Brewer’s Friend

Stir Plates and Growing Brewing Yeast Quickly

Creating a simple yeast starter as outlined here at brewersfriend.com will greatly increase your yeast cell count, but there is a way to easily supercharge this process. By using a stir plate you will be able to grow nearly 50% more yeast cells using the same volume of starter in the same amount of time. Basically, for the same investment in yeast, time and starter liquid, you will be able to double your yeast production with a stir plate.

A stir plate is a simple machine consisting of a base which houses a motor with variable speed control. Fig 1. This motor is used to spin two precisely spaced magnets, which in turn will spin a magnetic stir bar inside your flask. The intent here is to use this stirring motion to:

1. Keep the yeast in suspension
2. Release CO2 from the starter solution
3. Continuously aerate the starter liquid

Fig 1.

There are several important guidelines to follow when utilizing a stir plate to create a starter:

1. Ideally, boil the starter wort in the flask that you will be using to create the starter, this will help to sanitize the flask.
2. DO NOT use an airlock on a stir plate starter as it will impede the ability of the stir plate to exchange CO2 from the fermentation for O2, which will help grow yeast.
3. DO use foil to cover the top of the flask instead of an airlock.
4. Ferment the starter at 75F – 80F. While not optimal for flavor production, you are trying to grow yeast, and warmer temperatures will accelerate this process.

By doing the above, you are creating the perfect growing environment for your yeast cells, allowing them to grow at a much greater rate than they would with the use of a simple starter with no stir plate. After the starter has fermented out completely (approx 24 hours), remove the starter from the stir plate and chill so that the yeast may settle. After a short period of time (several hours), the yeast will form a nice thick slurry in the bottom of the flask, allowing you decant the starter wort prior to pitching. Decanting is required since the wort has been oxidized and fermented at a higher temperature than is optimal for most yeasts (75F-80F recommended). Though this temperature is excellent for yeast growth, it does not leave a very favorable flavor in the starter liquid.

There are calculators that will help you to determine the size of starter needed, and/or the volume of yeast slurry needed to pitch the optimal number of yeast cells into your wort. One such calculator can be found at www.MrMalty.com. This calculator will allow you to enter all of the vital statistics about your wort so that it can properly calculate the number of yeast cells you will require, the required volume of your starter wort and the volume of yeast slurry that you must pitch to meet your ideal pitching rate.

Below is a side by side comparison of the number of yeast cells created in a 2L simple starter (no stir plate) and a 2L stir plate starter. The results may surprise you!

Starter volume: 2L
Amount of Yeast used in starter: (1) 11g packet
Fermentation Time: 24 hours
Temperature: 75F (recommended to quickly grow yeast)

Using the above criteria, a simple starter will produce approximately:

201 billion yeast cells
82 ml of yeast slurry

Using the above criteria, a stir plate starter will produce approximately:

307 billion yeast cells
124 ml of yeast slurry

As you can see from the above comparison, utilizing a stir plate you are able to increase by 50% the yeast count with the same amount of starter wort while using the same amount of yeast pitched into said starter. Conversely, you can also create the same amount of yeast in a stir plate starter as you are able to with a simple starter, with nearly half the starter volume. The latter is of great interest to lager brewers who will routinely need to create very large starters to reach the proper pitching rate for their lager beers. Lagers will routinely require a 3L-4L stir plate starter. If you utilized a simple starter (no stir plate), you would be in the neighborhood of 6L to reach the proper pitching rate.