Purple agar jelly

suzyr · Post by **suzyr** » 19 Sep 2006, 10:53

Has anyone had a problem with Purple Agar not setting properly, is there a foolproof recipe please.
suzyr

Whspa · Post by **Whspa** » 19 Sep 2006, 11:15

Hi Suzyr,
The first time I made this jelly (and it was in my first month on the job) it did not set at all.
I thought perhaps the agar was old so I opened a new one and tried again.
I add 40g agar to 1L of nearly boiling 0.1M NaOH(with enough phenolphthalein to make it deep pink) and stir until it dissolves.This produces a really dense, firm jelly.
I hope this helps.
Carol

rae · Post by **rae** » 19 Sep 2006, 12:19

Hi,

I've not had a problem with it setting but I've had a problem with the gel going clear when storing it overnight. Does any one know why??

Ta Lorrae

suzyr · Post by **suzyr** » 19 Sep 2006, 14:10

Thank you Carol, that is a bit different to my recipe so here's hoping! I don't have to make any at present but since I have just been shown this site I have taken advantage of seeking help.
Suzyr

juliem · Post by **juliem** » 19 Sep 2006, 14:13

Hi Lorrae
like you I managed to have 4 trays 'dissapear' on me - 3 classes
- not enough 0.1m NaOH
- I made them in aluminium trays which reacted with the alkaline gel
so now I definately use plastic very clean well rinsed trays
and a different recipe hope this helps

450ml deionised water heat to about 60' mix in 9gm Agar bring to boil stirring boil 2mins and cool to below 60'
Add NaOH - 2gm in 50mls De' water
Add 1/8tsp phenolpthalein - mix well
pour into trays and cool cover with cling wrap
I make 1 tray and the students then cut to size in class

regards juliem

Lell · Post by **Lell** » 19 Sep 2006, 14:42

Hi Lorrae

I found with the pink agar,the water has to be boiling for the agar to set.Secondly, you could try a new bottle of phenolpthalein if your agar decolourises.Sometimes, the colour can be restored by adding a little more NaOH to the top of the set agar. Hope this helps.

gwen · Post by **gwen** » 19 Sep 2006, 15:03

Hi rae,

When storing phenolphthalein gel for 24 hrs or more I put it in a plastic tray containing a small wad of cotton wool dipped in ammonia and cover the tray with another inverted on top. This keeps it nice and pink and it seems to work well the next day.

Cheers,
Gwen

CarolF · Post by **CarolF** » 20 Sep 2006, 09:10

To avoid the agar gel not setting I always put a little extra in for good measure. I have had no trouble with the loss of colour and I always use stainless steel trays, cover with gladwrap and store in the fridge. It has lasted for two or three days this way before use and worked well. I would not put it into the fridge without being sealed with the gladwrap as the surface tends to dry out and crack if not covered.

PS Pam from Irrawang, is that the same Pam who attended the TAFE course about 20 years ago at Tighes Hill TAFE?

louie · Post by **louie** » 20 Sep 2006, 11:22

Dear All I make it in plastic trays .It seems to lose a lot of colour as it cools. I stored it in the fridge one time and it went nearly colourless so it 's room temp for me , cheers Loiue

Robbie · Post by **Robbie** » 21 Sep 2006, 09:17

Greetings all,

I have had all the problems you have mentioned but using the methods I now use I have no problems at all.

I set the agar in 3 litre (yellow) plastic ice cream containers. They have sharp sides and angles - not curved - which helps when cutting it up.

To make up 2 litres of gel, weigh out 36g (or a bit more) of agar and ONLY 2g NaOH. Dissolve the NaOH in 50 mL of water. Heat the 2 litres of water to boiling and then add the agar. Add this very gradually, stirring continuously, making sure it is thoroughly dissolved before adding more.

Allow the agar to cool and ONLY when the temperature falls below 60 deg. then add the NaOH stirring continuously as you do so.

Then add the phenolphthalein solution drop by drop, stirring continuously until the jelly is a deep pink. Don't just use the phenolphthalein solution you already have, probably in dropper bottles. Instead dissolve a fair bit in a very small amount of methys and use this. I think the more methys you add to the jelly the worse it becomes, so by making up some very strong phenolphthalein solution just for this purpose, you get the maximum phenolphthalein with the minimum amount of methys.

The problems I have had in the past have been caused by:

1. too much NaOH
2. too much methys
3. using water from the hot water tap which we later discovered was strongly acidic, instead of proper deionised water (in a hurry)!!
4. adding the NaOH without waiting until the temp falls below 60 deg.

Leave the gel on the bench to set overnight. No need for the frig.

I then invert the gel onto a white enamel tray and the students thoroughly enjoy chopping it up.

You can use 0.1 M H2SO4 to soak the cubes or 0.2M H2SO4 if you need a faster reaction.

The recipe comes from Web of Life Teacher's Guide Part 2 (1983 edn) pp60-61.

Hope this helps.

Robbie

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