How to make skim milk agar plates

Written by annie duchesne | 13/05/2017
How to make skim milk agar plates
Agar plates provide a nutrient rich environment for a variety of organisms. (Stockbyte/Stockbyte/Getty Images)

Skimmed milk agar plates are used to provide a nutritious medium for growing micro-organisms. Once prepared, the agar can be plated with a population of micro-organisms to test for the micro-organism's ability to digest casein protein. Casein is a large insoluble protein found in skimmed milk. As it is digested by an organism's enzymes, casein is broken down into small amino acids and peptides. Clear patches on the agar pate indicate regions where casein has been broken down. Skimmed milk agar is a relatively uncomplicated and inexpensive medium to use for an experiment like this.

You can buy skimmed milk agar from lab supply companies, or make it yourself.

How to make skim milk agar plates
If powder spills onto the scale surface, genty brush it away with a soft paintbrush. (Hemera Technologies/ Images)

Place a clean,dry watch glass on the scale and zero the scale. Measure out 5g of skimmed milk powder. Transfer the powder from the stock bottle to the watch glass on the scale with a clean, dry lab scoop.

How to make skim milk agar plates
Crush chunks of powder against the walls of the beaker with the glass rod to break them up. (Siri Stafford/Photodisc/Getty Images)

Pour 50ml of dH20 into one of the beakers. Add the skimmed milk powder and stir with the glass rod until it is completely dissolved.

How to make skim milk agar plates
Using clean, dry lab equipment ensures that your measurements will be accurate. (Hemera Technologies/ Images)

Tare a clean, dry watch glass on the scale. Measure out 1g of agar powder onto the watch glass using a clean, dry lab scoop.

How to make skim milk agar plates
Stir vigorously to ensure even suspension. (Thinkstock Images/Comstock/Getty Images)

Pour 50ml of dH20 in the other beaker. Dissolve the measured agar powder into it.

How to make skim milk agar plates
Autoclaving sterilises the agar to prevent contamination with other micro-organisms. (Brand X Pictures/Brand X Pictures/Getty Images)

Quickly pour the skimmed milk mixture into the agar mixture. Stir the mixture then autoclave at 121 degrees Celsius for 15 minutes.

How to make skim milk agar plates
Fill the dishes half-full for the best results. (Stockbyte/Stockbyte/Getty Images)

Pour the skimmed milk agar into the clean, dry petri dishes. Do not fill the dishes all the way to the top. Let the agar cool and solidify before you use it for your experiment.

Things you need

  • Agar
  • dH20
  • Skimmed milk powder
  • Autoclave
  • Empty petri dishes
  • Paintbrush
  • Scale
  • Two 150ml Beakers
  • Watch glass
  • Metal lab scoop
  • Glass rod

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