The more I read about food science, the clearer it becomes that protein interactions determine the fate of everything from simple scrambled eggs to the most delicate cake. And protein interactions are governed by chemical bonds. After finding myself referring to these interactions in every post lately, I decided that they need an explanation all to themselves. So fasten your seatbelts, because we’re about to delve into the details of bonding. It’ll be fun, I promise. And then everything will be so much clearer. (This post is really long, I know. I wanted to keep everything together. You can think of it like an encyclopedia–come back to reference it when other posts talk about bonding.)
It turns out that chemical bonds are just like interpersonal bonds: some are stronger than others, opposites attract, and an atom can be involved in more than one at a time (those scandalous atoms!). It helps to know a little bit about atomic structure before we start talking about how they interact. Every atom has two basic parts: a nucleus made up of protons (positively charged) and neutrons (neutral–no surprise there) and an electron cloud containing a bunch of negatively-charged electrons orbiting the positively-charged nucleus. They don’t just orbit at random, though–they fill up distinct levels. It’s like a stadium for a very strange sport. The nucleus is the field in the middle, and the electrons are the people filling up the seats around it. But in this stadium, the seats are arranged in a particular pattern: the first row has two seats, the next row up has eight, and the third has eight as well. The electrons want to sit as close to the field as possible so they always sit in the lowest row they can, and the atom is happiest when any row that has electrons is full. For example, lithium has three protons so it needs three electrons to be neutral. This leaves one electron all alone in the second row, though, so lithium is actually happier when it loses that electron–it has a positive charge, but it doesn’t have any partial rows of electrons.
Now that we know about how atoms are like stadiums, we can start talking about how they interact. We’ll start with the strongest kind of bond: covalent bonds are like your relationships with your family if you’re lucky, or with the people you hang out with to get away from your family. They’re strong and usually hard to break. They’re pretty resistant to heat, in the same way that you have to keep loving your family even when they cold-heartedly steal leftovers that were clearly yours out of the fridge (no, Dad, I’m still not over the chili incident). Chemically speaking, covalent bonds form when atoms share electrons–one electron has a seat at two side-by-side stadiums. Remember how atoms are happiest when they have full rows of electrons? Sometimes the easiest way to make that happen is to share (even on an atomic level, sharing is caring). For example, carbon has six electrons: two to fill up the first row and four hanging out in the second row. In most of its stable structures, carbon forms four bonds with atoms around it. Methane is the simplest example with four hydrogen atoms surrounding one carbon. In the diagram below, we see how the empty seats in the stadiums for carbon and hydrogen by themselves are filled up when they share (the purple squares are shared electrons). Instead of orbiting just one atom, the shared electrons now orbit both to make both atoms feel like their seats are filled. The atoms are happy, and the shared electrons mean that it’s harder to split the two apart, giving covalent bonds their strength. These are the bonds that hold a protein chain together, and that keep a sugar molecule in one piece.
Another strong bond is an ionic bond. Like the covalent bond, the goal is to give each atom filled rows, but this time there is no sharing. Instead of playing nice, one atom steals an electron from the other, leaving one with a negative charge and the other with a positive charge. Like opposite ends of a magnet, these two charged atoms (which we call ions) stick together. It’s like when the mean kid in preschool steals your truck, but then you’re forced to keep playing with him because you still really want the truck. This kind of bond is more common with atoms near the edges of the periodic table–like I explained before, atoms will lose or pick up electrons more readily if it leads to them having only full rows. This means that atoms that have a row with only one electron (like lithium) will lose it easily, while atoms that have a row with seven are hungry for one more. The resulting molecule is called a salt, and many of the atoms commonly involved are familiar from the everyday uses of salt: sodium, potassium, chlorine (sodium chloride and potassium chloride are two of the most common salts). Carbon, on the other hand, isn’t likely to form ionic bonds because it has four electrons in its outer row, which would mean having to pick up or lose four electrons–not a very good idea. Ionic bonds are strong by themselves but break in water, which is why it’s so easy to dissolve salt in water.
Now we’re moving on to the weaker interactions that are important in protein shape and connections between proteins. Hydrogen bonds are a little bit like weak versions of ionic bonds because they also rely on positive and negative charges coming together. This is the Sk8er Boi effect on an atomic level. The negative charge is a punk and the positive charge does ballet–what more can we say? Unlike ionic bonds, though, we’re not dealing with full-on electron transfer. These weak charges arise because of greediness within a covalent bond. Hydrogen is the weakest electron-grabbing atom (it has low electronegativity), and other atoms take advantage of that. The stronger electron-grabbing atoms hold the shared electrons more strongly–they spend more time around the greedy atom. This leaves hydrogen just a little bit positively charged, and the greedy atoms just a little bit negatively charged. In addition to the covalent bond it has with a greedy atom, the positive hydrogen can also bond to a different greedy atom with its negative charge. This kind of bond plays a big role in giving proteins their folded shapes and in keeping proteins together–like in cooking eggs or forming a gluten network in bread.
The last kind of bond we’ll talk about today is actually not a bond, it’s an interaction. Nevertheless, hydrophobic interactions can play an important role in the structure of proteins and in your final product. You can think of this as the sorting that happens in high school cafeterias: everyone walks in more or less together, in no particular order, but as soon as people are seated at tables, you have definite distinctions. Groups of friends cluster together, so you get the soccer team at one table while artists cluster at another. For atoms, there are really only two choices: hydrophilic (water-loving) or hydrophobic (water-hating). Just like high school cliques, atomic groups are very strict. Because water is all over the place in biological systems and in cooking, the hydrophobic molecules are not very happy. To make them feel better, they stick together–strength in numbers. This means that the hydrophobic molecules cluster in groups, like small cliques that stay separate from the main body of students in the cafeteria. In terms of cooking, this plays a part in protein folding (some protein bits are hydrophobic) and in oil-water mixtures like salad dressings.
So that’s the basics of bonding. Covalent bonds usually hold molecules together and are stable, while hydrogen bonds and hydrophobic interactions control how those molecules fold and interact. Heat can break the weak bonds and change how the molecules behave (as in the formation of a gluten network), and even higher heat can actually break molecules (as in caramelization). And now that we know all about the bonds that hold molecules together–in this case, egg proteins unfolding and networking to make a solid product–let’s make a pie.
Three-ingredient lemon pie
This should maybe be three-ingredient-plus-a-crust lemon pie, but that’s just not nearly as catchy. Egg yolks, sugar, and lemon juice make an incredibly easy but also delicious filling, and you just pop it into a pre-baked pie crust. If you happen to have leftover egg yolks from, you know, making meringues and delicious cakes (there’s an egg-white based cake recipe on Smitten Kitchen that will be up here soon), this is an excellent solution. And pie crust also happens to be a great thing to make and keep in the freezer–if you do that, this dessert is about 30 minutes away.
Yield: 1 pie
2 egg yolks
1 can sweetened condensed milk
1/2 c. lemon juice
Beat the egg yolks in a medium saucepan or double boiler, then stir in the condensed milk. Add the lemon juice. Heat over medium heat until it thickens (about 15 minutes). Pour into the pre-baked pie crust and cool for at least an hour.
1 1/4 c. flour
1/2 t. salt
1/2 t. sugar
1 stick butter, cubed and chilled
3-5 T. cold water
Mix together the dry ingredients, then cut in the butter using a pastry cutter (if you don’t already have one, get one. They’re amazing). When the pieces of butter are no bigger than peas, gently mix in the water. Do not overmix–we don’t want to form a gluten network like in the bread.
Refrigerate the dough for an hour (or stick it in the freezer for 15 minutes), then roll it out to 1/8 inch thickness–no need to measure, just make sure that the dough covers the pie pan plus at least an inch and a half around the edge. Fold the dough into quarters, place it in the pie pan, fold over the edges, and bake for 20 minutes at 350 degrees F, or until the crust is lightly browned.