Emulsions and foams
| Ice cream is a:
Foam of air bubbles,
Stabilized with small (yellow) oil drops,
In a matrix that is,
An emulsion of more oil drops
And a suspension of (blue) ice crystals,
In a continuous (grey) phase of surfactants, micelles, and solutes
In a sugar cone.
|Phase 1||Phase 2|
Cake batter is an emulsion and foam
A cake mixture is a complex, multi-component system, being simultaneously a foam, an emulsion and a complex colloidal dispersion. Of the main components of a cake batter - egg, flour, fats and sugar - only the sugar is non-colloidal. The process of transformation of these ingredients into cake (i.e. solid foam) is not completely understood, although it is known that for the success of the recipe it is vital to retain the air bubbles within the cooked batter.
To better understand the 'foamy' nature of cakes, let us break up the cake batter into its most essential components:
1) Flour, which is mainly composed of gluten and starch. Gluten was named by the Chinese 'the muscle of flour' due to its elastic nature. Gluten is composed of long protein molecules ( gliadins and glutenins) which are responsible for the elastic behavior of dough. Upon kneading and stretching of dough, gluten proteins unfold and align. The coiled, spring-like structure of proteins can unfold and store some of the energy of stretching, but when the stretching is stopped, the molecules spring back to their compact coiled form. This is macroscopically manifested when stretched dough creeps up to its original shape. Although bread preparation benefits from a strong, elastic gluten, excessive amounts of the protein are not desirable in puffy pastries and raised cakes. Ways to limit gluten presence in batter are the use of low-protein flours as well as adding water in the dough, which dilutes the gluten proteins and limits their bonds.
Starch makes up 70% of the flour weight. Therefore it is a structural component of doughs, especially the low-gluten cake batters. Along with water, starch interpenetrates the gluten network and break it up tenderizing the dough. During the baking of bread and cake, starch granules absorb water and set to form the rigid bulk of the walls that surround the bubbles of carbon dioxide. At the same time, their swollen rigidity stops the expansion of bubbles and forces the water vapor inside to pop the bubbles and escape, turning the foam of separate bubbles into a continuous network of connected holes. If this didn't happen, then at the end of baking the cooling water vapor would contract and cause the cake to collapse.
2) Eggs, which contain proteins, fats and emulsifiers. The proteins in eggs coagulate during cooking and supplement the gluten structure. The fats and emulsifiers in eggs work like starch, weakening the gluten network and stabilizing the bubbles in the dough.
3) Fats, contained in oils and shortening. Fats are an important component in pie crusts and puff pastries, where layers of solid fats separate thin layers of dough from each other so that they cook into separate layers of pastry. In cakes, fat and oil molecules bond to parts of the gluten protein coils and prevent the proteins from forming a strong gluten. This is the reason why in making bread, which requires a strong gluten, flour and water are mixed alone.
4) Gas bubbles make up as much as 80% of a cake's volume. These weaken the gluten and starch network and divide it into millions of delicate sheets that form the bubble walls. Baking yeasts and chemical leavenings are routinely used to fill baked goods with gas bubbles. A common misconception is that these products create new bubbles: in fact, the carbon dioxide in yeast is released into the water phase of the batter, diffuses to the pre-existing air bubbles and enlarges them. This is why the initial aeration of dough and batter through kneading, strongly influences the final texture of baked goods.
Information adapted from:
- Ian W. Hamley, 'Introduction to soft matter', John Wiley & Sons (2007)
- Harrold McGee, 'On food and cooking: the science and lore of the kitchen', Scribner NY (2004)
N.H Hyam, "Quantitative Evaluation of Factors Affecting the Sensitivity of Penetrant Systems". Materials Evaluation, pp. 31-38, February 1972.
A. V. Rode, "Electronic and magnetic properties of carbon nanofoam produced by high-repetition-rate laser ablation". Applied Surface Science 197–198: 644–649. 2002.
Ice Cream Floats
One interesting point building on the look into ice cream is the role it plays when you make an ice cream float. If you've ever made one, you'll remember that when the ice cream is added to the soda that it begins to rapidly foam up. You can see this in the image below:
Looking at this, it begs the question of why the ice cream causes an increase in foam in the soda. The reason is simple. As mentioned above, ice cream is an emulsion of various things like fat and sugar. As it melts, these tiny particles are released into the soda. As you know, soft drinks are liquids that have been supersaturated with carbon dioxide. Soda fizz comes from the nucleation of the carbon dioxide. The particles from the ice cream alter the solubility of the carbon dioxide and enable the creation of foam nucleated on the small particles. The amount of foam created is directly proportional to how much CO2 is diluted in the liquid. Furthermore its the fat serving as the instigators of nucleation that further the stability of the foam produced.