Thursday, April 22, 2010

Know your Foam

Ever experience the loss of a beautiful, fluffy, whimsy meringue from the top of a Lemon Meringue Pie. No? How about a Key Lime Pie? Well, I have….and it sucks! The meringue topping is the essence of the pie….it’s the show stopper that gives the pie height and dramatic effect. It doesn’t make much of a statement sitting next to a crust filled with lemon curd. Oh, and there’s no worse way to crush an ego than slicing into such a perfectly aesthetically pleasing pie and watching the essence slip and slide over an unattractive, soupy pool of watery egg protein.

Don’t sweat! This slippage can be prevented, but not cured. So, listen carefully as I explain the science behind perfectly whipped egg whites.

First, let’s look at the structure of whipped egg whites...

Foams consist of an air phase of tiny air bubbles dispersed in a continuous water phase. Proteins (albumin) are the molecules in egg whites that create a foam structure by forming a thin film or network around air bubbles. Upon physical stress, such as whipping, beating or shaking, proteins undergo transformations that allow them to bind together and arrange themselves around air bubbles. To make meringue, sugar is added to beaten egg whites at different stages to stabilize the foam.

There are three main culinary terms that are used to describe stages of foam development:

Soft peaks – refers to glossy foam edges that keep some shape but droop once lifted.

Stiff peaks – refers to glossy and defined foam edges that hold their shape. Peaks stand up straight once lifted. At (or sometimes just before) this stage is ideal for making souffl├ęs and sponge cakes, where further expansion occurs during baking. It is also perfect for shaping, decorating and piping.

Dry peaks – refers to a dull and firm foam that appears lumpy and crumbly. This is commonly used to make very soft-textured frothy meringue, such as floating islands, or stiff cookie batter.

There are 3 important ingredients that can help stabilize egg white foams and prevent weeping:
  • Sugar
  • Acid
  • Salt

Sugar:

Sugar is hygroscopic, which means that it binds water. When added to whipped egg whites, sugar binds the water between air bubbles in the foam and creates a thick syrup that prevents it from leaking out.

Acid:

Adding some form of acid, such as cream of tartar, vinegar or lemon juice, to egg whites before beating improves foamability. Firstly, acid helps to denature or unfold proteins to expose hydrophobic (air-loving) areas on the proteins that tend to arrange themselves around air bubbles. Acid also neutralizes the charges on egg-white proteins, which makes them less repulsive (I don’t mean less ugly, just less hateful of each other and more friendly I suppose) and more likely to come together and form a network around these air bubbles. The stabilizing function of acid may be due to an increased concentration of hydrogen ions (H+) which hinders the formation of strong sulphur bonds (S–S) between proteins. These sulphur bonds can be so strong that they squeeze water from between protein networks surrounding bubble walls.

Without Acid:

With Acid:
 

Salt:

Salt has a neutralizing effect on egg white proteins which increases their foamability and foam stability. When salt dissolves into the water phase of egg whites, it dissociates into sodium (positive) and chloride (negative) ions which neutralize the charges on egg proteins and increase their attraction to air bubbles.


If you've ever whipped egg whites without any acid or salt you might have noticed that shortly after you’ve put all your heart and soul into achieving perfect peaks, a puddle of water has developed in the bottom of the bowl underneath the all the frothiness. So, next time you're whipping up a batch of chocolate souffl├ęs or a meringue pie, remember that those tiny amounts of what seem like minor ingredients will greatly increase your chance for success! A good rule of thumb is to add 1/8 teaspoon of cream of tartar or 1/2 teaspoon of vinegar or lemon juice per egg white before beginning to whip or beat.
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