Rum Bubble Surprise




One of the great things about molecular gastronomy is it gives us techniques that allow surprising and unusual elemnts to be incorporated into a dish that would be otherwise impossible.

In this riff on bananas foster, a sphere of rum is served on top of a piece of cake and a slice of banana, after being covered in delicious meringue.

"What?!" I can hear you saying, "a sphere of liquid in close contact with meringue? Preposterous!"

Well, I can assure you that yes, it's true. Read on for the secret!

Step 1: Rum Spheres, the Science!

For this, you'll need two ingredients that aren't found in the average kitchen, sodium alginate and calcium lactate or calcium lactate gluconate. I got mine from, but there are a number of specialized food sites that you can locate them at.

These can also be used in the recipe for carrot caviar here on Instructables!

Making the rum spheres is called, unsurprisingly, spherification. The cornerstone of this process is the hydrocolloid mentioned above, sodium alginate.

Sodium alginate is a natural polysaccharide - a long chain made up of linked sugar molecules. As the sodium salt, the chains are loose and flexible, thickening up a water solution but leaving it mostly fluid. Expose the alginate to calcium, however, and neighboring chains are crosslinked together by electrostatic forces, which instantly creates a gel!

So what we want to do is make a tasty rum solution and add some calcium salts. Freeze that, drop it into a water bath containing sodium alginate, and let it thaw. The crosslinking reaction takes place at the interface, creating a thin membrane with your tasty liquid inside. Poke the membrane with a knife, or bite into it, and bam! Explosion of flavor!

The reason that you need calcium lactate or calcium lactate gluconate for this recipe is that they have a very light unobtrusive flavor. If a simple salt like calcium chloride was used, the taste would be unbearable.

The recipe I'm using for the rum balls is adapted from Grant Achatz's fantastic book, Alinea, which I would recommend for the beautiful pictures just as much as the fantastically creative recipes.

Step 2: Rum Sphere Ingredients

To start off, there are some tools that will make this a lot easier. If you don't have a scale, it'll be hard to measure out accurate amounts of the ingredients. I've provided my best guess as to the volumetric amounts of the ingredients you'll need in parentheses. Also required is an immersion blender to mix the alginate bath. If you don't have one, a regular blender will do in a pinch.

You'll probably want a turkey baster. You don't need one, but it'll help later on.

Also a requirement is a set of hemispherical candy molds. I have a set of cherry cordial molds that are set into a sheet of plastic - these things are really cheap, and you can find them at Hobby Lobby or any similar store.

Gather together the following:

For the rum spheres:

350 ml water
15 g (20 ml) of rum
100 g (~ half a cup, packed) light brown sugar
8 g (~2 tsp) calcium lactate or calcium lactate gluconate
10 g (~1 tbsp) molasses (optional, but does a lot of good for the flavor)
1/2 tsp Cinnamon
1/4 tsp Nutmeg
1/2 tsp Allspice

For the alginate bath:

1400 ml hot water
65 g (~1/2 cup) sugar
6 g (~1 3/4 tsp) sodium alginate

Prepare the rum ball solution by mixing the rum, brown sugar, calcium salt, molasses, and spices in the water. Heat in the microwave until everything has melted, and filter to remove the excess spices.

Make the alginate bath by pouring the sugar and sodium alginate into hot water, and blending with an immersion blender until all the solids have dissolved. This will whip a lot of air into the solution, so cover the container and set the bath aside until all the bubbles are out.

Step 3: Making the Spheres

Take your prepared rum sphere solution and use the turkey baster to transfer it into the candy molds. Freeze the filled mold overnight.

The next day, prepare the spheres by popping them out of the molds straight into the alginate bath. Gently ensure that the spheres are not touching each other or sticking to the bottom of the alginate bath container. Gently is the operative word here - when the surface begins to form it is prone to fusing to other spheres and forming ugly "tails."

Let the spheres thaw in the bath for between 5-8 minutes. Remove them from the alginate (again, gently!) using a slotted spoon and trasfer to a small container. Rinse them well with water, and they're done!

If you're storing the spheres for a while, cover them up with leftover rum solution. If they're covered with water, all the flavor will diffuse out, and you don't want them to dry.

Step 4: Cake and Meringue

There are three more components to this dessert: round slices of cake, meringue, and a banana.

For the cake:

Use your favorite white or yellow cake recipe. One 8" cake makes about 16 round bases. I used box cake, with some banana liqueur and cinnamon added for flavor.

Never made cake? Use box cake, and follow the instructions on the back. Easy as pie... Er, easy as cake, I mean.

For the meringue:

I'll cover how to make two different kinds of meringue - "easy" meringue, which involves a minimal amount of preparation, and a hot whipped swiss meringue, which requires more work but is more stable. For both, you'll need eggs, sugar, and banana liqueur for flavor.

For the banana:

You'll need a banana. Ripe, but not too ripe.

Step 5: Cake Time!

Bake the cake. The box cake I used yielded me two 8" cakes.

Pop out small circular sections of cake using a 2" cookie cutter or old tomato paste can. The cake needs to fit in your mouth in one bite, so don't go overboard.

Cut the sections of cake in half, and leave them on a cookie rack to rest while you prepare the meringue. Extra sections can be frozen.

I suggest eating the extra cake. You know, so you can be sure it's good.

Step 6: Meringue 101

A meringue is a delicate beast.

Or is it?

Conventional wisdom holds that egg foams are temperamental creatures, which will flounder, fail, and collapse faster than a mortgage bank in the face of a negative wall street rumor.

That's not really the case.

You'll hear lots of chatter about new versus old eggs, plastic versus glass versus copper bowls, room temperature versus refrigerated whites, and the like. The truth is, pretty much any clean bowl, whisk, and egg will work.

Though I would use a hand mixer instead of a whisk, for the sake of speed.

When you beat egg whites vigorously, the shear generated by the whisk starts denaturing the egg white proteins, while simultaneously introducing air bubbles. The hydrophilic (water-loving) portions of the proteins want to remain in the water, but the hydrophobic (water-hating) parts protrude into the interior of the air bubbles, stabilizing them. The proteins not tied up in the water/air interface tangle with each other, eventually producing a stiff light foam.

The quick meringue is a basic meringue - no frills, easy to prepare, but it will collapse after a night in the refrigerator. It needs a go in the oven to be safe to consume.

The swiss meringue is whisked until it reaches 130 degrees. It is safe to eat after this, since it has been precooked! This also greatly helps stabilize the finished product, and results in a stiffer, denser meringue.

The meringue you choose is up to you. I like a firmer meringue, so I prefer the Swiss. If you're just interested in fixing this quickly, for you or for friends, the other meringue is considerably easier.

Step 7: Preparing Quick Meringue

This meringue is softer and faster to make.

For the fast meringue, you'll need:

A clean container, the whitesof two eggs, 3/4 cup of sugar, and a tablespoon of banana liqueur (optional).

Put the egg whites and banana liqueur in the container and start whipping at medium-high speed. Once the foam begins to hold its shape, slowly add the sugar while whipping.

You're done when curved-over peaks in the meringue form when the beaters of the mixer are removed.

Step 8: Preparing Swiss Meringue

Get the following together:

The whites from four eggs
One cup of confectioner's sugar
A tablespoon of banana liqueur
A double boiler
A thermometer

Heat the double boiler until the water simmers. Add the egg whites, sugar, and banana liqueur, and whip on low speed until the whites have reached 130 degrees, about 7-10 minutes. Remove the upper bowl from the heat, and whip at medium-high speed until the meringue can hold its shape.

Step 9: Assemble and Consume!

You're in the home stretch, here!

Preheat the oven to 325 degrees.

Next, load the meringue into a piping bag or plastic bag with a corner snipped off.

Place cake rounds onto a small baking pan.
Put a banana coin onto that.
Balance a rum sphere on the banana. You might need to squash a dent into the center of the banana coin to get the rum sphere to stay put.
Pipe the meringue around the rum sphere. Try your best to not leave any large gaps or holes.

Pop them in the oven, and bake until the meringue turns a nice rich golden brown (7-12 minutes).


Since you don't want to burn your mouth off, let the assembly sit for five minutes, then pop one into your mouth.

When you begin chewing, you'll notice the crispness of the cake and the sweet soft texture of the meringue. when you puncture the rum sphere, hot spiced rum flows out, dissolving the cake round and the meringue instantly and unifying all the flavors.

In my opinion, it's an unforgettable dessert and worth all the work.

Bon appetit!

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    33 Discussions


    3 years ago on Introduction

    So apparently these comments are 5-6 years old. There's a good chance no one's reading this anymore. But I need to know... do you have any recommendations for replacements for someone who'd rather not use banana or, really, tropical fruits at all?

    Actually... as I've just watched your "make chocolate from scratch" video, I could go that route and use homemade dimpled chocolate rounds (plus chocolate cake and chocolate meringue). Oh god... I actually adore pairing chocolate and rum. That could be insanely good.



    8 years ago on Step 5

    Why not just use cupcake pans and only put a little bit of batter in the bottom of each. They'll cook super fast, you won't have much waste and you won't need to go through the extra effort of cutting them all out! You could even use the paper cupcake liners all just lined up on a cookie sheet. No pans to clean!

    2 replies

    Reply 8 years ago on Step 5

    That's a great idea! Man, that's going to be a huge timesaver.

    My way seems sort of stupid now. :)


    Reply 8 years ago on Step 5

    If you're looking for a particular size, your way definitely works. I've done it myself quite a number of times!


    9 years ago on Step 3

    Those are the tastiest looking liquor balls I've ever seen. Pause.


    10 years ago on Step 3

    I've done this a couple times now and I think it's awesome. I was just wondering if anyone felt like doing the math out and figuring out the proof of the resulting solution. Also does anyone know the highest proof you could freeze in a conventional above-your-fridge-freezer? I'd love to get a full shot out of these but the -80C blast freezer in the lab doesn't like it when we put food in him.

    1 reply

    Reply 10 years ago on Step 3

    Glad to hear you like it! I'll have to do the math on the spheres, Now I'm sort of curious about the proof...

    I'm pretty sure around 30% is the highest you could go in a conventional refrigerator, according to the Engineering Toolbox. Mine hovers around -15 to -20 C, and I assume that's a typical temperature. Do you have access to liquid nitrogen? I'm guilty of occasionally borrowing the dewar for a night to make ice cream. :)

    Also, at very high proofs, the alcohol might start interfering with the gel formation - at what point, though, I couldn't pretend to know.


    10 years ago on Introduction

    These look great! Willpowder, here I come! As soon my science project compounds arrive in the mail I will be making these. And I'm not completely sure I'm going to share them... Ooh -- I have a question! If I assemble them ahead of time (say, mid-morning for a late afternoon dinner party), what is the best way to store the little darlings until I'm ready to bake and serve them?

    2 replies

    Reply 10 years ago on Introduction

    I don't really know how well the assembled and uncooked dessert would keep. It might be just fine for a couple of hours, but my main concern would be the spheres - they tend to slowly leak liquid, and that could (in theory, anyway) make the cake soggy. Plus, the banana might get all brown and mushy without some sort of added protection. However, I do know that all the separate components keep pretty well in the refrigerator. The swiss meringue is pretty stable and even the "quick" meringue would probably keep in the fridge for an afternoon. The cake bases can be refrigerated or, for long-term storage, frozen. The rum spheres should be covered by reserved rum liquid for storage, so they won't deflate too much. Sorry I can't be more useful! If it helps, the assembly is pretty quick - I'm terrible at piping the meringue, but even my monkey's paws could manage to cover about two to three pieces in a minute.


    Reply 10 years ago on Introduction

    Oh wow. It never occurred to me to keep everything separated. I'm such a dork! Thanks for opening my eyes to the obvious. =)

    I think I'm going to spring these on my friends for dessert after my annual Christmas Eve dinner.


    10 years ago on Introduction

    Very cool i think i'll make these for mai (parents' ) Christmas party!


    10 years ago on Introduction

    Haha, congratulations on taking grand prize. I have yet to try this, but it's on my short list.


    10 years ago on Introduction

    Sounds and looks incredible, and the only comment I had was that it's a shame you're using dime-store spices, which are sometimes sweepings off the floor of the mill. (or so I've heard) I buy my spices from Penzey's, and believe me, it's more than worth the difference in cost. The most valuable part of this lovely dessert is your time, and it's a shame to cover that up with anything but the best in ingredients. All that being said, your dessert looks lovely, and would be an unforgettable ending to a dinner with very special friends, or perhaps just one very special friend. How long do the rum balls keep in the freezer?

    5 replies

    So, they don't seem to freeze very well. After I thawed the test spheres out, they leaked the remainder of their liquid out pretty quickly.


    I should have realized the sharp(ish) crystals from the freezing liquid would pierce the very fragile shell surrounding them. I'm sorry to have put you to the trouble. It's too bad in a way, because if you had the spheres in the freezer, the dessert would still be very, very special, but a whole lot easier to execute while your guest(s) is waiting patiently for your dessert course magic. On the other hand, it probably tastes a whole lot better than it would have if it were freezable, and the dessert is now reserved for even more special occasions. (In my case, that would probably the 25th wedding anniversary, or the kid's college graduation, which just happen to be the same year.) This really falls into the realm of "Genius Desserts," and I can't tell you how impressed I am with the recipe and technique.

    Different gels can have different syneresis behaviors - I wasn't optimistic, but I figured I'd give it a shot. :)


    You know, I feel guilty every time I use those spices in anything outside of cinnamon toast. Unfortunately, I inherited the packrat gene pretty heavily from both of my parents, so I think I'm stuck with them until I either can't stand the shame or my girlfriend throws them out when I'm not looking. On the other hand, after checking out the Penzey's catalog online, the temptation to refurnish my spice shelf is pretty strong... I'm actually not sure how the spheres would handle being frozen.They can keep in the refrigerator for a few days, but I've never been able to keep them around for longer than that! I'll try making and freezing some this afternoon and get back to you on that.