Chemistry in the Kitchen
Every time we crack open a cookbook, Google a recipe, or throw together ingredients for a quick bite to eat, we are preparing to conduct an experiment in culinary science. Unless you eat only raw foods, many of your meals undergo a complex series of chemical changes before becoming edible (and palatable)—and indeed, some foods are indigestible to us, or even toxic, before these changes are wrought. Many foods develop their smells and flavors only after their chemistry has been altered—as with heat, acidity, or fermentation—and many require a great deal of processing—both chemical and mechanical—before becoming recognizable to us as something fit to eat. We cook food to make it taste better, and sometimes to make it more nutritious for us; we process food to make it easier to eat, to combine dissimilar ingredients in delicious amalgams (think of baking), and to prolong shelf life. Every time we cook, we are experimenting: the ingredients are the test subjects, the recipe the procedure, and the kitchen is the laboratory. These lessons are designed to explore a few aspects of the endlessly fascinating, deliciously gratifying, incredibly varied ways we explore science wearing aprons in lieu of lab coats.
Lesson 1: Rising to the Occasion (Chocolate Chip Cookies)
Gluten forms when two proteins in wheat flour—glutenin and gliadin—combine with water. A gummy, stringy substance, gluten gives form to baked goods by creating a latticed structure to capture carbon dioxide. In these chocolate chip cookies, the leavening—the chemical reaction that causes the cookies to rise—comes from baking soda (an alkaline substance) reacting with the butyric acid in butter (a mildy acidic component of milkfat). This acid-base reaction produces carbon dioxide gas, which bubbles up in the dough and is trapped by the gluten structure, “lifting” the cookies as they bake.
Makes 60 cookies
For the dough:
2 ¼ cup flour
1 tsp baking soda
1 tsp salt
2 sticks butter
¾ C granulated sugar
¾ C brown sugar
1 tsp vanilla extract
2 large eggs
2 C chocolate chips
1 C nuts
Directions: Preheat oven to 350 degrees F. Cream together butter and sugars. Add in eggs and vanilla. In a separate bowl, combine flour, baking soda, and salt. Mix the wet ingredients with the dry (do not overmix—the more the dough is mixed, the stronger the gluten strands become, which makes for tough, dense cookies), add in the nuts and chocolate chips, and drop tablespoonfuls onto greased baking sheets. Bake for 8-10 minutes, or until golden brown at edges.
Lesson 2: Setting an Egg-cellent Example (Broccoli-Cheddar Quiche)
Quiche is essentially an egg-filled pie. When the proteins of egg whites are beaten vigorously and subjected to heat (as in a 350-degree oven), the globular strands of amino acids come apart, or denature, rearranging themselves into long, crooked chains. These chains latch onto one another and form a lattice, which, with the continued application of heat, gels into a solid. This solidified lattice constitutes the substance of a quiche, the medium in which all the other yummy bits are suspended.
Makes one 9-inch quiche
For the filling:
8 eggs, beaten
½ C milk
1 small onion, diced
3-4 cloves garlic, minced
1 head broccoli, pared
1 C shredded cheddar cheese
1 9-inch pie pastry, unbaked
Directions: Preheat oven to 350 degrees F. Lightly sauté the onion and garlic. Beat the eggs and milk together, and line a 9-inch pie dish with the pastry. Cover the bottom with half of the cheese, sprinkle the onions, garlic, and pared broccoli over it, and pour the egg-milk mixture over it all. Sprinkle the remaining cheese on top. Bake for 30-40 minutes, or until a knife inserted into the center comes out egg-free.
Lesson 3: Sold on Cellulose (Veggie Stir-Fry with Peanut Sauce)
Cellulose is the substance comprising the primary cell walls of all green plants. Almost every vegetable we consume comes from a green plant, so our diets—if veggie-based even in the slightest—contain lots of cellulose. But here’s the rub: cellulose is all but indigestible to humans. To utilize the myriad vitamins and minerals sequestered by these plants, we need to break those cell walls and release the contents—a result achieved partially by chewing; or, to a greater extent, by cooking. Depending on the vegetable, cooking times vary considerably; some are more fibrous, and thus take longer to cook, than others. Also, some nutrients (such as vitamin C) are extremely volatile: even the lightest touch of heat will break the compounds down and render it useless to our bodies. Others, such as beta-carotene (vitamin A) are more heat-stable and survive a moderate level of cooking. It’s a fine balance between how much nutrition you wish to get from your veggies, and how long you’re willing to chew.
Makes stir-fry for four (steamed rice would be an excellent side-dish)
For the peanut sauce:
½ C crunchy peanut butter
2 tbsp soy sauce
2 drops hot pepper sauce (Sriracha or similar)
1 clove garlic
½ C water
For the stir-fry:
3-4 carrots, diced
2 medium yellow onions, diced
2-3 stalks celery, diced
1 green bell pepper, diced
1 large sweet potato, cubed
Directions: For the sauce, mix all the ingredients together and set aside. Heat 2 tablespoons oil in a large skillet at medium-high heat. Add the onions and garlic; cook until translucent. Add the celery, sweet potato, and carrots. After five minutes or so, throw in the peppers and cook until all the veggies are al dente. Toss with the peanut sauce and serve over rice.
Lesson 4: Lollipop, Lollipop (Homemade lollipops)
Molecules of table sugar (sucrose) are composed of two separate sugar molecules: glucose and fructose. In a dry, room-temperature state, sucrose assumes a crystalline structure—the glucose and fructose lock solidly together like Lego pieces. Under a microscope, sucrose crystals appear brick-like and blocky, as if they were the rubble of some Lilliputian skyscraper. This structure works fine when things are dry, but add a little water to the mix, and everything gets…well, sticky. As anyone who’s made Kool-Aid knows, sugar is eminently water-soluble, readily dissolving into most liquids. When sugar dissolves in water, it’s said to have gone into solution. But you can’t dissolve an infinite amount of sugar into a finite volume of water: when as much sugar has been dissolved into a solution as possible, the solution is said to be saturated.
The saturation point varies at different temperatures—the higher the temperature, the more sugar that can be held in solution. This principle is of utmost importance in the business of candy-making. Candy usually involves cooking sugar, water, and various other ingredients to immoderately high temperatures. At these extremes, the sugar remains in solution, even as much of the water boils away. Because there is, at this point, more sugar in solution than is normally possible, the solution is said to be super-saturated.
Supersaturation is a highly unstable state! As the solution cools, the sugar molecules will begin crystallizing back into a solid at the slightest agitation—even pouring and stirring can set them a’ settin’. Working fast becomes imperative—but before we get started, let’s look at the two main classes of candy-making.
There are essentially two categories of candies: crystalline (candies which contain crystals in their finished form, such as fudge and fondant), and noncrystalline, or amorphous (candies which do not contain crystals, such as lollipops, taffy, and caramels). The ingredients and procedures for noncrystalline candies are designed specifically to prevent the formation of sugar crystals, because these give candy a grainy, inconsistent texture. One way to prevent crystallization is to make sure there are other types of sugar—fructose and glucose, for instance—to jam up the process. Crystals of sucrose form less readily when molecules of glucose and fructose are in the mix. Think again of the Lego pieces, and imagine that each type of sugar is a different-sized Lego piece. Because the pieces are of varying sizes, they don’t lock together well, and the crystals don’t form.
Another measure to take is adding an acid. Acidic substances like lemon juice or cream of tartar will break sucrose molecules down into their constituent parts—again, fructose and glucose—allowing other sugar molecules to drift by and interfere with crystallization. Fats in candy serve a very similar purpose: just another substance that gets in the way of crystals forming, albeit one that happens to taste delicious as well. Whew! Now we’re ready to make some candy.
For the lollipops:
1 C sugar
1/3 C corn syrup
½ C water
¼ tsp cream of tartar
¼ to 1 tsp flavoring
Liquid food coloring
1 to 2 tsp citric acid
-nonstick saucepan, preferably with spout
-cooking oil spray
Directions: Prepare the cookie sheet by covering it with parchment paper and spraying with oil. Lay out the lollipop sticks so that they’re well-spaced on the sheet. In the pan, over medium heat, stir together the sugar, corn syrup, water, and cream of tartar with a spoon until the sugar crystals dissolve. Continue stirring, using the pastry brush dampened with warm water to dissolve any crystals clinging to the sides. As the syrup begins to boil, stop stirring and place the thermometer into the solution, letting it boil until it reaches 300 degrees F. Remove the pan from heat and let the syrup cool to 275 degrees before adding flavoring. Once flavored, quickly pour the syrup in roughly two-inch circles over the sticks, having a helper twist the sticks in each circle to ensure an even coating. Let rest for ten minutes, or until solid.
Lesson 5: Not Yo’ Ordinary Nacho (Beef Nachos)
Often one hears about “browning” foods to ensure that they are properly cooked. Meats, breads, and many vegetables will take on a brownish hue after sufficient exposure to high temperatures, and usually the food starts to smell different, too—a useful hallmark for “doneness”. This lesson explores why, exactly, this phenomenon occurs, and how we cooks use it to scrumptious effect. (To be clear, there are two distinct phenomena that describe “browning” by heat: carmelization—which deals mostly with sugars and starches—and the Maillard reaction, which is protein-specific.) Also known as the “browning reaction”, the Maillard reaction occurs between proteins, which are composed of chains of amino acids, and sugars, such as glucose. When proteins (such as those found in ground beef) are exposed to heat (think 300-400 degrees Fahrenheit), the chains of amino acids unravel, or denature, allowing the acids to bind with the sugars present, creating the smells, colors and flavors we associate with cooked food.
For the nachos:
1 onion, minced
1 pound ground beef
Taco seasoning (maybe one packet)
Shredded cheese (cheddar, pepper jack, etc.)
Chopped veggies (peppers, olives, etc.), if desired
Directions: Preheat oven to 400 degrees. Sauté the onions and beef in a large pan at medium heat, until thoroughly browned. Drain off the excess fat and dispose of it. Add the chopped veggies if desired, pour in the taco seasoning, and stir until evenly coated. Spread chips on baking sheets, cover with the beef-veggie mixture, and top with cheese. Bake for ten to fifteen minutes, or until cheese is fully melted.
Lesson 6: The Need To Knead (Pepperoni Pizza)
We return to the wonders of gluten again, but this time, instead of utilizing a chemical leavener, we’ll rely on the respirations of a minute organism to provide the lifting. Yeasts are unicellular fungi, related to mushrooms and mold. For millennia humans have used yeasts in cooking—to ferment beverages like beer and wine, to culture cheeses, and to provide leavening for bread. Yeasts eat sugars. Sugars are marginally present in most flours, and in many baking recipes sugar is added to further provide for the yeasts. As these tiny organisms munch away, they produce carbon dioxide as their waste, and voilà—the carbon dioxide is trapped by the gluten lattice and the dough rises, just as it did with the cookies. What’s different about yeasted dough, though, is that because the yeasts are alive, they will continue to eat sugar and pump out gas for as long as they’re able. This allows for multiple “rises”: letting the dough greatly increase its bulk—swelling with gas, stretching and developing the gluten strands—and then punching it down to rise again. Most dough recipes call for one to two rises; some even more. This pizza dough will rise once, rest, and then get quickly baked at 450 degrees.
(double this recipe to make 2 large crusts)
1 (.25 ounce) package dry yeast
1 C warm water
2 C bread flour
2 tbsp olive oil
1 tsp salt
2 tsp white sugar
1 package sliced pepperoni
2 bags shredded mozzarella
1 large jar pizza sauce
Other toppings for variety, if desired
Directions: Dissolve the sugar and yeast into the warm water. Allow five to ten minutes for the yeast to “bloom”, or become active. Then add in the salt, oil, and flour. Knead gently for ten minutes or so, until the dough becomes supple and stretchy. Roll into a ball, place in a bowl with oiled sides, cover loosely with a clean rag and place in a draft-free, warmish spot for about an hour and a half, until the dough doubles in size. Once risen, punch the dough down, roll it flat and lay it on baking sheets. (Dusting them with cornmeal will help prevent any sticking.) Preheat the oven to 450 degrees. Top the pizzas and bake for ten to fifteen minutes, or until cheese is bubbly.
Lesson 7: Oxidation Station (Apple Crisp)
Apples provide one of the more illustrative examples of enzymatic browning, a chemical reaction that occurs when its flesh is exposed to air. Enzymatic browning is chemically complex and occurs in a number of different foods—sometimes improving taste, sometimes contributing to spoilage—but it essentially boils down to enzymes reacting with oxygen to produce new colors and flavors. Incidentally, the lemon juice in this recipe helps to stave off the detrimental effects of browning on the apples—but mostly we add it for its tartness. Lemon juice contains citric acid, or vitamin C, an antioxidant that inhibits the enzymes involved with browning. Since we’re throwing the crisp right into the oven, however, there’s no real need to try and prevent spoilage.
Makes one 9×13-inch crisp.
For crumb topping:
2 sticks butter, cold
1 C bread flour
1 ½ C quick oats
½ C brown sugar
2 tbsp milk
5-7 Granny smith apples
1 tbsp lemon juice
¾ C brown sugar
1 ½ tsp cinnamon
½ tsp nutmeg
1 9×13-inch baking dish
Directions: Preheat the oven to 350 degrees. To prepare the crust, cut up the butter as small as possible and mix with the flour, oats, and sugar. Add the milk and gently combine. Thinly slice the apples and toss in a bowl with the lemon juice, sugar, cinnamon, and nutmeg. Grease the 9×13-inch dish, pour the apple mixture in, and top with the crumble. Bake for twenty to thirty minutes, or until apples are cooked through.
Lesson 8- All Dressed Up (Wonder-salad with balsamic vinaigrette)
Emulsification describes the blending of two or more liquids that are normally immiscible, or “un-mixable”. Oil and water, for example, are famously immiscible, but adding an emulsifier such as honey or mustard helps to disperse the molecules so that they can’t clump together, like with like. An emulsion, when done right, consists of evenly-coated molecules suspended in liquid, such as with milk: a matrix of milk-fat molecules suspended in water. Emulsions “break” when too many of one kind of molecule—say, a hydrophobic, oily molecule—are allowed to bind together, separating from the matrix and floating to the top. The dressing for this salad is an emulsion, mixing oil and vinegar with honey and Dijon mustard.
For the dressing:
½ C balsamic vinegar
1 C olive oil
3 tbsp Dijon mustard
3 tbsp honey
1 tbsp dried oregano
Salt to taste
For the salad:
2-3 bags mixed greens
Blender with a lid (to emulsify dressing)
Directions: Pour the mustard, honey, oregano, and salt into the blender. Add the vinegar. Turn the blender on the lowest speed and, while it is blending, remove the lid and slowly pour the oil in. Continue blending until the dressing reaches a uniform color and consistency, a couple seconds at most. Toss with the greens and seeds.