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Mayonnaise and other mysterious mixtures
If you stir an egg-yolk into a cup of water, it will mix - not an experiment you?ll want to do more than once, because the result is of no real culinary value. Stirring it into milk, on the other hand, produces the base of a custard.
If you stir some olive oil into the same cup of water, it will disperse into little droplets and then gradually float to the top and form a completely separate layer.
So egg-yolk is watery and oil is oily and never the twain shall mix - right?
Wrong, because if you mix olive oil with egg-yolk (albeit a little more carefully) the two stay mixed. Better still, in the course of doing so they create one of the greatest of all sauces: mayonnaise.
So is mayonnaise oily or watery?
It must be oily, because if you stir in more oil it stays mixed.
But wait a minute. I?ve published two recipes here in which watery things like tinned salmon and cream and tomato ketchup and Greek yoghurt are mixed into mayonnaise, and they stay mixed too. And mayonnaise has watery things like vinegar and lemon juice in it already. So it must be watery.
Actually it?s both. It?s an emulsion - a mixture in which watery and oily ingredients are mixed and stay mixed.
Emulsions that don?t work
Basically, an emulsion consists of a liquid with lots of tiny drops (droplets) of another liquid dispersed in it. In the case of mayonnaise, we have a small amount of watery egg-yolk with millions of oil droplets in it.
Let?s start by thinking about why an emulsion might not work. If you put some water and oil in a blender (the original blenders developed in the USA were actually called ?emulsifying machines?) and run it, you will produce a liquid that is thicker (more viscous) than either the oil or the water alone and milky-looking rather than clear. If you were to look at this through a microscope, you would see tiny spherical ?beads? of oil fairly evenly spread out with thin layers of water separating them.
The mixture is viscous because the oil droplets behave almost like solid particles. A similar effect is seen if you make a cold paste of water and flour. The viscosity is caused by the friction between the particles.
The mixture appears milky because the water and the oil interfere with (refract) the passage of light in different ways, so it scatters in many directions rather than passing through in more-or-less straight lines, as it does through either pure water or pure oil.
Because the blender was running at very high speed, it has broken the oil up into very tiny droplets, producing a moderately stable emulsion. If you make a vinaigrette (oil and vinegar) dressing in a blender, rather than just whisking it by hand, it will stay emulsified for longer - perhaps long enough for you to eat the salad - but eventually the emulsion will break down and the ingredients will separate.
Gradually, because the oil is less dense(lighter) than the water (or vinegar) the individual oil droplets float upwards, concentrating together and squeezing out the thin layer of water that separates them until they combine into larger droplets. This process continues until eventually there is a layer of pure oil floating on a layer of pure water.
What scientists say is happening is that the oil in the container has a tendency to reduce the total area of its surface that is in contact with the water. They call this phenomenon surface tension. When there are millions of oil droplets in the blender, the total surface area of the oil is enormous. But when all the oil is in a single layer its surface area is far smaller. I?m not sure that this explanation is particularly helpful to ordinary people like you and me, though it does explain why, when two droplets join up, they form a larger spherical one rather than staying in a dumb-bell shape, which would have a larger surface area.
Making emulsions work
To make an emulsion stay an emulsion, something has to stop the tiny droplets combining into bigger ones. This is normally done by adding a third substance. Not surprisingly, substances that do this are called emulsifying agents or simply emulsifiers. Even those of us who do not habitually make our own mayonnaise meet emulsifiers many times each day.
Soaps and detergents
When you do the washing-up, you put greasy dishes in hot water and the grease mysteriously vanishes. Where does it go? Into the water, obviously. But how? Grease doesn?t mix with water - it floats in nasty puddles on hot water and in disgusting lumps on cold water. The answer is, of course, the washing-up liquid. This contains detergents, which are powerful emulsifying agents. Soap is one kind of detergent, which is why it can emulsify greasy dirt and remove it from your hands. Shampoos contain detergents that emulsify the natural oils on your hair (though hopefully not too much).
Soaps and detergents taste pretty awful, but some powerful emulsifying agents can be found in foodstuffs. Egg-yolks contain several, the most notable one being lecithin, and as we can see from the process of making mayonnaise they are very powerful, allowing a great deal of oil to be emulsified with a relatively small amount of watery material to produce something we don?t find oily at all. This is important, because it is the high oil content that makes the mayonnaise thick and creamy, or viscous. And deceptively fattening.
How do emulsifiers work?
Scientists, being prone to glibness, tell us that emulsifiers work by reducing surface tension. Since I didn?t find the surface tension idea very helpful to begin with, this doesn?t enhance my understanding one little bit. The mystery for me is how they reduce surface tension.
Soaps, lecithin and other emulsifiers rely on substances called fatty acids, whose molecules have a fat-soluble tail and a water-soluble head. The fatty ends attach themselves to the oil droplets and present the watery ends to the outside world - in this case to the surrounding water. This prevents the surfaces of the oil droplets meeting and stops them combining, allowing the modified oil droplets to live in peace with their watery environment.
Apart from mayonnaise, the two most common emulsified sauces are hollandaise and béarnaise. Unlike mayonnaise, these are made with butter, and because butter is solid at room temperature they have to be made warm. This makes the whole process pretty nervous, because if you cook the egg yolks you end up with no emulsifiers and a lump of scrambled egg floating in a puddle of melted butter.
So risky is the process that a 1977 issue of the leading scientific journal Nature contained an article with a intimidating title of Interparticle Forces in Multiphase Colloid Systems and the much more human-friendly subtitle The Resurrection of Coagulated Sauce Béarnaise. ?Coagulated? in this context means what we cooks call ?curdled?.
The practical application of all this is that mayonnaise can be mixed with both oily and watery ingredients. We will be looking at the culinary implications of this in the section on salad dressings.
Mayonnaise and the other sauces are quite spectacular examples of emulsions, but many of the other things we eat and drink are emulsions.
Milk and cream are less obvious examples in which the water content is higher and the fat content lower than in mayonnaise. You sometimes see fat globules floating on your cup of tea when the milk is getting a bit past its best. This is because some of the first products of spoilage (the milk ?going off?) are starting to break down the emulsion.
Butter is also an emulsion, but it is fundamentally different from all the others because it consists of water droplets in a continuous matrix of fat, rather than the other way around. When I was young and cream was still an outrageous luxury in the age of post-war austerity, my Mum bought something called a Bel Cream Maker at a jumble sale. This consisted of a goblet with a small lever-operated pump, like a tiny bicycle pump, at the bottom. You put warm milk and (preferably) unsalted butter in the goblet and pumped the lever up and down. A kind of vane attached to the pump agitated the mixture to stop it separating and the pump squirted it out through some very tiny holes. What emerged was quite thick cream. I guess that what was happening was that the two emulsions - the milk and the butter - were combined by the pressure into a single new one.
Much later, when Wife Number Two was given her mother?s old Kenwood Chef mixer and cream was still for special occasions, we discovered that you could buy an equivalent device to fit on that - it just pushed onto the drive at the front of the mixer. On one memorable occasion, Wife Number Two fitted the cream-maker to the Chef, filled it with warm milk and butter and switched the mixer on - only to discover that, with her customary lack of mechanical aptitude, she hadn?t pushed it on quite far enough. The whole accessory began revolving at high speed, and centrifugal force ensured that the warm, greasy contents covered an amazingly large area of two walls, the floor and the ceiling of the small kitchen that I had just finished fitting, not to mention the worktops, the many bits and pieces with which she had already begun to clutter them - and, of course, herself.
The kids thought this was hilarious. I wasn?t quite so amused. Wife Number Two was seriously unamused.
Personal site for Paul Marsden: frustrated writer; experimental cook and all-round foodie; amateur wine-importer; former copywriter and press-officer; former teacher, teacher-trainer, educational software developer and documenter; still a professional web-developer but mostly retired.
This site was transferred in June 2005 to the Sites4Doctors Site Management System, and has been developed and maintained there ever since.