The Chemistry behind a Cheeseburger The first cheeseburger was created between 1924 and 1926 by a 16 year old chef from Pasadena, California named Lionel Sternberger. The anecdote goes that a passing homeless man suggested Sternberger should add a slice of cheese to his hamburger order. Sternberger then added this to his main menu at the Rite Spot and the cheeseburger was born.1 Today the cheeseburger is marketed by not only fast food chains like Steak-N-Shake and McDonald’s but also steakhouses such as Longhorn and Logan’s Roadhouse. Whether you choose to cook your cheeseburgers at home on the grill or grab one on the go, cheeseburgers are an American tradition that everyone can enjoy. However, not all who devour this scrumptious piece of American history actually know the chemistry behind the ingredients that come together to make this delicious masterpiece. A cheeseburger would not exist without the bun to hold all of the ingredients together. The bun is made of flour, a starch or long chains of sugars molecules known as polysaccharides, yeast, salt and water. Flour, or grains are comprised mainly of wheat. When these ingredients mix, the flour gums up and absorbs all the water to produce a protein matrix called gluten. Gluten is elastic and holds the bread together to give it structure. As you knead the dough, the yeast, a living single cell creature, begins to eat the starch. When the starch is consumed by the yeast, carbon dioxide is produced. The process of microbes producing gas from starches and sugars is known as fermentation. When the CO2 gas is heated, it expands, making the bubbles inside the dough larger and inflates the elastic gluten. When the dough is placed in an oven at 400°F, the yeast become more active and produce CO2 faster. But when it becomes too hot, the yeast microbes die and expansion stops. The air pockets set and give the buns a fluffy texture and a light taste. The main ingredient of the cheeseburger is the beef patty. Originating in the German city of Hamburg, researchers believe that Americans consume some fourteen billion hamburgers a year.2 The meat is composed of ground muscle tissue of cows, which contains a large amount proteins, fats and water. Proteins are chained molecules made up of amino acids. The main component of protein in meat is myoglobin, which is composed of helices or coiled regions of the protein chain. These helices are bundled together within the protein structure and deep inside is an iron atom. The red color of meat actually comes from the myoglobin molecules. Meat changes color when heat is applied through a series of chemical reactions. As the meat is cooked, the myoglobin proteins in the meat vibrate faster and faster, denaturing the proteins, unfolding them and changing their shape. The denaturing, particularly around the iron atoms, causes the color change from red to dark brown. The sizzling sound that is heard when the meat is placed on a hot surface is actually the water molecules on the surface of the meat boiling off. The browning on the outside of the fried burger is known as the Maillard Reaction, or the Browning Reaction. This comes from amino acids and sugars being heated at high temperatures to produce the dark, brown crust that gives the burger its flavor. The cheeseburger gets its name from adding a piece of cheese to the old fashioned hamburger. Cheese is a concentrated form of milk with the water removed. The process of making cheese involves a phase change from liquid to solid evoking a nonreversible chemical reaction. Milk is composed of proteins, fats, sugars and water and the process of making cheese involves removing the water so that all is left is a concentrated mass of the proteins and fats. Milk is an emulsion with millions of globular protein molecules and droplets of fat suspended in the liquid. In order to separate the proteins in milk from the water, they have to be denatured, relaxing them in a long chain. These long chains of proteins coagulate or stick together and can then be strained from the milk. There are three ways to denature proteins which involve adding heat, salt or acid. Using an acid such as citric acid to denature the proteins allows the liquid or whey to be strained through a cheese cloth keeping behind the solids, or curds, which firm up into a ball of fresh cheese. A majority of cheeses are made by adding an enzyme and a bacteria. This enzyme is called rennet and it is found in the stomach lining of most animals. It further breaks down the proteins to create a nice gooey mess to give cheeses of different textures. The processes of fermentation, oxidation, dehydration and bacterial growth are all chemical reactions that turn liquid milk into solid cheese to turn hamburgers into cheeseburgers. A cheeseburger is rarely served without nice plump slices of ripe tomatoes. A tomato was classified as a vegetable is 1893 in Nix v. Hedden, however it is actually a fruit. This is because it has seeds and ripens in much of the same ways as fruits. Tomatoes start off small and green and as they ripen they become large and red. The ripening process relies on ethylene, C2H4, an unsaturated hydrocarbon or alkene. Ethylene is the smallest, most simple alkene. Also a gas, ethylene can seep out of the ground as a component of natural gas. Ethylene is a plant hormone that triggers changes in living things. Ethylene gas is also emitted from fruits, such as bananas, apples and peaches, as they ripen. Receptors in the tomato plant can sense when ethylene gas is produced, the chemical process of ripening begins. This causes starches in the sour, green fruit to be converted into fructose. Fructose are the natural sugars found in fruits. The presence of the ethylene gas also introduces the lycopenes, which give the tomato the nice red color. Lycopene is a natural color pigment and a carotenoid or antioxidant. The chemical changes induced by ethylene and lycopene tell animals and humans that the tomato is ripe and ready for consumption. Tomatoes scientific name, lycopersicum, comes from the term lycopene. The tomato industry has been able to control the ripening process of green tomatoes picked off the vine early by exposure to ethylene gas. The green tomatoes can be stored in greenhouses until it is time to be sold and then ethylene gas is pumped in, which causes the gas to diffuse and spread throughout the air. It only takes a part ethylene per one million parts of air to trigger the ripening process. Tomatoes can ripen quickly by being placed next to another fruit that emits ethylene gas. However, fruits can be overexposed to ethylene which can cause them to rot. Another ingredient that gives variety to any cheeseburger is the pickle. A dill pickle starts out as a cucumber. Cucumbers, like tomatoes, are technically a fruit because they ripen and have seeds. Cucumbers contain bacteria that can cause them to rot in about ten days if refrigerated. With a short life cycle, pickling allows for cucumbers to be stored and enjoyed for longer periods of time. Pickling is the process of preserving and keeping the cucumbers from spoiling. Pickling comes from the Latin word servo meaning to save or to keep. Cucumbers are placed into brine. Brine is salty water, made from sodium chloride, NaCl. The word pickle is believed to come from the Dutch word pekel or piquant meaning sharp-tasting. The process of pickling is a battle between good and bad bacteria or microbes. The good bacteria gives rise to the flavor profiles that we find appealing, while the bad bacteria can cause diseases such as botulism and salmonella. The fermentation process enhances the growth of the good bacteria as it decreases the growth of the bad bacteria. The good bacteria is known as lactic acid bacteria. During the process of fermentation, both the good and bad bacteria eat and digest starches to convert them into sugars that they use to grow and multiply. As the good bacteria grow, they continue to consume more and more starches leaving very little for the bad bacteria to feed off of. Also during fermentation, the good bacteria produce bi-products such carbon dioxide and alcohols or organic acids like lactic acid, C3H6O3. Lactic acid produces more lactic acid, good microbes that inhibit bad bacteria from having a chance to grow. This changes the acidity of the brine and the cucumbers soaking in it. The pH is a way to measure the concentration of the hydrogen ions present in a solution. These hydrogen cations, H+, decrease the pH and increase the acidity, giving pickles the sour taste. Bad bacteria generally like a more basic pH. The more acidic the pH of the brine, usually around a pH of 4.6, the less ability for the bad bacteria to grow. The last and final ingredient of the cheeseburger is the addition of the condiments. Ketchup, mustard and mayonnaise are all mixtures of solids and liquids, known as suspensions and colloidal dispersions. Suspensions are heterogeneous mixtures of particles suspended inside that are large and dense enough to not stay suspended for long. Ketchup and mustard belong in the category of suspensions. They are the particles of vegetable materials such as tomatoes or mustard seeds. They are ground up and dispersed throughout an aqueous water phase to give us sauce. They will begin to separate into the liquid and solid particles if left alone long enough. Colloids or colloidal dispersions are also mixtures similar to suspensions except they do not settle out. Colloid comes from the Greek word kolla meaning glue. They are made up of substances that are immiscible or unblendable, this is known as emulsion or a dispersion of one liquid in another. Mayonnaise is made of vegetable oil, vinegar, acetic acid and water. The oil being hydrophobic, separates, moving from the aqueous vinegar layer. The oil molecules, made of long hydrocarbon chains, move away as quickly as possible to reduce any interface or contact from the water. Water is a polar molecule and since oil is non-polar and neither has a positive or negative side so water is not attracted to them at all. A coating is formed around the oil droplets to keep them from forming larger droplets and separating from the water. An emulsifying molecule known as lecithin that comes from egg yolks is used in this process. The egg yolks are mixed with the vinegar and then the oil is added slowly while mixing to break the oil up into small droplets. Lecithin is both hydrophobic and hydrophilic. The mixture becomes very dense and semi solid, not allowing for any separation to occur. The chemistry behind the cheeseburger is very interesting. From the bun that holds the yummy ingredients together to the condiments, ketchup and mayonnaise, that give that extra added flavor, chemistry is the building block that makes this creation possible. The next time a commercial for McDonald’s comes on or you’re invited to a cookout, remember all the chemical processes and reactions that make this American icon achievable. The history of the cheeseburger is interesting but the chemistry is even more remarkable. Lionel Sternberger probably had no idea about the immense scientific circumstances that brought about the cheeseburger the day that he added this creation to his menu.
Resources
1 History of the Cheeseburger. Cheese-Burger.net. 2015. http://www.cheese-burger.net/history.
2 Cheeseburger Chemistry. NBC Learn. NBC Universal Media, LLC. 2015. http://www.nbclearn.com/portal/site/learn/chemistry-now/cheeseburger-chemistry.