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Pre-cook your agar and warm it up again in the microwave

  • timer  5 Minutes to read
  • 12 January 2022
  • Written by Michèle Heidemeyer
  • Life Science

There is a microwave in almost every molecular biology laboratory – so let's use it for more than just a few agarose gels!

Preparation of agar plates

Just as work is about to finish in the lab, before you finally go home, you want to quickly plate out a few transformed Top-10 cells. As luck would have it, all the agar plates have been used up and, as always, none of your kind laboratory colleagues have thought of preparing new agar plates. So you have to spend valuable overtime on autoclaving.

So that you don’t have to constantly repeat this annoying scenario, we have prepared some instructions for pre-cooking and subsequently warming up agar plates ready for impromptu use.

Recipe for 200 ml emergency supply of agar:

Pre-cooking the agar:

  • First, the agar is weighed into a 1 L bottle (we have listed three of the most frequently used recipes).


    For 1 L Medium


    10 g Tryptone, 5 g Yeast-Extract, 5 g NaCl pH 7.0


    20 g Tryptone, 5 g Yeast-Extract, 0.5 g NaCl pH 7.5


    10 g Tryptone, 5 g Yeast-Extract, 8 g NaCl

  • The weighed powder is then suspended in 1 L of distilled water. The medium must be mixed well, and if necessary, it can be briefly heated (on a hot plate) while stirring frequently.
  • When all the powder is completely dissolved, aliquot 200 ml of the mixture into a 250 ml Duran bottle (use a measuring cylinder for this).
  • Now fill 4 bottles with 200 ml medium in each. The last 200 ml are never used (residues in the original bottle would prevent an exact 200 ml filling!).
  • For sterilization purposes in the autoclave, the lid should be only loosely screwed onto the bottle thread!
  • The agar media are now autoclaved at 121°C for 20 minutes.
  • After autoclaving, the laboratory glass bottles must be tightly closed (to prevent contamination).
  • The pre-cooked agar can be stored for a long period of time at room temperature without any problems.

Warming up in the microwave:

When you need agar plates quickly, it’s so easy to take one of these pre-cooked 100 ml bottles and boil them up in the microwave.

Place the agar in the microwave
  • To do this, put your bottle in the microwave with the lid loose (!) – as with autoclaving – for two minutes at full power.
  • As soon as a foam starts to form, take the medium briefly out of the microwave and swirl the bottle to dissolve the foam.
  • Then continue to heat the agar in the microwave until it is completely liquid. The agar should briefly boil at least two or three times.
  • To be on the safe side, please only touch the bottle when wearing oven gloves, otherwise there is a real risk of burns!
  • As soon as the agar is liquid, begin to carefully cool it down to 55 to 65°C while swirling in a cold water bath.
  • Now stir while adding the required antibiotic.
  • Your plates can then be poured.

Tip: Lay out the plates to dry individually instead of piling them up (which is often the common practice), because this dries the agar much faster!


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