Course 1: Making Yogurt the Scientific Way

Lesson Goals

  • To see an example of a bacterial growth medium with familiar ingredients.
  • To learn the names of some common lab equipment.
  • To chill out and get a feel for being in the lab.

 

 Key Terms

  • Milk
  • Canned tomatoes
  • Water
  • Agar
  • Autoclave
  • Centrifuge
  • Gloves
  • Pasteur Pipette
  • 50 mL Falcon tubes
  • Kim Wipes
  • Petri Dishes

 

 

Protocol

SBone Protocols: Tomato-Milk Plates

 

Script

Yogurt bacteria are lactic acid bacteria and they have some special growth requirements that more commonly used lab bacteria like E. coli don’t have. Yogurt bacteria won’t grow well on media like LB, and we need to add things to the media that will help the lactic acid bacteria grow and inhibit the growth of other more fast-growing bacteria.

In the 1920s, scientists trying to grow cultures of Lactobacillus acidophilus experimented with adding milk and other sorts of foods to their media. Empirically, they found that media made with milk and tomato juice was the best for growing the lactic acid bacteria. The milk provides the carbon source (lactose sugar) and the nitrogen source (milk protein). The tomato juice gives a mixture of salts, acids, and growth factors that improve the bacterial growth.

To make tomato juice agar you need:

  • Milk
  • Canned tomatoes
  • Water
  • Agar
  • Autoclave
  • Centrifuge
  • Gloves
  • Pasteur Pipette
  • 50 mL Falcon tubes
  • Kim Wipes
  • Petri Dishes

First, collect the juice from the canned tomatoes. To separate the juice from the sauce, you can either filter the sauce in the can through a coffee filter, or centrifuge the sauce at 4000 RPM and collect the supernatant. You’ll need 100 milliliters of juice per liter of media.

Mix 100 milliliters of tomato juice with 100 milliliters of skim milk and 20 grams of agar. Add water up to 1 liter and shake to mix.

Optionally, you may want to add 5 grams of yeast extract to the mix. This will produce an enriched media that allows faster bacterial growth.

Autoclave the media to sterilize and melt the agar, then pour the liquid media into empty petri dishes after it’s cooled to about 60 degrees celsius.

After the agar has hardened, you’re ready to isolate and study lactic acid bacteria from yogurt.

That’s all for now, happy culturing!

 

 Optional Links and Further Reading

 

 

Course Curriculum