Overcooked Agar?

[RosalieM]

Hi to all the Agar Gurus out there!! I have a slight problem... I've just made up a batch of Agar in preparation for a prac on Monday. I had it almost boiled, cleaned the bench tops and went to get the petrie dishes (we use the plasic sterile ones) and the box was empty!! We have enough glass ones, but they aren't clean and sterile, so I'll have to do that. Can anyone tell me if it is ok to keep the Agar simmering for a few hours (Don't know how long it will take to sterilise all these glass ones) or will I need to make a new batch? It's only 1.5L but still, I'd rather not waste it! Should I turn it off and then re-boil it when I'm ready? I've never had this problem before!! I always just make it and pour it straight away... Thanks in advance!
Rosalie

[Labbie]

I have just kept ours warm at 50c for about 2 hours before, that was fine. We use our incubater. Give it a stir every so often.

[noona]

Hi Rosalie
I have left it for some time just on a simmer and they turned out OK
Noona

[RosalieM]

Thanks for that I ended up just washing the petrie dishes, pouring the plates, letting them set, and now they are in the pressure cooker to steralise

Now for my next trick...

I'm making phenolphtalein agar (separate prac and class - why does everyone want it all at once??) and I've boiled it, and added NaOH. It has gone almost black. Haven't seen this happen before. Did I add it too soon? Perhaps not cooled enough? Any ideas???? Today is not my day...

[Sassi]

Black??? I know you have to let it cool down to same temp as you do when you pour agar plates, 50 I think it is... But I don't know if it makes it go black if it is too hot. I guess there mus t be a reason you have to let it cool down, so

[matchstick]

hi
Your problem has happened to me aswell, I worked out that the phenophthlien (sorry if the spelling is wrong) was too strong you need a weaker solution.
Goodluck
Matchstick

[RosalieM]

OK so now it is totally black... not looking good at all... why is it doing this to me??? Just poured some into a beaker and added phenolphthalein and it has gone purple but i can only tell that because my white stirring rod is also purple... then i poured some of that into an evaporating basin (white surface so i could see better) and poured in a heap of 1M HCl (as in 1/4 of a dropper bottle... not really a heap...) and it went yellow... Any opinions on whether i should use it for the prac? I'm thinking I should just start again... SO FRUSTRATING!! I won't be here on Monday so I have heaps to do in order to have the day off... Don't have time for re-making agar!

Sassi and Matchstick, I wrote the above before your replies came through... Thought I'd submit it anyway.

Matchstick - I hadn't even added the phenolphthalein yet... :(

[fibreweb]

I have at times, when I don't have time to completely make, sterilise and pour the agar plates, made the agar mixture and then poured it into the conical flasks ready to go into the microwave.
It has then spent the night in the fridge, going solid. The next morning I have put it into the microwave and done its 15 minutes of sterilising then poured it into the petri dishes.
This has saved me on a couple of occasions when the have been ordered for the next day and I didn't have time to do the lot in the afternoon or the next morning.

They still worked.

Wendy

[RosalieM]

Thanks Wendy... If only I had a microwave!

I'd just like to report that after getting rid of my black agar, I started fresh again and I believe I have just made the most beautiful batch of purple agar ever made by me! Finally, a success story for the day!

[RosalieM]

Continuation of my agar saga... Ok, so I got home from my Melbourne trip at about 2:40am... and dragged myself to work this morning... My perfectly purple agar had turned a beautiful pale shade of pink (not at all what I wanted!!!) and about the top 3mm were set (probably just dried out from the air) and the rest was still liquid!! So I poured it all back into a flask, reheated it, added a bucket-load of agar (ok, so maybe not QUITE that much...), re-boiled it and poured it out again into the containers (washed while the agar was heating up). Unfortunately I hadn't thought to check it early enough so this was done before lunch (for a prac right after lunch!). I told the teacher where i put it in the fridge and when i went in a little while back it still wasn't set!! I hope they have enough work to cover them until it is ready to go!!! I feel so bad, but what can you do? It's not like I didn't try - 3 goes to get a batch of phenolphthalein agar ready!! Grrr...

[dime]

The phenolphthalein agar goes blackish cause the Sodium hydroxide is added when the agar is too hot. Cool to 50deg or below before adding NaOH and phenolphthalein. There was a good thread on this not long ago. But the thing is the blackish agar should still work with the acid, just not as pretty

[RosalieM]

Well you would think I would learn the first time!! I had a horrendous week last week, as you probably read in other threads... and Friday afternoon was no exception. I needed phenolphthalein agar for period 1 today (Monday) and had heart/lung dissections going on and all sorts of other things. I put it on and once again added the NaOH too soon. Black agar again :( As soon as I tipped it in I regretted my actions, even before it started to change colour because I knew it was going to. I just couldn't reverse it!! I had no way of making up more because I was needed in the heart/lung prac and couldn't watch the agar in the preproom, and I was leaving for a trip to the coast for a church 'ladies retreat' with others from work at 3:30. I decided to go ahead and finish making it and hope for the best! As it was we didn't get away till about 3:45 thanks to me but we still arrived in time for dinner Came in early this morning (after having a workman knock on my door while i'm still dressed in pjs... and sitting behind a car doing 40km/hr the entire length of the 80 zone, and the 60 zones either side of it...) and checked the agar worked - had to make 0.1M acid cos I hadn't checked we had enough on friday!! Thankfully it worked, although it went a yellowish colour instead of almost clear because by that time there was no way I would get more agar made this morning with all those setbacks! But thankfully I enjoyed my 'ladies retreat' and am feeling relaxed and a lot less flustered than I would have after an ordinary weekend at home

[Kay]

I am, about to prepare some agar plates and have noticed that some of you use a microwave. I am unfamiliar with this method. Would someone be kind enough to forward the method to me.
Thanks
k

[RosalieM]

Hi Kay, if you use the search tool and enter 'microwave agar' into the seach field you will come up with some answers. I don't have a microwave (other than in the staff lunch room!) so I can't help you...