Chapter I

Emmanuel: (to Jean-Louis) I am going to ask you a question that occurred to me while I was making a mayonnaise. I warn you: I already asked the same question to an experienced cook to have another point of view.

The cooks mention binders which indeed are also present in painting (egg, flour, gelatin, etc.); and nothing prevent you from painting with mayonnaise or from making a tempera « sauce poulette ». Sometimes, however, the cooks’ binding agent doesn’t bind anything in particular. It does not seem to coat but rather to come in between in the manner of a simple colloidal charge (I wonder what  would give a  “colloidal silica Blanquette”), whereas in painting it coats the pigment, holds it in place. On the other hand, in both disciplines, the binding agent seems to have some plastic properties.
I am starting to wonder what really defines the term “binder”. Its plastic properties, its sticking and colloidal properties, with which physical reality does it, match, finally? Unless it is only a common term, a holdall. And yet, we find approximately the same products in these disciplines.

What does an interfaces specialist say to that?
And an experienced cook? What is a culinary binding agent?

Anne : Hum, the binder allows to bind, do you see? To bind two or more elements that, without its help, would refuse to mix together. The binder can be a surfactant, an emulsifier (egg yolk for instance). But, the binding agent can be also what gives body to a preparation.  A thickener like flour or corn flour.
This is what is coming to my mind.

Jean-Louis : As always, nothing is simple. The term alone of binder does not give an account of all what happens "behind". To sum up, we may say that there are two categories of problems: 1) making miscible two products that are not (ex. water and oil) or stabilizing suspensions (ex. Indian ink); 2) gluing (i.e.: binding, sticking together, coating ...) solid grains on a surface (paint).

First problem:

The very well-known example of the vinaigrette: if you mix oil and vinegar, the two components separate as soon as you stop stirring. Magic: when you add mustard, it is already more stable. With egg yolk you will even get a mayonnaise. Another famous example: if you scatter soot into water you obtain an outstanding ink, but very quickly, the soot flakes stick together, fall down to the bottom of the inkwell and it is lost.

I. Miscibility :

Empedocles (450 BC), then Aristotle assumed that two forces, Love and Hate, were sufficient to account for all natural phenomenons: Love causing the rapprochement of the objects (ex. magnetite); Hate causing them moving away. This theory has been meeting success during several centuries.

And today still, we have to admit that it could remain tempting, because the molecular interactions often sum up to attractive or repulsive interactions, and there are two part molecules, made with love and hate, the soaps or amphiphiles ( from the Greek : who likes both).

An amphiphiles is a molecular compound of which one part of the structure is polar (or hydrophilic, who likes water and the liquids known as polar) and another one likes lipids – that are apolar -, known as lipophilic or hydrophobic. Mustard grains, egg yolk, contain such amphiphiles molecules. When mixing, amphiphiles place themselves at the interface between water and oil, and it stabilizes the sauce; at that time, this is an emulsion. We generally emulsify oil in water:  and we have then the following structure :

In the case of Indian ink, the problem originating the flocculation is that the ink particles are electrically charged and attract each other. Arabic gum (extracted from the sap of an acacia tree) is a water-soluble polymer playing at once a role of neutralization of the electric charges (the polymer winds around particles) and of solubilization (Arabic gum is also used in industry as an emulsifier. Cf. apega.bf/gomar.htm).

The case of frescoes: this is a bit particular, since in this case the colours are applied over a fresh rendered surface, i.e. which has not finished to harden, to set. When the mineral matter of the coating (lime, plaster, cement) hardens, it traps the particles of pigment that have been applied and which diffuse on a few tenths of millimeter. We can say that the medium, the binder, is the wall!

Emmanuel : Let’s get back now to the  mayonnaise.

Upon Anne’s latest advice, I read an article where a cook calls on Hervé This. I was trying to find some answers to the question: "is it necessary to whip it?" (Or "can we whip it?"), which does not really go without saying, depending on the sources. Well, yes, we whip it. See the "parent" article and the actual recipe (external links, new windows). This is quite related to our subject.

 Hervé This claims that this is not the phospholipids which are responsible for the good consistency of the emulsion, but the proteins. What do you think of that?

Jean-Louis: If he is saying so. Most of the organic molecules and especially the very big ones like proteins have hydrophile and hydrophobe parts. They can therefore be used as a surfactant in the same way as the phospholipids.

Read a more detailed passage in Chapter III

Emmanuel : It is  the second time that you are using the term "surfactant". How do you define it?
 

Jean-Louis : A quasi-philosophical question. This is a molecule able to modify the interfacial energies, the surface tensions. So, a molecule which likes the interfaces between two bodies. The molecules of a given body, the easiest way is to imagine it a liquid body, enjoy staying with each other. A molecule located at the core of the liquid is happy, all her neighbours are like her; a molecule located "at the edge" is not happy, half of her neighbours are of a different nature. So, the molecules from the surface whish that their friends come and surround them; and this attractive force creates the surface tension.

A surfactant is a molecule generally amphiphiles that changes the « force ratio » between two bodies in contact: liquid-liquid (emulsion), liquid-gas (bubbles, foams), liquid-solid (wetting and capillarity).

But what you are trying to do, finally, is to evaluate the siccativity of mayonnaise...?

The surfactant,