wort at 1.048 SG (12°P). This is a pitching rate of 5.3 million cells per milliliter, which is close to the pitching rate many professional breweries begin with when starting a new pitch of ale yeast. This rate works well because the health and vitality of fresh laboratory cultured yeast are superior to yeast harvested from normal fermentation. Both companies also concur that higher gravity worts, especially once they exceed a specific gravity of 1.060 (15°P), larger wort volumes, and lager fermentations all require higher pitching rates (or a starter) for optimum results.
-Here is the simple math to calculate the number of cells needed. For an ale, you want to pitch around 0.75 million cells of viable yeast, for every milliliter of wort, for every degree Plato.
-(0.75 million) X (milliliters of wort) X (degrees Plato of the wort)
-There are about 3785 milliliters in a gallon. There are about 20,000 milliliters in 5.25 US gallons.
-One degree Plato is close to 1.004 of specific gravity (SG). Just divide the decimal portion of the SG by 4 to get the approximate degrees Plato (e.g., 1.060 is 15°P).
-The proper amount of yeast for 5.25 US gallons of 1.060 wort is around 225 billion cells if you are pitching 0.75 million per milliliter.
-(750,000) X (20,000) X (15) = 225,000,000,000
-Another way to put it, you need about 3 ¾ billion cells for each point of OG when pitching into a little over 5 gallons (20 liters) of wort. Double that number for a lager.
-You might ask why not pitch as much yeast as possible? Over pitching causes other problems with beer flavor, such as a lack of esters. Changes in the flavor profile are noticeable when the pitch rates are as little as 20% over the recommended amount.
-In summary, the products from White Labs and Wyeast are excellent, yet when making a bigger beer or when the yeast has been abused or is old, a clean, properly prepared starter will result in consistent, well fermented beers. Always try to keep your process simple rather than complex and always ask yourself if the beer is the way you like it. If you’re making starters, keep track of your process and keep track of the results. In the end, keep the big picture of yeast handling in mind, which is to have healthy yeast first and proper quantities second.
Originally Posted by Chimone
you give an explanation, then not but 3 posts later someone comes by after completely ignoring your post and posts something contradictory to what you just said.
didn't give an explanation, just started out with a "do it". That doesn't help anyone learn. If you don't want to answer or don't really know the answer, why post? Just want to whine about having to answer a question you've answered before? Seems kinda silly.