Science Buddies: "Ask an Expert"

I have been trying to make Agar plates for the last two weeks, and they do not want to solidify. The first time I tried making them, I left them undisturbed in a room at room temperature. I was told by my teacher to put the lid on, but have it to where it is slanting so the condensation coming from the Agar doesn't disturb it. When I came back to check on, it had a water-jelly like consistency and had form some sort of crystals. I redid the agar, but instead of leaving it in a room temperature room undisturbed, I left them in an incubator at about 23 degrees Celsius. I put the lids on the way I had them on the first time. Yet, when I came to check on them, they were almost exactly identical to those I had left at room temperature. What can I do to successfully make Agar plates and to prevent this crystallization?


Here is my recipe for my Agar Plates in case this is the problem:
Ingredients:
500 mL of Beef broth
15g of Agar
10g of sugar
50g-200g of non-iodized salt
Distilled H2O

Recipe:

• Assemble all the ingredients in the recipe

• In the glass container, put all the ingredients in

• On the glass container, loosely put aluminum foil over the container (enough to where steam can escape)

• Sterilize the solution by boiling it on a hot plate for at least 15 minutes. Boil it until the agar is completely dissolved, and the media is clear.

• While boiling, swirl the media often to prevent the media from burning

• Cool the media until it is just cool enough to handle, about 20–30 minutes. One should be able to comfortably hold their hand against the container for a few seconds. If the media is too cool, it will solidify in the container. If it is too hot, it will leave excess condensation on the lids.

• Remix the media before pouring. Be careful not to over mix to not add bubbles to the media

• Pour into the plate until it covers the bottom using heat-resistant gloves, approximately 25 mL

• Incubate at 20-25 °C for 2–3 days. Bacteria grow more successfully at lower temperatures.

Hi there!
Agar is a temperamental thing. If not precisely following directions, you can get bad media. Squishy or fragile media can be a result of a couple things. Are you heating until all of your the mixture is completely dissolved? Are you overheating? Both will results in water-like media. How much salt are you using? Depending on the % concentration can determine how long you will heat your mixture. How long are you letting your plates sit at room temperature? How much melted agar are you placing in your dishes?

Here is a link you can find through ScienceBuddies to help with troubleshooting
https://www.sciencebuddies.org/science- ... leshooting