This NBC Learn video, one in a 6-part "Cheeseburger Chemistry" series, uses ketchup, mustard and mayonnaise to explain two different types of mixtures: suspensions, and colloidal dispersions (emulsions).
The Chemistry of Condiments
AL ROKER, reporting:
It’s the last step in making a cheeseburger.
KENT KIRSHENBAUM (New York University): It wouldn’t really be a cheeseburger without the condiments, without our favorites: ketchups, mustard, and mayonnaise, maybe embedded in that secret sauce.
ROKER: As it turns out, these three neighbors in refrigerator doors nationwide can help us understand a critical part of chemistry: mixtures. Most things in nature are mixtures of matter, in one common state or another. Solids and liquids, for example, like mud, a mix of dirt and water. Or solids and gases, like smoke, a mix of solid, if tiny, particles and air which is itself a mix of gases: nitrogen, oxygen and water vapor. We can use ketchup, mustard and the making of mayonnaise to explain the basics of two important types of mixtures: suspensions, and colloidal dispersions. First, suspensions. Suspensions are heterogeneous mixtures or mixtures of unlike substances that have particles suspended or hanging inside particles large and dense enough that they won’t stay suspended.
KIRSHENBAUM: A suspension will eventually settle out. If you think about sand in water, if we wait, that sand will settle to the bottom of that mixture. It’s because the sand particles are more dense. I would put ketchup and mustard into the category of suspensions. They are just particles of vegetable materials, mustard seed or tomatoes, that are ground up and then distributed, dispersed, throughout an aqueous water phase. If we wait long enough, we’ll see the liquid components separate from the solid particles.
ROKER: You won’t see that with colloidal dispersions or colloids, for short. These are mixtures much like suspensions, but involving smaller particles. Unlike suspensions, colloids don’t settle out. In fact, the word “colloid” comes from the Greek kolla, meaning glue. What’s cool about colloids, though, is a seeming oxymoron, fancy word for contradiction in terms. Colloids are mixtures of substances that don’t mix…that are immiscible, fancy word for unblendable.
KIRSHENBAUM: That’s called an emulsion: a colloidal dispersion of one liquid in another. And one great example is mayonnaise.
ROKER: That’s because these are the makings of mayo: vegetable oil and vinegar, which is a water-based mixture, H2O and acetic acid. Oil and water: the very definition of what doesn’t mix.
KIRSHENBAUM: Before too long, we see that the oil and the vinegar will separate. The oil droplets want to stick to one another, and want to segregate, or move apart, from the aqueous vinegar.
ROKER: Far, far apart from anything aqueous, or watery, because oil is hydrophobic, that means afraid of water. So afraid that the oil droplets naturally move as quickly as possible, to reduce any interface, or contact, between with any of its surfaces and the water.
When those separated oil droplets coalesce, or unite, into one oily layer, the minimum number, only the unfortunate oil molecules on the very bottom, are still in any contact with the hated water. All the others are, as they prefer, completely surrounded by their own kind, except for the top layer, exposed to the air, which is ok. Oil isn’t aerophobic.
Water molecules, too, are strongly drawn to their own kind, which explains another reason oil and water separate. H2O is a polar molecule: one side has a slight negative electrical charge, the other a slight positive electrical charge. The positive sides of every H2O molecule are going to attract, and be attracted to, the negative sides of other nearby H2O molecules in all directions until they’re like people in a dance club packed so close together they can hardly move, but still turning and moving as wildly as they can.
It isn’t always easy for another substance to make this dance party a mixer and mix in with the H2O. It’s especially hard for oil droplets to do, first because they’re big. Edible oils are made of very long hydrocarbon chains. And second, because oil molecules are non-polar; they have neither a slight positive nor negative side. So H2O molecules aren’t attracted to them nearly as much as to other H2O molecules. Unable to break into the tight H2O clique, the oil droplets congregate, like wallflowers, at one edge. And what is mayonnaise again?
KIRSHENBAUM: Mayonnaise is a distribution of oil particles throughout water.
ROKER: So how are you going to separate pools of oil into individual particles, let alone get them distributed in water they hate, amongst water molecules that don’t want to bond with them? In a kitchen lab at New York University, Kent Kirshenbaum, a chemist funded by the National Science Foundation, shows how.
KIRSHENBAUM: We can provide a coating around the oil droplets to prevent them from meeting each other, from bumping into each other, from associating and forming larger and larger oil droplets and finally separating out into two different phases. The way we’re going to provide that coating is we’re actually going to use molecules that come from egg yolks. These are called emulsifying molecules.
ROKER: Specifically a substance called lecithin. Kirshenbaum mixes the egg yolks and the vinegar together, then introduces the oil.
KIRSHENBAUM: The trick is to begin very, very slowly. Very small quantities with vigorous whisking breaking apart the oil into small droplets. Those small droplets can then be covered by the emulsifying molecules that are present in the egg yolk.
ROKER: Think of the lecithin mixture coating individual oil droplets in full-length coats. Here’s the thing about lecithin: part of it, like oil, is hydrophobic, avoids water. But another part of it is hydrophilic - loves the water. That’s what makes the lecithin coat perfect: the lining, touching the oil droplet, is hydrophobic, too, so it’s a fit. And the water-loving outside is not only attracted to H2O molecules - H2O finds it attractive, and bonds with it.
KIRSHENBAUM: And it’s only because these oil droplets are covered with those egg yolk proteins that they’re sort of in stealth mode. They don’t see one another, and they’re completely distributed in a very, very small size.
ROKER: No longer grouped apart at the edge, but mixed in throughout the vinegar, more and more of them, as Kirshenbaum adds in more oil.
KIRSHENBAUM: Over a cup of oil has been distributed in minute little droplets all throughout the vinegar and lemon juice. This is going to become pretty dense. Take two liquids and mix them together and get something that’s a semi-solid. And what we have is a beautiful, nice, dense mayonnaise – a stable emulsion. And this is stable enough that we can sit it here on the table, and wait all afternoon long, and we’re not going to get any separation.
ROKER: So there you have it: some basics on the chemical structure of ketchup and mustard, suspensions, and mayo and mayo-based sauces, emulsions, to add to the mix.
As you're sifting through your Halloween bounty this year, consider all the forms candy can come in. Some will be smooth and chewy, such as caramel. You'll surely have some gummy animals in the mix, along with a rainbow of lollipops. All of these desserts can be made with the same main ingredient: sugar. It's through the wonders of chemistry that confectioners make the sweet stuff take so many different forms.
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