Food Lorists

Emulsion under the microscope

As you all know it is hard work to make an emulsion. The reason that justifies these frustrating minutes of whisking, is: when two liquids, that don't mix for chemical reason, are disturbed (whisking) and let to stand, they spontaneously take a configuration that minimize their contact with each other: they coalesce. They form two separate large masses, to expose less surface area to each other (see Picture). This behaviour is the expression of the force called: surface tension.

Fig. 1 Fig. 2 Fig. 3
Fig. 4 Fig. 5 Fig. 6
In conclusion it helps to start with as viscous a part of the continuous phase as possible and dilute in it, any of the other ingredients required in the recipe, after the emulsion has been established.

An emulsifier is a molecule that lowers the surface tension of a dispersed liquid in an another. The way that emulsifiers work is that they create a coating on the droplets that shields them from the continuous phase. Such molecules are partly soluble in each of the different antagonists in an emulsion: one part of the molecule is water soluble and the other part soluble in fats. It is said that they have a hydrophobe (afraid of water) side and a hydrophile (that likes water) side.

• The first ones are, like the phospholipid lecithin in the egg, are small molecule that have an hydrophobe side that attach itself in the fat phase and an hydrophile head that is electrically attracted to the water molecules of the emulsion.
• The second one are proteins which are much larger chains of amino-acids that have a number of hydrophobe and hydrophyle regions. The yolk proteins in the eggs and the casein in dairies are the best protein emulsifying agent.

Another type of molecules that can help in the stabilization of an emulsion are: the stabilizers. They are starches and plant particles. There are many kinds of particles and molecules that can help to stabilize an emulsioned sauce once its formed. They all have in common the property of getting in the way, so that the droplets that are getting towards each others meet the stabilizer instead. Large molecules like proteins or starches are very efficient in this role, pectins, gums and particles of pulverized plants too. Ground, white mustard seeds are a very efficient stabilizer thanks to its particles and gums that are released when wetted. Tomato paste too, is a really interesting stabilizer and emulsifier with its large protein content (3% !) as well as its cell particles. There is a but, though, once the emulsion has formed, the droplets may be so crowded that they bump against each other, then the force of the surface tension may pull them together and cause them to coalesce again.

Now, after the theory the practical stuff. Always keep in mind that emulsion sauces are very fickle ones. There is always a risk that the sauce splits. Three reasons for that:

• The first ingredients in the bowl must be the continuous phase and at least one emulsifying or stabilizing element. The dispersed phase must, ALWAYS, be added to the continuous phase. Otherwise it cannot be dispersed.
• The dispersed phase must be added to the continuous phase very gradually, to begin with. Then, it requires an energetic and continuous whisking all the way through the making of the sauce. ONLY, when the sauce has started to thicken and become more viscous that the oil be added more rapidly.
• Finally, the proportions of the two phases must be kept in balance. The right proportion is that the dispersed phase should not exceed three times the volume of the continuous phase. If the emulsion stats looking stiff it is the sign that it is time to add more continuous phase to the sauce.

When the sauce is finished, it should not be stored at a temperature that is either too hot nor too cold. It should not be kept at a temperature exceeding 60 degrees Celsius. They should not be stored at a low temperature either, under 15 degrees Celsius the surface tension increases making it more likely to coalesce. Butterfat and some oils, solidify at room temperature or in the fridge. This results in sharp-edged fat crystals rupturing the layer of emulsifier on the droplets. Then the sauce will coalesce and separate when stirred or warmed.

In the event of the sauce separating, there are two ways to reemulsify it.

• The first one is to put the sauce in a blender to break the dispersed phase apart again. This has its limitation. It will only work if the sauce still has enough emulsifier left intact. It won't work in the event of the sauce being overheated. Especially, sauces like bearnaise or hollandaise that contains eggs, their proteins may have been cooked thus destroying their emulsifying properties.
• The second one and more reliable one, is to start with a small amount of the continuous phase, adding in an egg yolk (optional), and carefully beat the broken sauce back into it. If the proteins in the sauce have coagulated (cooked) they must be strained out of the sauce first, then it is a good idea to add an egg yolk at the beginning of the process.
  

Whole milk ..................5
Semi-skimmed milk ..................15
Light cream ................25
Double cream ................70
Double cream reduce by a 1/3 ................160
Egg yolk ..................65
Mayonnaise .................400

Water-in-Fat emulsion