Why do we want to make a starter? There are at least five good reasons:

1. to ensure a clean and complete fermentation
2. to improve fermentation performance and consistency
3. to allow for fine tuning
4. to improve reproducibility of our beers
5. and because of the versatility, since by building our own starters we are not limited to commercial strains.

If we don't make a starter, we risk severely underpitching the wort. When we underpitch, we risk increased yeast cell growth, which leads to increased esters and increased fuel alcohol generation, incomplete fermentation, which leads to unreduced dimethyl sulfide (DMS) and acetaldehyde, and our wort is more susceptible to infection. Since bacteria growth rate is six times faster than yeast cell growth rate, we want to provide enough yeast to get the wort fermenting quickly.

To hit the desired pitching rate, we need about 200 billion cells for 5 gallons of beer. Pitching rates for ales are 6 to 10 million cells per milliliter multiplied by the factor (OG-1)/0.048, which increases the pitching rate as the wort gravity increases. For lagers, we want 10 to 15 million cells per milliliter with the same multiplicative factor.

Here is a table of experimental results on yeast cell count and viability for various sources of yeast, looking at both the package, and the package used to make a starter:

Item	Pkg	Cells/ml	Viability
Wyeast shampoo tube	150ml	4.25 x 106	96.8 %
	1 L Starter	7.10 x 106	95.6 %
White Labs vial	35ml	1.80 x 106	81.8 %
	T1 L Starter	5.70 x 106	92.5 %
Slant	50 ml	0.20 x 106	93.3 %
	1 L Starter	9.00 x 106	99.9 %

The results show that the cell count and viability of the starters are within the desired pitching range in terms of cell count. The packages themselves, though, are not sufficient to get the desired amount into the beer at the start of the fermentation. Note that these data are for the Wyeast shampoo tubes, which are 150 ml in size, compared to the White Labs vials, which hold 35 ml of slurry. Note the very high cell count and viability of the starter built up from the slant.

For yeast propagation, we want increased cell growth, while for beer production, we want a good rate of fermentation and the production of a balanced sensory profile. The yeast manufacturers may produce their yeast under conditions that are optimal for growth rates, not beer production. Making a yeast starter provides the conditions to prepare the yeast for beer production.

Four factors influence yeast growth: A. Oxygen / aeration; B. Temperature; C. Wort composition; and D. pH.

A. Oxygen / aeration:

Oxygen is essential for yeast growth, and, in general, the more the better. Since it's needed for the growth phase, it's only important in the first stages of fermentation. With an aquarium pump, it will take about 80 to 90 minutes of aeration to reach the point of oxygen saturation in the wort. Using direct injection of oxygen with a 2-micron stone, 5 gallons of wort needs only 90 seconds at full throttle to reach 10 to 12 parts per million of oxygen. A one-liter starter only needs 2 to 4 seconds, and Maribeth finds that the oxygen makes a big difference on the starter performance. An oxygen system can be purchased from a number of suppliers, and the oxygen bottles can be purchased at the hardware store. [Home Depot sells them for just under $7.]

About six years ago, Maribeth conducted an experiment on the effect of starter method on yeast cell count. She found the following:

Method	Cells/ml
Airlock	5 x 106
Shaken	60 x 106
Aerated	90 x 106
Stirred	275 x 106

For NHC 2001, she presented data from another yeast cell count experiment:

Stage	Cell Count	Time for Kreusen
Initial	2.84 x 106	--
No Shaking	50 x 106	ND
Shake	100 x 106	> 60 hrs
Shake with oxygen every 4-6 hrs.	137 x 106	60 hrs.
Stir with initial oxygen	172 x 106	36 hrs.
Stir	147 x 106	24 hrs
Stir without nutrient	95.5 x 106	48 hrs.

Here, the term "stir" refers to growing the yeast starter on a magnetic stirrer; a Teflon coated magnetic spin bar is placed in the starter flask, and this bar is rotated by a second magnet spinning under the surface of the plate.

The results of the experiment show that the cell count for the stirred plate starters are far better than either letting the starter sit undisturbed or shaking it periodically during the fermentation.

B. Temperature

Yeast is sensitive to temperatures. For lager yeast, the optimal fermentation range is 35° to 55° F; for ale yeast, 55° to 70° F.

Yeast will grow at temperatures up to 98 F. The presenter grows her starters at room temperature, about 75° to 77° F, and not at the desired fermentation temperature. In fact, she prefers to pitch at a lower temperature than desired for the fermentation, and to let the fermentation warm up the wort. For her Old Fornicator barley wine, she started the fermentation at 50° F, which is just under the lower limit given for ale yeast fermentation range.

C. Wort Composition

The composition of the wort determines yeast growth and fermentation performance. The presenter believes that the closer the starter composition is to the wort being fermented, the better. Yeast growth in OG 1.020 wort results in higher yeast masses, but is not recommended (this is used by the yeast companies). A wort at OG 1.040 works well with most beers, but OG 1.065 starters should be used for high-gravity beers.

For yeast starters, we want to use nitrogen supplements in the wort. Both ammonium phosphate and amino acid / peptide formulations are available from homebrew supply stores. The presenter uses a BrewTek formulation for propagation, and a Crosby and Baker product for fermentation. The starter wort also benefits from zinc at a level of 0.5 parts per million.

A good starting point for a yeast starter is an OG of 1.040, with 1% vitamin-based yeast nutrient and some hops. Boil the wort for half an hour, transfer to canning jars, and pressure cook to sterilize.

D. pH

The typical wort pH range is 4 to 6. Cooled wort should have a pH of about 5.2, and this will drop to about 4.2 during fermentation.

Some final notes: You should allow 1 to 2 days to grow each step of the starter. It's OK to refrigerate harvested yeast for 1 or 2 weeks-just feed them in a starter before pitching. When propagating yeast, it's important to work in a draft-free area with a flame source, and to have containers open as little as possible.