From MrMalty
Quote:
Proper fermentation is what sets apart great beers from just OK beers and starters can help ensure proper fermentation. Proper fermentation results in a beer with the correct appearance, flavor, body, and aroma profile. It is also clean, complete, consistent, and reproducible.In order to have proper fermentation, you must have the right amount of clean, healthy yeast to ferment your wort.
Q: What is a starter and what is its purpose?
A: starter is a small volume of wort that yeast use as an initial step to become healthy, multiply and prepare themselves to ferment a batch of beer.
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. As Logsdon puts it, “Would you rather have an excess of 80 year-old people building your house or a smaller amount of really healthy 20 year-olds?”
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Yeast starters can reduce yeast stress reducing "usually" undesireable off flavors such as esters. I say "usually" because some styles require certain flavors that underpitching can produce.
Overall making a starter will reduce lag time, possibility of off-flavors, and promote a healthier fermentation.
Some will argue that a Wyeast activator pak, or a White Labs vial can be directly pitched into a wort of 1.050 or less without any ill effects, however, if you look at the pitching rate calculators such as the one found at
Mr Malty you will find that you will normally be underpitching even with a moderate gravity.
Another consideration to make when using liquid yeast; usually the culture will not have 100% viability of all cells contained within it, therefore, pitching from a Wyeast pak or WL vial does not mean you are actually pitching the ~100 billion (viable) cells they claim to contain.
Now without taking your original question into account making a starter also "proofs" your yeast ensuring you aren't pitching a culture of dead cells into your wort that will not ferment, this is especially important if you are mail ordering your liquid yeast cultures due to the extreme temperatures the yeast may encounter during shipping.
Yeast starter question
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