Recipe: Agar Jellies that Fluoresce under Blue Light

They are also vegan, gluten-free, and very tasty. And not at all radioactive, despite their alarming appearance. But let me go back to the start of this story.

Next weekend, I and a bunch of my colleagues will be participating in the Cancer Council Relay for Life at Lakeside Stadium. This is a fundraising event, where teams do laps of the stadium over a 24 hour period to raise money for cancer research and patient care. The goal is to always have someone from your team walking (or running). We did this for the first time last year, walking in our lab coats (or in my case, a lab coat borrowed from my lab, since I’m not a real scientist), and the Cancer Council and other participants were super excited to have Actual Cancer Researchers there. So we are doing it again this year – we get a lot of support from the Cancer Council, and we like to give back to the community.

Each team gets a marquee, where they can hang out or also do fundraising. Last year, we had a Science Never Sleeps stand, with various messy science experiments that kids could participate in, and this was very popular. But we were the only team that wasn’t fundraising on the day, so we’ve been having a bit of a think about what we could do for that.

I suggested making agar jellies and putting them in Petri dishes to sell (everything looks more sciencey in a Petri dish, and we do, in fact, use agar gels in the lab, so that’s double the science right there!), and one of my scientists mentioned that, speaking of sciencey foods, did I know tonic water fluoresces under black light, because of the quinine.

Readers, I did not know this.

But once I did know this, I became utterly CONSUMED with the idea of making fluorescing agar jellies to sell.

My first batch was a failure. While the results did fluoresce (the fluorescence is caused by the quinine in tonic water, which also gives it its bitter flavour), the jellies just didn’t set properly. They achieved a sort of jammy consistency – definitely not confectionery levels of firmness.

So I consulted with my scientists, and someone suggested that perhaps tonic water is too acidic, and I looked it up and yes, that was almost certainly the problem. Agar doesn’t like acidity, which I did know, and tonic water has a pH of around 2, which I did not. 2 is pretty acidic, by the way – about the same as lemon juice. The scale goes from 0 to 14, with water being a neutral 7. (And now I have become distracted by this chart which tells me that baking soda has a pH of 9 and bicarbonate of soda has a pH of 12. I thought baking soda had a pH of American cookbooks and bicarb a pH of English and Australian cookbooks, but that they were otherwise the same thing. Hmm.)

But how to get the acidity out of the tonic water without losing the fluorescence? I didn’t want to dilute the tonic water, because that would reduce the amount of quinine in the gel, and also, I just wasn’t sure how much difference this would make once I concentrated everything again by boiling it down.

Bicarb of soda was suggested but tonic water is, to be frank, not the easiest flavour to work with, and adding bicarb to the mix was not going to improve that.

The internet told me that if I let tonic water go flat, its pH might rise to as much as 4, and also that agar is happy to gel between a pH of 4.5 and 7. This made no sense to me, because I gave up on chemistry in year ten, but another of my scientists kindly explained that no, it wasn’t elves. Apparently, the bubbles (which are carbon dioxide) reduce the pH of the solution. So basically, as the bubbles pop, the acidity of the solution reduces. (This probably also explains why flat soft drinks taste so much sweeter than fizzy ones.)

(But who is popping those bubbles overnight? Elves, obviously.)

Another of my scientists suggested heating the tonic water to reduce its acidity. And of course, there was always the possibility of adding more agar, to increase the overall gelling tendencies of the solution.

I decided to do all three of these things. Which is not good scientific practice – having changed three things in the recipe simultaneously, I have no way of knowing which of these factor had the desired effect, or whether it was all of them together – but it’s more efficient, especially when agar is a bit hard to come by and I didn’t want to have to keep buying more.

And… it worked! This morning, I woke up to perfectly set agar jelly that fluoresces brightly under black light (yes, I bought a black light torch specifically for the purpose of making and testing fluorescent jellies).

And they taste awesome. I made gin and tonic jellies, and lemon lime and bitters (the bitterness coming from the tonic water, of course) jellies, and several more conventional lolly flavours, and they were all pretty good. The flavour of the tonic water did get rather lost, but since this was really about the fluorescence, and I’d normally make the jellies with tap water, I’m pretty OK with that.

Use these for your next science themed party – or maybe as a spooky Halloween treat.

Note: This recipe is fairly simple to make, but you will need a sugar thermometer. You will also need to start between 36 and 48 hours before you want to use it – the tonic water needs 24 hours to go flat, and the jellies themselves need 12-24 hours to fully set.

Your shopping list

400 g tonic water (I used the Schweppes brand)
25 g agar agar powder
600 g white sugar
350 g glucose syrup
10 g citric acid
flavours and colours of your choice (go easy on the colours, or skip them entirely – the fluorescence works better if there isn’t much pigment going on)
caster sugar for dredging

Now what will you do with it?

Pour the tonic water into a jug and leave it for 24 hours to go as flat as possible.

It is important to shine your black light torch on the tonic water multiple times during the 24 hour standing period so that you can admire its fluorescence properly.

The next day, heat the tonic water in a saucepan until just before boiling point, then take it off the heat and leave it to return to 30°C or less.

Once the tonic water is back to a sensible temperature, sprinkle over the agar agar powder, and stir gently, then leave for 20 minutes to soften.

While the agar agar powder is softening, and even while the tonic water is heating in the first place, I recommend getting everything else measured out and ready.

Start by making the citric acid solution, which you do by dissolving the citric acid in 10 g of water. I know, that’s a pain in the neck. I usually make a solution of 50 g citric acid and 50 g water by stirring the two together, then microwaving for about 20 seconds and stirring again until the solution is clear. Then I measure out the 20g of citric acid solution that I need for this recipe, and keep the remaining 80 g for future recipes.

Decide what flavours you want to use, and how many flavours you want from this batch. I usually divide a batch into two or three flavours. My favourite options so far have been lemon essence and the zest of a lime (lemon lime and bitters!), a few tablespoons or rhubarb and ginger gin (gin and tonic!), and raspberry flavour with a little rosewater. In terms of tins, I find that this amount makes enough to fill a single 23cm square tin, or two loaf tins. Or, really, I’ve been doing two smallish loaf tins and a 12x18cm plastic container, and making it a bit shallower. It really does depend a bit on how thick you want your jellies.

Anyway, whatever you choose, line it well with baking paper, because agar jellies are very, very sticky.

Does this look non-sticky to you?

Measure out your sugar and glucose into separate bowls. Have 2-3 bowls ready for you to divide your mixture between for flavour purposes. Yes, you will be using every bowl in the kitchen for this recipe.

Bring the agar agar and tonic water solution slowly to the boil, and simmer for 2-3 minutes until clear-ish, then add the sugar and boil to 105°C. Move the thermometer around a bit as you boil it, because

Immediately remove from the heat and add the glucose syrup, stirring it in well. Let the mixture cool to 65°C. You want to be fairly precise about this – if the mixture is too hot when you add the citric acid solution, it won’t set, but if it’s too cool, it will be setting already and the citric acid and other flavours won’t distribute properly. So stir the solution occasionally, and move your thermometer around a bit, because it cools at different rates, and you want it to be 65°C on average, at least.

When the mixture hits 65°C, stir in the citric acid and divide between the bowls. Quickly add your flavours and colours, stir well, and then pour into the prepared tins. (65°C isn’t too hot for you to taste test the solution and see if you are happy with the flavour, so I encourage you to do this. Especially with the gin, be cautious, because you are adding liquid, which reduces how tightly the jelly will set, so you want to add it a bit at a time, and stop as soon as you have a decent hit of flavour.

Exhibit A: what happens when you accidentally add too much blue food colouring after waiting too long and don’t stir it in very well…

Let sit at room temperature for 12-24 hours to set.

Sprinkle with caster sugar, and put more caster sugar into a clean bowl. Cut the jellies into squares or shapes, and drop into the sugar, rolling them well – the sugar will stop them sticking to each other and everything else.

Serve in a petri dish under blue light for extra scienceiness!

Without the blue light…

Also, if you like this recipe and want to chuck some money in the direction of cancer research, I would love it if you would sponsor me or sponsor my team for next week! (Donations close in mid-November and are very much appreciated).

This may be my favourite photo ever.


This recipe is vegan, gluten-free, nut-free, and probably very slightly protective against malaria (but take your anti-malarials anyway, because the amount of quinine in here is really very small and not enough to deter more than one mosquito at most). It is not and never will be low GI. I mean, it is literally made of sugar. But you can use any flavours you like, and you probably should.

As mentioned at the start, this recipe, while very easy, does take some advance planning. But if all you want is a glowing dessert, you can definitely make up any jelly mix with tonic water instead of water and get the desired result. Personally, I feel that Frog In A Radioactive Pond is the next big thing in Halloween desserts, but I could be wrong about that.

I’m still stuck on the fact that they still put quinine in Schweppes tonic water. I really would have assumed that they would be using an artificial flavour by now, but my black light says nope to that theory. I am also wondering how much tonic water one would have to drink to have eerily glowing intestines if one were cut open, and also whether mosquitoes don’t like quinine flavoured blood because they are quite small and really don’t want to glow under black light, which is the sort of train of thought that happens when I start writing blog posts at eleven at night.

Print Friendly, PDF & Email

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.