Digestible Science: Foams and The Baked Alaska

The Baked Alaska's diversity of temperature and texture makes it a unique treat, and one worthy of further inspection. (Photo by Finny Philip)

The Baked Alaska’s diversity of temperature and texture makes it a unique treat, and one worthy of further inspection. (Photo by Finny Philip)

The ethereal mouthfeel of a meringue makes it a prime candidate for desserts such as the Baked Alaska, a dish traditionally made with soft meringue piled on silky ice cream piled on moist sponge cake. Whisking egg whites integrates a bit of the ether into the egg’s protein matrix, creating foam that is part liquid and part air. Add to this mix the element of fire, which browns the meringue but is forbidden from warming the ice cream inside, and the balance of the elements is complete.

The Baked Alaska is dependent on two types of foams: ice cream and meringue.

The Meringue:

The great divide between foams is whether they are made of fats or proteins. Meringue is essentially bubbles of air trapped by egg white proteins and water. The proteins involved, mainly ovalbumin, are chains of amino acids with various affinities for water. Normally the proteins are in a three dimensional tangle, but whisking unravels these proteins and also adds air into the mix. Water-loving parts of the proteins will tend to stick to themselves and hydrophobic parts of the proteins will be at the protein-air interface. Unraveling these proteins and allowing them to come back together in a new configuration is what gives meringues stability and airiness. The addition of fat or the enzyme amylase from egg yolk means death for the protein-based meringue, just as the addition of protein to a fat-based foam would be detrimental.

An overworked meringue can lead to an emotional breakdown, especially if made by hand. Just as proteins unraveled and began interlocking again, they squeezed themselves together too tightly and forced out water molecules. This happens because proteins in eggs are prone to forming sulfur to sulfur (disulfide) bonds with each other, but this can be stopped with the addition of hydrogen ions from acids. One definition of an acid is how well it can give off ions, which can cap a sulfur group and prevent it from bonding to another sulfur group. Lemon juice or cream of tartar are commonly used to help prevent over-whipping.

Gravity and water’s tendency to attract more water to itself are two of the meringue’s natural enemies. If the protein matrix is not set by cooking the meringue, it will eventually leak fluid, a process called “weeping” or “syneresis”

According to Harold McGee’s On Food & Cooking,egg white is 90% water. Since proteins make up the backbone of the structure and excess water tends to leak out of it, concentrated egg white powders are often used to create better foams.

Adding sugar to the meringue will help stabilize it, prevent weeping and give it more body, but only if it is added gradually as the meringue is in its soft peak stage. When sugar is integrated into the liquid, it becomes syrupy and less likely to weep; this viscous syrup won’t allow for thin walls in between bubbles, so the end product is a meringue that has less volume but is more stable. Ordinary granulated sugar won’t dissolve into the foam very well, so extra fine sugar works best. Powdered sugar contains up to 10% cornstarch which can absorb moisture but also change consistency[F2] . Accounting for this I made a meringue with 28 grams of powdered sugar; the result was a dense foam that wouldn’t set as easily as the other foams. Italian meringues use syrups, which can be infused with flavors. For the sake of keeping the Baked Alaska under a million calories (yes, those are kilocalories, science pedants), I wanted to cut back on the sugar content while also achieving the best consistency. 40 grams of sugar was the most I could put in the meringue without having it become cloyingly sweet.

Foams have interesting properties when it comes to transferring heat. Aerogel—used by NASA to catch comet ejecta moving at speeds up to six times the speed of a bullet—is a silica-based nanostructure that approaches the density of air. Bubbles in a meringue are arranged geometrically in such a way that liquid and air mix as little as possible (called a Weaire-Phelan structure for you geometers). Layers upon layers of bubbles acting under gravity will eventually cause bubbles at the bottom to become spheres, which have a lower surface area for the same amount of air and will cause foams to weep as more liquid is forced to come together. Unlike meringue, Aerogels don’t weep and have a porous, dendritic structure that keeps its solids and its air from either conducting or convecting heat.

There are three ways which heat can be transferred: conduction, radiation and convection. Conduction occurs when two objects touch each other, transferring thermal energy directly. Radiation transfers heat through the electromagnetic spectrum and is how we feel the Sun’s heat through the vacuum of space. Convection is a mix between conduction and advection, the movement of a bulk of substance.

That little caveat about advection is what allows solids such as the ductile rock in the Earth’s mantle to slowly convect, redistributing heat as the giant mass moseys near the Earth’s crust and back down again.

Since the air in a meringue is trapped in stable pockets, advection can’t occur throughout the entire egg foam.  This allows the top of the meringue to brown, the proteins in the interior to firm and saves the ice cream is saved from a soupy fate. Think of it like a thin, but oh-so-warm down comforter. The fluffy feathers in down form a three dimensional structure that traps air and uses it as insulation.

Photo by Finny Philip

Photo by Finny Philip

The high temperatures needed for the browning to occur are what make the existence of the Baked Alaska extraordinary. Despite the flames of a hot broiler or butane torch threatening to lick its surface, the meringue insulates its iced payload. At temperatures of 145°F/63°C egg white proteins begin to solidify; the major protein in egg whites, ovalbumin, will begin to coagulate at 180°F/80°C. The result is a crisp outer edge encompassing a moist, eggy solid enveloping creamy ice cream dropped unceremoniously on crunchy graham cracker crumble.

Ice Cream:

Photo by Finny Philip

Photo by Finny Philip

Ice cream is a foam, albeit a set foam. So is bread. For some reason, this revelation is more earth-shattering than the fact that solid rock undergoes convection. Air can account for up to half of ice cream’s volume. Ice cream starts out as a creamy liquid that is mixed thoroughly with air. As it freezes, water crystals form and leave the creamy components behind, creating a more concentrated cream. The ice crystals become the backbone for the cream and air, creating scoopable foam.

While meringue is a good insulator, ice cream does begin melting soon—at 22°F half of its water is in liquid form. A unique way of making ice cream is mixing in liquid nitrogen, which boils at -320°F/-196°C. I couldn’t get my hands on any liquid nitrogen for Baked Alaskas, but if I did, some serious science would have happened.

In any case, serious science does happen whenever you’re in the kitchen. Even something as simple as the Baked Alaska is a complex world where the disciplinary walls between physics, biology and chemistry break down. It’s a world where chefs become polymaths and curiosity and hunger are the driving forces of new science.

Rocky Road Baked Alaska

By Finny Philip Published: January 22, 2013

  • Yield: 4 Servings

The Baked Alaska is traditionally made with cake, ice cream and meringue, but as a Rocky Road fan, Finny has taken the liberty of creating a s’mores version that replaces cake with crumbled graham cracker pie crust.

Ingredients

Instructions

  1. Crumble graham cracker crust and create a thin layer of graham cracker crumble in a heat-proof serving container. If you’re using an over-fired broiler, preheat it.
  2. 2. Add your acid of choice (lemon juice or cream of tartar) to your egg whites. Beat the egg whites until soft peaks appear and your meringue starts sticking to the walls of the bowl. Slowly add sugar and stop beating the mixture when stiff peaks begin forming.
  3. Add ice cream on top of the graham cracker crumble and drizzle honey over the ice cream. Using either a pastry bag or a mixing spatula, place meringue on top. For completely frozen ice cream and a warm meringue, it is possible to make everything but the meringue ahead of time and have it ready in the freezer. Just be aware that placing an ice-cold container under a broiler may cause it to shatter, so carefully use a blowtorch in this case.
  4. 4. If you don’t have a blowtorch, place the containers on a cookie tray and put the whole thing under the broiler until the meringue browns. This will happen very quickly, so keep an eye on it. If you do have a butane blowtorch, follow its instructions carefully. While blowtorches can heat the meringue with precision, placing the Baked Alaska under a broiler subjects it to even heating that is more likely to set the meringue (but also more likely to melt the ice cream by heating from below).
  5. Serve it forth.

About Finny Philip