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Transcript
Name __________________________ Assn ___________ The Science of Cooking with Organic Molecules From “The Science of Cooking” at www.exploratorium.edu The Science of Eggs Egg proteins change when you heat them, beat them, or mix them with other ingredients. Understanding these changes can help you understand the roles that eggs play in cooking. Proteins are made of long chains of amino acids. The proteins in an egg white are globular proteins, which means that the long protein molecule is twisted and folded and curled up into a more or less spherical [ball] shape. A variety of weak chemical bonds keep the protein curled up tight as it drifts placidly in the water that surrounds it. Suppose this is a chain of amino acids: Draw what it might look like when twisted up into a spherical protein: Uncooked Egg Proteins Heat ’em When you apply heat, you agitate those placidly drifting egg-white proteins, bouncing them around. They slam into the surrounding water molecules; they bash into each other. All this bashing about breaks the weak bonds that kept the protein curled up. The egg proteins uncurl and bump into other proteins that have also uncurled. New chemical bonds form—but rather than binding the protein to itself, these bonds connect one protein to another. Draw two uncurled proteins, and draw lines between them representing new bonds: Cooked Egg Proteins After enough of this bashing and bonding, the solitary egg proteins are solitary no longer. They’ve formed a network of interconnected proteins. The water in which the proteins once floated is captured and held in the protein web. If you leave the eggs at a high temperature too long, too many bonds form and the egg white becomes rubbery. Add some water molecules to your drawing above, inserting them in the web of protein bonds. Draw the water molecules showing one oxygen and two hydrogen atoms, like this: I’m making an omelet with 3 eggs. Explain why I have to cook them to get them to stick together: ______________________________________________ ______________________________________________ ______________________________________________ I have cooked my eggs too long, and they are like rubber! Explain why this happened: ______________________________________________ ______________________________________________ ______________________________________________ Beat ’em When you beat raw egg whites to make a soufflé or a meringue, you incorporate air bubbles into the waterprotein solution. Adding air bubbles to egg whites unfolds those egg proteins just as certainly as heating them. To understand why introducing air bubbles makes egg proteins uncurl, you need to know a basic fact about the amino acids that make up proteins. Some amino acids are attracted to water; they’re hydrophilic, or waterloving. Other amino acids are repelled by water; they’re hydrophobic, or water-fearing. Hydrophilic means ___________________________ Hydrophobic means ___________________________ Egg-white proteins contain both hydrophilic and hydrophobic amino acids. When the protein is curled up, the hydrophobic amino acids are packed in the center away from the water and the hydrophilic ones are on the outside closer to the water. If protein is in water, where will the: hydrophobic amino acids want to be? __________________ hydrophilic amino acids want to be? __________________ Draw a curled up protein with water molecules outside of it. Label where the hydrophobic and hydrophilic amino acids are. Protein in water with hydrophilic and hydrophobic amino acids labeled When an egg protein is up against an air bubble, part of that protein is exposed to air and part is still in water. The protein uncurls so that its water-loving parts can be immersed in the water—and its waterfearing parts can stick into the air. Once the proteins uncurl, they bond with each other—just as they did when heated—creating a network that can hold the air bubbles in place. Draw a web of connected proteins, with air bubbles trapped in the web: When you heat these captured air bubbles, they expand as the gas inside them heats up. Treated properly, the network surrounding bubbles solidifies in the heat, and the structure doesn’t collapse when the bubbles burst. Explain why heating this in my oven will make the egg whites expand (get bigger): _______________________________________________ _______________________________________________ _______________________________________________ Mix ’em up Everyone knows that, left to their own devices, oil and water don’t mix. But for many recipes, you mix oil- based and water-based liquids—and want them to stay that way. Often, egg yolks come to your rescue by creating an emulsion. Most food emulsions are known as the oil-in-water type, which means that oil (or fat) droplets are dispersed throughout the water. Put oil and water in a jar, shake it vigorously, and you’ll disperse [spread out] the oil. To prevent the oil droplets from coalescing [clumping back together], however, a substance known as an emulsifier is required. Egg yolk contains a number of emulsifiers, which is why egg yolks are so important in making foods such as hollandaise and mayonnaise. Oil belongs to which group of organic compounds: proteins, lipids, carbohydrates, or nucleic acids? __________________ Name a food that depends on oil and water being held together with eggs: ______________________ Many proteins in egg yolk can act as emulsifiers because they have some amino acids that repel water and some amino acids that attract water. Mix egg proteins thoroughly with oil and water, and one part of the protein will stick to the water and another part will stick to the oil. Draw a sketch that shows how egg proteins can keep oil and water molecules mixed together: Lecithin is another important emulsifier found in egg yolk. Known as a phospholipid, it’s a fatlike molecule with a water-loving “head” and a long, water-fearing “tail.” The tail gets buried in the fat droplets, and its head sticks out of the droplet surface into the surrounding water. This establishes a barrier that prevents the surface of the fat droplet from coming into contact with the surface of another fat droplet. Lecithin is another molecule that holds fat and water together. It is called a “phospholipid,” so it must belong to which group of organic compounds? ___________________________ The Science of Candy What is sugar? **Extra Credit** The white stuff we know as sugar is sucrose, a molecule composed of 12 atoms of carbon, 22 atoms of hydrogen, and 11 atoms of oxygen (C12H22O11). Like all compounds made from these three elements, sugar is a carbohydrate. It’s found naturally in most plants, but especially in sugarcane and sugar beets—hence their names. Sucrose is actually two simpler sugars stuck together: fructose and glucose. In recipes, a little bit of acid (for example, some lemon juice or cream of tartar) will cause sucrose to break down into these two components. Sucrose is made of a molecule of ________________ and a molecule of ________________ bonded together. Sucrose has ____ carbon atoms, _____ oxygen atoms, and ____ hydrogen for a total of _______ atoms. What happens when you heat a sugar solution? When you add sugar to water, the sugar crystals dissolve and the sugar goes into solution. But you can’t dissolve an infinite amount of sugar into a fixed 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 is different at different temperatures. The higher the temperature, the more sugar that can be held in solution. When you cook up a batch of candy, you cook sugar, water, and various other ingredients to extremely high temperatures. At these high temperatures, the sugar remains in solution, even though much of the water has boiled away. But when the candy is through cooking and begins to cool, there is more sugar in solution than is normally possible. The solution is said to be supersaturated with sugar. How does cooking ingredients to a high temperature allow us to make a sweeter candy (hint: “sweeter” = more sugar)? ______________________________________________ ______________________________________________ ______________________________________________ ______________________________________________ Supersaturation is an unstable state. The sugar molecules will begin to crystallize back into a solid at the least provocation [disturbance]. Stirring or jostling of any kind can cause the sugar to begin crystallizing. Why are crystals undesirable in some candy recipes—and how do you stop them from forming? The fact that sugar solidifies into crystals is extremely important in candy making. There are basically 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). Recipe ingredients and procedures for noncrystalline candies are specifically designed to prevent the formation of sugar crystals, because they give the resulting candy a grainy texture. How do crystalline and noncrystalline (amorphous) candies differ? ______________________________________________ ______________________________________________ ______________________________________________ One way to prevent the crystallization of sucrose in candy is to make sure that there are other types of sugar—usually, fructose and glucose—to get in the way. Large crystals of sucrose have a harder time forming when molecules of fructose and glucose are around. Crystals form something like Legos locking together, except that instead of Lego pieces, there are molecules. If some of the molecules are a different size and shape, they won’t fit together, and a crystal doesn’t form. A simple way to get other types of sugar into the mix is to "invert" the sucrose (the basic white sugar you know well) by adding an acid to the recipe. Acids such as lemon juice or cream of tartar cause sucrose to break up (or invert) into its two simpler components, fructose and glucose. Another way is to add a nonsucrose sugar, such as corn syrup, which is mainly glucose. Some lollipop recipes use as much as 50% corn syrup; this is to prevent sugar crystals from ruining the texture. Explain how adding cream of tartar to my cooked sugar mixture will prevent crystals from forming: _____________________________________________ _____________________________________________ _____________________________________________ Fats in candy serve a similar purpose. Fatty ingredients such as butter help interfere with crystallization—again, by getting in the way of the sucrose molecules that are trying to lock together into crystals. Toffee owes its smooth texture and easy breakability to an absence of sugar crystals, thanks to a large amount of butter in the mix. Explain how adding butter to my cooked sugar mixture will prevent crystals from forming: ______________________________________________ ______________________________________________ ______________________________________________