The Food Lab: Better Home Cooking Through Science

small bits of fat will refract light to a much greater than a big
pool of fat, which is what gives mayonnaise its opaque-
white appearance. Think of it like the windshield of a car.
When it’s whole, light passes through it easily. But get a few
cracks in it, and it becomes difficult to see through. Crack it
enough times into small enough pieces, and it becomes
opaque. Same thing with the fat in mayonnaise.
Normally when you mix fat molecules with water, no
matter how thoroughly you combine them, like MIT nerds at
an all-girls-college mixer, they eventually separate
themselves and regroup. Because of their shape and
electrical charges, fat molecules are mutually attracted to
each other while simultaneously being repelled by water.
This is where egg yolks come in. Egg yolks—which are
complex fat and water emulsions in themselves—contain
plenty of emulsifiers (agents that aid in getting fat and water
to behave), the most important of which is lecithin, a
phospholipid found in both the low-density lipoproteins
(LDLs) and high density lipoproteins (HDLs) abundant in
eggs. Emulsifiers are long molecules that have a hydrophilic
(water-loving, fat-hating) head, and a hydrophobic (water-
hating, fat-loving) tail.