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I'm a high school science teacher, and am starting at a new school. I'd like to do some labs with bacteria and so I'll need to make plates with agar agar. I found 3 huge jugs of powdered agar in the storage cabinet, but I'm wondering if they're still good. The only one with a date said "1995" and I think that may be a bit much, but I could be wrong. Does anyone know if I should buy new agar or use the old stuff?

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    $\begingroup$ There's a similar post on our sister-site for cooking: seasoned-advice. $\endgroup$ Jan 20 at 0:58
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    $\begingroup$ I have used 5 year expired agar, no dramas. I expect your old agar will be fine. $\endgroup$ Jan 20 at 1:25
  • $\begingroup$ I don't see why it shouldn't be ok as long as it has been stored dry. If it is still powdery and white it should be ok and personally, I would have any problems to use it in the lab. $\endgroup$
    – Chris
    Jan 20 at 6:48
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    $\begingroup$ I'm assuming these labs are going to be mostly qualitative in design? If you're able to use it to make an agar plate and grow bacteria on it, seems like it's still "good". If it won't form a gel then something's wrong. If it forms the gel and doesn't grow the bacteria... save that stuff and maybe consult with someone to find what new antibiotic you've discovered :) $\endgroup$
    – Bryan Krause
    Jan 20 at 16:13

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Use science to figure it out!

Order some new jugs. Tell the class about the old jugs you found. Make 2 batches of plates: old and new. Make dilutions of bacteria. Does the old agar support as many colonies as the new?

Even better would be to have some other kinds of agar plates. You can have old and new. You can have agar with extra salt. You can have agar with extra sugar. You can have agar with a little iodine. You can have agar made with old coffee. You can have agar made with water from the fish aquarium. Each student group will have a group of plates one of each type.

Then you can pool the plates of each type. Graph number of colonies. Use statistics.

If you determine the old agar is good then you can use it again next year and after that. If you determine it is bad then it gets trashed. Either way you are using the scientific method. Yay science!

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Test it first, but it should work fine

Agar Agar powder has a pretty long shelf life, even decades expired agar still works fine.

But if the container was opened it might have picked up some moisture from the air which will weaken how well it gels. It will be pretty obvious if you mix a test batch, just mix a bit in a paper cup and see if it gels. As long as it gels it will still work fine for any lab in a school setting. Any change will be uniform so if you used it alongside fresh agar there might be a noticeable difference in say PCR, but as long as you only compare it to itself even slightly bad agar will work fine.

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