I had a big old chat about spherification the other day. One of the first things I wanted to explore was how we can use reverse spherification to create liquid centre rhubarb and custard jelly. Spherification allows us to seal off liquids and purees into spheres, meaning we can start hiding these liquids in places they previously wouldn’t have been allowed to play. Imagine bubbaloo chewing gums, or lava hot chocolate cake, but you’re no longer restricted to the cooking process! Do read through the spherification post for full details of how the process works!


My Nan loves her fresh veg and gardening, but she’s in her later years so taking care of her garden is largely past her these days. So although I help out often, we’re now restricted to whatever is left growing wild in her yard from yesteryear, and the small things we can plant and not have to manage very much. Very fortunately, three of my favourite things from her garden have survived the weeds – rosemary, mint, and rhubarb. And by-George there is tons of all three.

Freshly picked rhubarb, sous vide with plenty of sugar and lemon until tender, is a thing of beauty. Adding some fresh egg custard or crème anglaise is heaven in a bowl. As far as flavour pairings go, these two are bunked up, married for thirty years, and the kids have grown up and gone to college. It’s a happy time in rhubarb and custard land. There was even a crazy kids cartoon named after the pair (although you may have a seizure while watching that animation…) No comments on how old this makes me, please!

Alas, the season is no longer upon us, and fresh rhubarb is long gone until next year. But blogland refuses to wait for the sun to come up in England (could be a few years, giving our weather), so I’m going to have to use tinned rhubarb for this liquid centre rhubarb and custard jelly. To be fair, for the purposes of testing the process and experimenting, it’s far easier and more flexible to use store bought rhubarb. The same rules apply for the custard – homemade fresh egg custard might be unrivalled in flavour, but it’s no place for wanton experimentation where things could go catastrophically wrong. It would break my heart to see my slaved stirring time over the stove going down the drain as a miserable failure.


This recipe does use gelatine, so it isn’t vegetarian. There are alternatives in the form of agar-agar and other products. If you’re confident with substituting, then feel free to; we’re just making a standard jelly, the gelatine is doing nothing fancy. I’m not a fan of the texture and bite that agar has, but this can be mitigated using locust bean gum. I’ve only used this method to make gels, so I’m not well versed in translating it to a full jelly. Definitely something for the lab though – if the recipe can be vegan then it should be, especially when on the surface the dish appears vegan friendly.

So my process was fairly straightforward. I use reverse spherification to make custard spheres; I make a rhubarb puree, dissolve gelatine into it, then pop a sphere into the centre before chilling. The jelly would set, the liquid centre would stay liquid, and I would delight at my liquid centre rhubarb and custard jelly.


Two things surprised me. 1) The fact that it worked exactly, precisely as I’d imagined; and 2) how darn gorgeous the rhubarb jelly was considering I’d never made one from scratch before. I do hate blowing my own trumpet, but I was so shocked that it worked so well, I called my mum into the kitchen to devour the jellies, totally forgetting to take any photos! Another batch later, and things were back under the lens.


There is still room for improvement here. The custard could be sharper – I’m using store bought, so that in itself is half the reason. Adding some fresh vanilla or some essence would be a step in the right direction. The fact that the store bought custard was so thick meant I needed to add milk to loosen it enough to form spheres. I’m sure this was to blame for the weak flavour. I also wanted to consider better moulds rather than the ramekins I used. My spheres were large, so that forced my hand somewhat. Although probably a perfect full dessert size, much smaller and daintier would be more aesthetically pleasing and camera-friendly. It would also be far more approachable for anyone unsure of the process.

Lastly, a little confession. The tinned rhubarb was awfully pale and would have looked anaemic as a jelly, so I’ve added about 1tsp of red food colouring. This adds nothing other than to the eye, so I’ve left this out of the recipe.

Liquid Centre Rhubarb and Custard Jelly

Prep Time 30 minutes
Cook Time 10 minutes
Total Time 40 minutes
Servings 4 large jellies, more if smaller


  • 400 g tin Rhubarb drained
  • 2 Tbsp Sugar
  • Squeeze lemon juice
  • 4 leaves Gelatine
  • 1 litre Bottled water
  • 5 g Sodium Alginate
  • 60 ml Milk
  • 2 g Calcium Lactate
  • 120 ml Custard from a tin or carton


  1. Pour 1 litre of water in to a flat bottomed container
  2. Blend in 5g Sodium Alginate using a hand/immersion blender – (Alginate Bath)
  3. Place in fridge for at least 1 hour until bubbles have gone
  4. Mix 60ml milk with 2g Calcium Lactate until calcium is dissolved
  5. Add to 120ml custard and stir well to combine
  6. Spoon custard in to Alginate Bath to form spheres (a teaspoon gives a good size)
  7. Remove after 1-2 minute with slotted spoon (leave for longer if spheres burst easily, less if too thick skinned)
  8. Rinse in a bowl of water to remove excess gel, then set aside
  9. Blend drained rhubarb to a puree, then blend in the sugar and lemon. Adjust to taste
  10. Soak gelatine leaves in cold water to soften
  11. Pour puree into a saucepan, place over a medium heat and bring to a simmer
  12. Shake off excess water from gelatine leaves, then add to simmering puree. Mix to dissolve
  13. Pour a small amount of puree into ramekins, then carefully add a sphere of custard. Completely cover with remaining puree.
  14. Allow to cool, then refrigerate until completely set. Remove from ramekin and serve.

How do these little spheres form? The sodium alginate turns into a gel when it comes into contact with calcium. So by placing a calcium-enriched custard into an alginate bath, the alginate around the edge of the custard starts to form a gel film. This is our baby and makes everything work so mouth-burstingly delicious. Read more about it in my opening blog on mango spherification, which also covers things like why we use bottled water and why the spheres can’t touch each other when in the alginate bath.


Why are we adding calcium lactate to the custard? Isn’t the custard already rich in calcium? You’d think this wouldn’t you. But when I tried it (albeit many moons ago) without the added calcium, it didn’t want to play ball (pun intended). Maybe it just wasn’t enough calcium, or perhaps it’s the store bought custard. Either way, the calcium lactate was necessary.


And the milk? As mentioned above, this was to thin the custard. Another factor of using store bought custard is that it is heavily thickened. As much as this is prized in the mouth, it doesn’t help when trying to form spheres in an alginate bath (how inconsiderate of the manufacturers!). So we loosen the mixture up just enough to allow spheres to form.


Why are many of your balls disfigured and look nothing like spheres? Making them perfect spheres is VERY tough! Usually your very first attempt will be the best one. After that, alginate attaches itself to the spoon and starts forming gels all over the place, making the spheres more and more wonky. If you have 10 spoons to use would stop this. There is a process called frozen reverse spherification that we can employ whereby you freeze the custard into perfect spheres, place it in the alginate bath, then as it thaws the external wall creates the gel while holding its perfect shape. Would the custard freeze ok? If so, then this is an excellent method. But truth be told – we’re hiding these centres in the middle of big old jellies. It doesn’t really matter how pretty or symmetrical they look as we won’t see it when we’re deep into the dessert. So let’s just have wonky ‘spheres’ and be done with it!

Why didn’t my jelly set? I’ve gone on the packet instructions that came with my gelatine, which uses 4 leaves per pint. If your gelatine needs more, then adapt as necessary.

Can I have this dessert hot? No, as it will melt the jelly. Gelatine dissolves at around 40C, and ‘hot’ is usually in excess of 60C. However there is a substance called methyl cellulose that is solid when hot and turns to a liquid as it cools. I’d like to play around with this to see how it could work!


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