Week 8: Emulsions & Foams, Harvard Style

Did you know: Truffle centers made of chocolate ganache are an example of an emulsion?

In Week 8 of the Harvard online food science class, we finally get into something that looks familiar from culinary school. It's the idea that not all foods dissolve into each other. Some exist in suspension, that is, when forced together, they make an emulsion. You've eaten an emulsion if you've ever had mayo or hollandaise, but cake batters, cookie doughs, souffles, vinaigrettes, whipped cream and even chocolate ganache are also emulsions.

Let's define emulsion - it's a state in which drops of one immiscible fluid are mixed into a second fluid. If you pour oil into vinegar and  shake the bottle vigorously, you suspend droplets of oil into the vinegar  to make a vinaigrette. They'll mix together so you can dress your salad, but they won't stay together forever. Oil is lighter than vinegar, so little oil droplets that were in suspension end up floating to the top, where they begin sticking together with other oil droplets on the same journey. Eventually this oil mass separates from the solution and stays on top until you do something about it. (That's when you have to shake shake shake shake the bottle again to break the oil back into small droplets.) In this case, the vinegar is known as the continuous (usually fluid) phase, and the oil is known as the dispersed phase (because it gets dispersed in droplets throughout the vinegar). That's one kind of emulsion - but not the only one.

Now why do bottled dressings from the fancy food store seem to hold together and avoid separation? They may contain surfactant ingredients. A surfactant - surface active molecules - likes both water  and oil. Surfactants sit at the interface, or the surface, between the two fluids and help hold onto both the oil and the water in a chemistry kind of way. They also prevent droplets from colliding with each other and forming larger droplets which, if left to their own devices, can start to separate out of the solution. Lecithin in eggs is an example of a good surfactant - and it's why egg yolks can help hold an emulsion together (one of the go-to fixes of saving a broken Hollandaise is to beat in another egg yolk).

A foam represents the same idea of one substance suspended in another, but instead of a liquid dispersed in another, you whip air bubbles into a liquid, as in a souffle. These bubbles can also "cream," or rise to the top, where they destabilize more easily and pop, there goes that emulsion.

Did you know: Nitrous oxide, which is in those cool canisters that The Buck uses to add whipped cream to your latte, turns into liquid under pressure? When forced through the canister of cream, it becomes an emulsion of two liquids.

Chocolate ganache is another type of emulsion, where particles are dispersed into a liquid. In this case, cocoa solids in the chocolate are mixed with the continuous phase of cream. Solid particles in a fluid are called colloids. These colloidal particles like to exist at an interface between two fluids (think of melted chocolate and cream as the two fluids).

If you've ever tried to mix cream into chocolate and watched it turn oily, you've seen an emulsion break. The go-to fix is to bolster the continuous phase - the cream - by adding more cream and stirring heartily while whispering encouragement.

How is it that emulsions fail anyway?

In the liquid in liquid emulsion, oil and water have different densities. Eventually the oil will do what's known as "creaming," by rising to the surface of the water. The Week 8 review sheet calls this "separation of phases by gravity due to density differences." Oh, gravity. To get the oil and water back together, you have to whip this solution to break the oil into small droplets that recombine into the continuous phase of water.

Then there's "coalescence," where two oil drops come together and form a larger drop; it's small droplets that give an emulsion its character and smooth mouthfeel. Too much heat can also break an emulsion, which you know if you've ever left a Hollandaise over a bain marie of steaming water and your eggs coagulated, forcing liquid out of the egg protein network like wringing a sponge.

And then there's something called Ostwald Ripening. It's the transfer of the fluid from one drop to another. There's a driving force and increased pressure to drive the fluid from the small drops into large drops. My notes here say: Aggregates of surfactant can swell slightly, absorbing some oil, and can transfer that oil across the continuous phase from one small drop, where the pressure is large, to a large drop where the pressure is small. Small droplets get smaller and large drops get larger. Eventually the small droplets disappear. I had to look at the Week 8 review sheet on this. It says, "Despite an energy barrier, individual molecules from the dispersed phase occasionally dissolve in the continuous phase, through which they can travel to other droplets/bubbles. Over time, there is a net movement of molecules from small bubbles/droplets to larger ones." There you have it.

Sometimes the continuous and dispersed phases change. The continuous becomes the dispersed, and the dispersed becomes the continuous. You've seen this if you've badly made a cookie batter, where you had a water-in-fat emulsion while creaming your ingredients, suddenly the emulsion breaks and you have the reverse - a fat-in-water emulsion where bits of fat are floating around in your solution.

Each type of emulsion break can often be fixed, and it usually involves agitation to break big drops into little drops via whisking, sometimes includes heating over a hot water bath so the continuous phase becomes more fluid, along with vigorous whisking (good for buttercreams); and sometimes you just have to add more continuous phase, such as adding more cream to ganache, to help suspend those cocoa solid colloids in the cream.

It is not scientifically established that hurling curses at the mixture can right the wrongs, but encouraging words are never discouraged, so try it.

Equation of the Week: E = 0/r (phi - phi c), where E stands for emulsion, r is the radius of the droplets, phi c is the critical volume fraction and o is the interfacial tension. To understand this, you need a Harvard professor, so take the course and do the legwork.

Ah-hah Moment of the Week: To freeze egg yolks, add 1/4 teaspoon of simple syrup per yolk. (Simple syrup with a ratio of two parts water to 1 part sugar). This lowers the freezing point of the eggs and prevents ice crystals that disrupt the protein network.

Next Week: It's Baking!