The mysterious world of colloids and emulsions
Have you never tried to mix oil and water? I bet you did, anyone tried at least once in their life! As you might know, as harder as you stir, there is no hope to mix the two: they eventually separate into two phases.
Oil is a pure fat and hydrophobic, which means it repels watery stuff. Just hydrophilic substances, which have an affinity for water, can be dissolved in water. A moin que…
There is a way to obtain a homogeneous blend of water and oil, which probably you are familiar with. Add some liquid soap to your water/oil system and stir it now. What you obtain is a milky stable mixture (that is not a solution though), better known as emulsion.
In order to talk about emulsion, I must take a step backwards and introduce colloids. What’s a colloids? Simply, a system where two different phases (e.g. liquid and solid, liquid and gas) coexist. We see colloids every day, in particular in our kitchen. Milk, butter, marshmallows, whipped cream are all example of colloids.
In the figure above, you can see a picturesque representation of a foam, which is a common type of colloid (gas dispersed in liquid).
Emulsions are specific types of colloids. In an emulsion, two liquid phases (water and oil) coexist and one is dispersed into the other. The dispersing agent is called emulsifier (the soap in the example mentioned above), a compound with both hydrophobic (=affinity for fat) and hydrophilic (=affinity for water) parts. There are several types of emulsifiers, but proteins and phospholipids (e.g. lecithin) are probably the most common in the kitchen. Below, you can see how a phospholipid looks like:
An emulsifier basically acts like a bridge between oil and water, the polar part takes contact with the aqueous environment, while the apolar one with the fat:
There are two types of emulsions:
· Oil in water (O/W): the oily phase is dispersed into the watery one (e.g. mayo, yes!)
· Water in oil (W/O): the watery phase is dispersed into the oily one (e.g. butter)
There are also more complex systems (W/O/W or O/W/O), but I won’t go so much into details.
An important feature of emulsions, which actually makes them different from suspension, is the stability. An emulsion MUST be stable. If not, something went wrong...either you mess up with doses or with the procedure or both ;)
Why an emulsion breaks down?
That is the sad part: you prepare your emulsion (e.g. your mayo) and (you think) you did everything right, but, despite that, your mixture starts to separate. S@1!!t!!!!! No, it wasn’t because of the weather, neither because you were sad (as my grandma used to claim :D), nor because there was a full moon…you just did something wrong, you did not use the right method.
Here the most common reasons why an emulsion breaks down:
· The emulsifier wasn’t enough or wasn’t the best (not all emulsifiers behave identically)
· The stirring power wasn’t enough (colloids do not form spontaneously, they require energy)
· pH and temperature can affect your outcome
This list is far to be complete. There other reasons causing emulsion breakdown (e.g. salt concentration), but let’s say those listed above are the most important factors to control in emulsions as egg custards, mayo, fondue and so on.
Emulsions are not easy to prepare: they are indeed very delicate and their preparation requires accuracy and experience. Fortunately, technology is on our side and we can benefit from robots with enough stirring power to solve at least one of our problem (see point 2). In the next posts, we will see how to master the common emulsions found in our kitchens.
References:
L. Stryer, Biochemistry, 284-287, Freeman and Company
D. J. Shaw, Introduction to colloids and surface chemistry, 262-270, Butterworth Heinemann