1. How to Make a Yeast Starter
By Scott Bailey

2. What is a Yeast Starter and why do it?
A yeast starter is used to start cell activity or increase cell count
You need to increase cell count for higher gravity beers or cleaner, quicker fermentation in standard gravity beers
A yeast starter saves you from having to buy additional packets of yeast for the same batch.

3. What do you need to do a Yeast Starter?
DME (Dry Malt Extract)
Glass container - preferably a pyrex or Erlenmeyer type
Stir plate – optional
A stir plate makes it easier and more efficient, but not required
Stir plates can be easily made obtaining parts from a hobby store for about $20
You can buy one already made for about $40
You can find refurbished laboratory types for $50 and up or new ones for around $100
With a stir plate you will need a magnetic stir bar

4. The Math Disclaimer – I use BeerSmith so I don’t do the math
(0.75 million) x (milliliters of wort) x (degrees Plato of the wort)
There is about 3785 milliliters in a gallon. There are about 20,000 milliliters in gallons.
A degree Plato is about of original gravity. Just divide the OG by 4 to get Plato (e.g., is 12 degrees Plato)
So a beer with wort and 5.25 gallons you would need 180 billion cells.
This math is all for ales, if you do a lager it is basically double.
Resources

5. Yeast Facts
White labs claims that you don’t really need a starter for a beer under but you can still make one
Most beers in the to range would call for two packets of yeast or 1 packet of yeast and 1 liter starter. Anything over should probably get three packs, or a huge starter. A packet of yeast is approximately $7.
A standard packet of liquid yeast contains about 100 billion viable yeast cells. The number of viable yeast cells in a package decreases over time, which will affect the size of the starter.

6. The Process There is no one magic recipe for making a starter.
I typically use 5oz DME per liter of water.
Another recipe uses 2 pints of water to ½ cup of DME.
Another recipe uses 6 oz of DME to 2 quarts of water.
Still another one is a 10:1 ratio of water to DME.
All recipes are dependent on the age of the yeast.
Some recipes recommend adding about 1/4 teaspoon of yeast nutrient to the boil vessel. You can use slight less for starters under 1-2 L and slightly more for ones larger.
In a saucepan, bring your mixture to a boil for about 10 minutes to sterilize.
Next transfer this into your flask.
Cover it with a piece of sterilized aluminum foil. Put this in an ice bath and cool to degrees.
Some people use a bung with an air lock instead of the aluminum foil.
Both let Co2 escape.

7. The Process
At this point add the yeast. Place on a stir plate for hours.
If you don’t use a stir plate, shake the container as often as possible.
After 18 hours, the yeast will have consumed all the nutrients and oxygen in the starter.
Some people put the entire starter solution into their wort.
Some people decant as much water as possible before adding to the wort.
It can also be refrigerated and saved for use later with the foil or airlock left in place.
For dry yeast just do a proper rehydration in tap water, do not do a starter.

8. Stir Plates
The purpose of stir plate is to help the yeast propagate by aerating the wort. Think of a starter as a small beer.
Yeast propagation with a stir plate can increase cell count by times over manual agitation and an air lock.
It helps the yeast starter process by exposing more surface area of the starter to oxygen. Yeast then use that oxygen to reproduce. Since the yeast are exposed to more oxygen then they would be without the stir plate, they reproduce more. Since the yeast reproduce more, you get a larger amount of healthy yeast using the stir plate then you would without.
Resources
Make Your Own
Buy One

9. Healthy Yeast
Healthy yeast leads to better attenuation. Attenuation is the efficiency of the yeast eating the available sugars. Once the yeast has consumed the sugars they clump or flocculate and sink to the bottom.
Unhealthy yeast or under pitching of yeast causes the yeast to be stressed and not consume sugars, possibly leading to not achieving the final desired gravity and also possibly producing off flavors.
Excess levels of diacetyl
Increase in higher/fusel alcohol formation
Increase in ester formation
Increase in volatile sulfur compounds
High terminal gravities
Stuck fermentations
Increased risk of infection
Creating a starter that is too large (too many yeast cells) is equally bad because the yeast cells fight over the available sugars, becoming stressed and also possibly producing off flavors.
Very low ester production
Very fast fermentations
Thin or lacking body/mouthfeel
Autolysis (Yeasty flavors due to lysing of cells)