Wednesday, 26 September 2012

Understanding Membrane Biology and Surface Tension with Meringue

I am particularly interested in Molecular Gastronomy.  As a biophysicist and amateur chef I love thinking how each protein is becoming denatured, how there is a change in the hydrophobic properties of pasta, or how the phase properties of chocolate changes in a double boiler.  It is amazing and scientific.  People are still exploring new ways to make and manufacture food.  In fact we do have a lot of interesting things to discover in food properties that we never thought imaginable unless science and experimentation is introduced.  Things like eating new kinds of species (e.g grasshoppers), discovering a new process to make something e.g. space icecream or using a different kind of bacteria or yeast for Kimchi or beer production.   We have a lot of things to discover in food since for me making  

Inside-Out Proteins Bubbles

At some point people (possibly French people I should check in Jen Gardner's book Meringue) started beating egg whites, a pinch of sugar and some vinegar so much that they foamed.  The hydrogen bonds were broken then reformed repeatedly until they made a new kind of structure.  The ovalbumin and lysozyme in egg are normally in a water soluble medium so they have their 'water-loving' part on the outside and their 'water-hating' parts on the inside.  This is also their minimal energy state.

However, when you beat these proteins into submission half the proteins are formed with water and the other half are exposed to air.  A new minimum energy configuration is made where you 'water-hating/hydrophobic' proteins are on the outside and the 'water-loving/hydrophilic' proteins are on the inside.  It is as if you were to blow a soap bubble and one part of the soap is exposed to the water in the middle whereas the other part is exposed to the air.  Meringue is these bubbles with two layers of protein separated by water in the same way as the soap bubbles.  Sugar is also mixed into the egg whites to increase their viscoscity.  Cooking helps remove some of the water so the layers become stiff with the sugar forming the hydrogen bonds both sides of the proteins.  If one were to look at a a cross section of a meringue bubble you would see: air outside, hydrophobic amino acids of outside protein, hydrophilic amino acids, sugar, hydrophilic amino acids of inside protein, hydrophobic amino acids, inside air.

Colloidal Mediums 

So we understood the actual molecules (the proteins) behave but let's look at the bubbles as a whole.
The surface tension in the layers of water molecules on the inner and outer surfaces of membranes, hold the water and protein bubble membranes together to create a foam.  The foams are called colloids.  A colloid consists of a  dispersed phase (protein) and a continuous phase (water).

From a thermodynamic point of view,  all emulsions and food colloids are unstable e.g. the free energy of food is higher in the emulsion or colloidal state than it would be if the food were to separate fully into two (or more) macroscopic regions. For example, a meringue is only in a medium of the proteins and the sugar for a certain period of time before the foam breaks down (baking it makes the colloid more stable).

Once you stop whipping the eggs and leave the foam you will see it separate.  The internal interface area of the system creates the excess free energy of an emulsion.  The excess Gibbs free energy of creating a surface of area, dA, can be written as dG ) γ(dA), where γ is the surface free energy density or the surface tension γ ) ((∂G)/(∂A))T,p.  Mixing other ingredients like sugar, emulsifiers or surfactants can help to change the free energy of the system and potentially stabilize the colloid mixture so it won't separate so you can get the Meringue Pie for whatever occasion!


Molecular Gastronomy: A New Emerging Scientific Discipline Peter Barham et. al  (2010) Chem. Rev. 110, 2313–2365