Figure 1. Mayonnaise, something that necessarily requires the emulsification of fat and water.

Emulsification is the word scientists give to mixing liquids together. Specifically, it is the process by which immiscible liquids, liquids that cannot normally mix, are mixed together. The chemical agent used is called an emulsifier. Emulsifiers are not unique to food, we use emulsifiers to clean ourselves (soap), and in order to produce medical injections.

Emulsifiers in Food

The most familiar emulsifier is probably the egg. Eggs are used as an emulsifier in everything from cakes and custard, to mayonnaise (Figure 1) and from hollandaise, to soufflés. What the egg is doing chemically is allowing the other ingredients to form a stable emulsion (mix). Interestingly, egg itself actually contains two types of emulsifier: one is protein, the other is lecithin (Figure 2). They both have chemical properties that are shared by both water and fatty substances. This helps mix things up sufficiently well to make a homogeneous mixture of water, fat and lecithin, that looks not unlike baby sick. There are others that are used regularly in cooking too. As well as the egg in mayonnaise, mustard powder is also added to many recipes. This also helps it to stay homogeneous.

Figure 2. The structure of lecithin. The blue section is water-liking.


Industrial Emulsification

In industrial food production, several harmless emulsifiers, not common-place in the home, have been used for some time. One such is xanthan gum. The name is perhaps misleading as it is not a gum as such when bought, but an off-white powder that is usually accused of being either cocaine or flour. The usage of the word gum is perhaps clearer when we consider the properties it has on being mixed with fat and water. Figure 3 shows the transformation. Here, I include weights and volumes so you can do this yourself if you want to.

Figure 3 – mixture of oil, water and xanthan gum (E415) forming an emulsion that is also thickened. A: oil and water, B: xanthan gum as available commercially, C: xanthan gum with oil and water before shaking, D: water, oil and xanthan gum mix after shaking, showing solidification, E: solid mixture as compared to xanthan gum.

In part A we see the oil (clear yellow layer, 10 mL) above the water (colourless layer, 25 mL).  Adding the xanthan gum (B, 5 g) appears to do little initially (C), however a brief agitation of the system leads to homogenesis of the three substances (D).   Not only do we have an homogenous mixture, but also one that is thicker than it was previously – squeezing it out gives the appearance of an off-white turd (E).  Said turd goes brilliantly with a sprig of basil or rosemary and glass of chilled white Sancerre.