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Why should I make a yeast starter and not just sprinkle the yeast on top of the wort like the packet says? I am sure there must be some good reason why you would do this.

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A yeast starter is essential to making sure there are enough live yeast at pitch time to complete fermentation. You might want to make a starter for several reasons, including:

  • Unsure of yeast viability due to age or temperature swings. If you've ordered liquid yeast during the summer months and you're unsure if the high heat may have killed some off, making a starter is a great idea.
  • You're making a high gravity beer. These are particularly exhausting to yeast and you want to pitch a pretty big starter into it (at least 2 liters if not more) to ensure there are enough viable yeast in there to finish all that fermentation

However, these concerns are generally only applicable to liquid yeast cultures. The dehydrated yeast packets contain a lot of yeast, usually enough to get the job done either by dry pitching or rehydrating it according to the package instructions. The rehydration wakes the yeast up and gets them used to working at fermentation temperatures before they actually have to go to work. You're not going to be doing your dry yeast any harm by making a starter, but it's also fairly unnecessary. If you're really worried about pitch rates with dry yeast just throw another packet into the fermenter at pitch time, or make another yeast slurry with the pack contents and double pitch that way.

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You should make a starter to ensure that you get a healthy and vigorous fermentation. Rehydrating yeast can kill a large number of the cells, so by creating a starter you can get your brew off to a healthy start.

This is especially important for stronger beers:

"For strong beers and barleywines, at least 1 cup of yeast slurry or 1 gallon of yeast starter should be pitched to ensure that there will be enough active yeast to finish the fermentation before they are overwhelmed by the rising alcohol level."

http://www.howtobrew.com/section1/chapter6-5.html

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