Emulsions
An emulsion is the even dispersal of one liquid throughout another immiscible liquid. If the drops of the dispersed liquid are small enough and well separated from each other, the mixture will be thicker than either liquid and will remain in solution, for a time. The amount of time for which the emulsion is stable depends on what is preventing the dispersed phase from coming together or (pooling) and separating from the other liquid.
It is the natural tendency of like molecules to attract each other. If you shake oil and vinegar in a jar it will form what is known as a temporary emulsion. Then the drops of oil in the jar begin to attract each other. As the drops get bigger, they grow faster, incorporating more drops of oil until there is only one very large drop of oil, the upper phase of the jar.
The way to make a more stable emulsion is to physically prevent the drops from getting together. This is accomplished in one of several ways. The use of emulsifiers, finely divided (powdered) solids, polysaccharides, proteins, or even thick liquids, can help stabilize an emulsion.
Emulsifiers have the unique physical property of having two distinctly different areas on one molecule. Each of these areas would prefer to be dissolved in a different substance. This is known as the hydrophilic/lipophilic (water loving/fat or oil loving) balance. Lecithin, which is found in egg yolks, is the primary emulsifier in both mayonnaise and hollandaise. It works because one part of the lecithin molecule attracts the oil or fat and the other part attracts the water and water compatible parts of the emulsion. Finely divided solids or powders such as finely ground-dried herbs can stabilize an emulsion by absorbing oil and water at the interface, (the point where the oil and water meets). Polysaccharides, such as certain gums, can interfere with the pooling process of the dispersed phase or form a matrix that traps the dispersed droplets. Gelatin is an example of a protein that can act as a stabilizer. Proteins that are naturally present in foods can make effective emulsifiers. Another method of stabilizing an emulsion is to add a thick liquid (such as honey) to the two immiscible liquids.
Many flavoring agents are added to emulsified sauces. Some serve only to add flavor. Others, such as vinegars and citrus juices (acids), can also change the physio-chemical properties of the emulsifier. Proteins can be denatured, creating a greater barrier between the dispersed droplets. In the case of mayonnaise and hollandaise, the acid changes the chemical properties of the emulsifier so that it can take up more lipid (fat or oil) before becoming saturated. Plant particulates such as chopped onion and citrus peel are effective in the initial breaking up of droplets when the mixture is whisked or blended. They also act as flavoring ingredients.
There are three main reasons that emulsions separate.
1. Liquid is added too quickly:
When the liquid that is to be dispersed is added too quickly, the emulsion may never form or may quickly separate. This occurs because there is not sufficient time for the liquid to disperse fully. The tiny drops cannot form, and pools of the liquid are created instead. There are two solutions for this problem. One is to slowdown the stream of liquid, giving it time to disperse into the continuous phase. The second is to mix the item faster.
2. The mixture becomes saturated:
When an emulsified mixture becomes saturated, it is similar to when the liquid is added too quickly. The difference is that no amount of whisking will incorporate all of the liquid. With saturation, there is simply too much oil. A towel can only hold a certain amount of water without dripping. When it becomes totally saturated with water, it begins to release some. This is what happens in an emulsion when too much of the liquid to be dispersed is added.
When saturation occurs, the solution is to put some of the continuous ingredient into a separate mixing bowl and slowly add the broken emulsion into it. When the emulsified mixture nears saturation it is easier to break it by over- whisking. Adding a small amount of acid (lemon juice, for example) can increase the saturation point of the mixture.
3. Temperature:
Temperature abuse is a major cause of broken emulsions. In the kitchen, the most common emulsion is between oil or fat and a water based solution (vinegars, fruit juices, wines, stock, etc.). Often the emulsifier is a protein or is part of a protein system. Proteins are relatively large molecules that exist in their native state folded in a certain configuration. When the protein is exposed to heat, acid, or agitation, it can be unfolded. This unfolding (a change in shape) is called denaturation. Excessive heat, agitation, or acid can catalyze chemical reactions that can destroy the protein molecule’s emulsifying properties.
The thickness of the liquids being emulsified is affected by temperature. When oil gets cold, it gets thick. When it gets hot, it becomes thin. Water does not do this to any noticeable extent. This presents a problem in making an emulsion because, not only are you trying to combine two immiscible substances, you are often trying to mix two solutions that are of different thickness. If you are trying to make a mayonnaise in a very cold room, you encounter the problem of trying to incorporate a thick oil into a relatively thin egg mixture. When this happens, the oil molecules tend to clump together. If the oil clumps together, it will not be able to disperse effectively. Butter sauces are emulsions that are formed using the proteins and natural emulsifiers found in fresh whole butter (strengthened by acid in wine or citrus juice). This emulsion is very temperature dependent in that, if it is allowed to cool and is reheated, it will break.