Yeast Culturing: Plates

One of the best ways to maintain a yeast culture is on an agar plate. This method can be used to keep yeast health in your fridge for about a month or two. It also happens to be very easy and this post will be about making your own plates to use at home.

First, what is an agar plate? Agar is a compound derived from sea weed. It has many applications in science but is basically like jello that sets up firmly at room temperature. It creates a nice substance for microbes to grow on. By adding compounds to agar it can be customized to create whatever type of a plate is needed. In this case we will be adding nutrients and sugar to the agar to make it suitable for growing yeast.

Finished Plates

   What you will need:

1.    Petri Dishes
2.    Agar
3.    Dry Malt Extract
4.    Yeast Nutrients (optional)
5.    Sterile Loop or Toothpicks

Directions: YM Media

Food Grade Agar

1.  Add 9 grams of food grade agar to 400 mL water.
2. Add 1.5 grams of Dry Malt Extract
3. Add 10 grams Table Sugar
4. Add 1 gram of yeast nutrient (like servomyces)
5. Bring the volume up to 500 mL
6. Boil the mixture in a microwave, watch it carefully so that it does not boil over, when all the solids have dissolved allow it to cool (covered) to a temperature in which it is comfortable to be held
7. Pout the agar mixture into the petri dishes. You only need enough to cover the bottom of the dish with about 1/8 of an inch of liquid. Pour quickly and place the lid immediately on the plate. Stack the plates on top of each other as they cool. This will help dissipate the moisture that likes to accumulate on the top of the lids.
8. When you are ready, take a small sample of yeast on a loop or a sterile toothpick and smear it on the plate, you do not need much and you do not need to press hard. You do not want to break the surface of the agar. Place the plate upside down in a warm room and allow the yeast to grown 2 days. You will see white colonies form, these are the yeast.