Key concepts
Physics
Viscosity
Starch
Emulsion
Science of cooking

Introduction
There's nothing like your favorite comfort food after a long day, right? Maybe it's beef stew, a really great salad or many people's favorite—macaroni and cheese. The sauce has to be just the right texture, though, or your noodles are either swimming—or just one big glob! So how can you get just the right texture? Get ready to invite some friends over to taste test. With this science activity you can be a scientist, a cook and a crowd-pleaser all at the same time!

Background
Sauces cling to food and to our palates, but how do you get just the right thickness or stickiness? Scientists describe the thickness, or gooiness, of a fluid by its viscosity. The more viscous a fluid is, the more it resists flow due to internal friction. This typically happens when enough particles are suspended in the fluid. When you pour a fluid with higher-viscosity, these particles bump into one another; the watery part cannot simply run down but needs to elbow its way through the crowd of particles, dragging them along. The more particles—and the bigger the suspended particles—the more viscous the liquid is.

Cooks have a variety of ways to make a sauce more viscous. One way is to add starches. When heated together with a liquid, starches absorb the liquid and expand. Their volume then increases by a factor of about 30. This explains why a small amount of starch can have a big effect on viscosity. Another way to thicken a sauce is by creating an emulsion. An emulsion is a mixture of two liquids that do not usually mix well. In cooking, this typically means tiny droplets of oil or molten fat dispersed in a watery solution. These tiny droplets act like the particles when a liquid is made viscous.

In this activity you will explore what thickens a cheese sauce and measure how the viscosity changes when you eliminate the different thickening agents, one at a time.

Materials

• Scale to measure 100 grams of each type of cheese (If you do not have a scale available, buy a little less than half a pound—about 200 grams—of each cheese; half of the purchased amount will be approximately one 100 grams of cheese.)
• 200 grams of American cheese
• 200 grams of mild cheddar (or your favorite melting cheese)
• Knife (Have an adult help with the cutting.)
• Cutting board
• Three clean plates
• A little less than one stick of butter (three eighths of a cup, to be exact)
• Two tablespoons of all-purpose flour
• Measuring tablespoon
• Three cups of milk (preferably whole milk)
• Three liquid measuring cups in the following sizes: one cup; three-quarter cup; one eighth cup
• Three cups of cooked and drained macaroni
• Saucepan (Be sure to have an adult help when heating items on a stove and handling them while hot.)
• Stove
• Three tall glasses
• Whisk
• Spoon
• Timer
• Clean containers in which to save leftovers
• Soapy water and towel to rinse and dry your utensils
• Food thermometer (optional)
• Ground black pepper, ground nutmeg or other spices you would like to use in your sauce (optional)

Preparation

• Gather all of your materials.
• Cook and drain the macaroni (with assistance from an adult)
• Make sure you have an adult on hand to help with cutting and any steps involving the stove and hot items.
• Measure 100 grams of American cheese.
• Cut the American cheese into cubes, and put them on an empty plate.
• Measure one 100 grams of mild cheddar.
• Cut the cheddar cheese into cubes, and add them to the plate with the cubes of American cheese.
• Repeat the previous four steps, preparing a second plate with cubed American and cubed mild cheddar cheese.
• Cut off one eighth a cup of butter and add it to the plate that is still empty. Repeat this two more times, adding butter to the two plates that have cheese on them.
• Fill the three tall glasses with the cooked macaroni, leaving about one to two inches at the top for the sauce. In a moment you will pour a three-quarter cup of sauce over one of the cups of macaroni to measure how easily the sauce flows.
• Study this ingredients list for the macaroni and cheese sauce:
• One eighth of a cup of butter (the amount of butter on one plate)
• One tablespoon of all-purpose flour
• Ground black pepper, ground nutmeg or other spices you might like
• One cup milk
• One hundred grams American cheese and 100 grams mild cheddar cheese, cut into cubes (the amount of cheese on one plate)
• When mixed together in the right way, these ingredients yield a nice thick sauce that sticks to your macaroni and lingers in your mouth. Milk is very runny. Do you think it would stick to your pasta? Which ingredient(s) do you think will help the sauce thicken and stick better to the macaroni? Can you explain why you think one or more of these ingredient(s) will make the sauce thicker?

Procedure

• Now that you have made a hypothesis (or a guess based on your current knowledge), you can start the test. You will make three sauces, one with all the ingredients as listed, a second with everything except the flour and a last one with everything except the cheese.
• Note that the directions instruct you to create one sauce after the other. If you have enough utensils and pans available, you can make two sauces simultaneously with someone else's help.
• Follow the instructions below to create your first sauce. This sauce will include all the ingredients. Note the right amount of butter and cheese is sitting on a plate, ready to be used. Be sure to have an adult help when using the stove.
• Melt the butter from one of the plates in a saucepan over low to medium heat.
• Blend in one tablespoon of all-purpose flour, stir well and cook for a few minutes until the butter–flour mixture looks smooth and the flour has absorbed the liquid.
• Add black pepper, nutmeg or any other spices you would like (optional).
• Slowly stir in one cup of milk. Use your whisk to gently beat out any lumps.
• Heat the mixture until it is gently bubbling.
• Lower the heat setting and add the cubes of cheese from one of the plates.
• Leave on low to medium heat and stir continuously until all the cheese has melted and you have a smooth sauce.
• How does this sauce feel as you stir it in the pot? Do you feel any resistance? How does this sauce run as you let it flow from the spoon? Does it look nice and thick or rather thin and runny?
• Let the sauce cool a little.
• If you have a food thermometer, use it to register the temperature of your sauce just before you pour it over the macaroni (see next step). (Tip: Having all sauces at the same temperature when performing the measurement will yield more exact results.)
• In a moment, you will pour a three-quarter cup of sauce over the macaroni. Do you think this sauce will pile up on top of the macaroni or will it slowly or quickly run down and accumulate near the bottom? In this activity you will measure the time it takes to flow down as an indication of how easily the sauce flows. Be sure to have an adult help you when you are handling the warm saucepan and sauce.
• Take one tall glass of macaroni, pour a three-quarter cup of warm sauce over it and start the timer. Watch the sauce make its way down and stop the timer as soon as some sauce reaches the bottom of the glass. Maybe the sauce never makes it all the way down, which is fine, too. Were you surprised about how fast or slow this sauce made its way down? Wait! No eating from this macaroni yet! Be patient; at the end of your tests you—and your helpers—will be able to taste all your sauces and vote for the best taste.
• You can save any leftover sauce that is still in the saucepan. Wash and dry your saucepan, whisk, measuring cup and spoon and get ready to make your second sauce.
• Follow the instructions to create your second sauce, as described above, with one single change: Do not add any flour. After melting the butter, go directly to adding spices (if you used any) and milk. Which differences do you notice while making this sauce? Does this sauce feel different when you stir it in the pot? Does it run differently as you let it flow from the spoon?
• While you let your sauce cool down a little, think about what you expect will happen when you pour this sauce over the second glass of macaroni. Do you think this sauce will pile up on top of the macaroni—or slowly or quickly run down and accumulate near the bottom?
• If you have a food thermometer, use it to measure the temperature of the sauce and wait until it reaches the same temperature as the sauce in your first test.
• Measure how this sauce flows by pouring a three-quarter cup of sauce over the macaroni in the second glass, then time how long it takes for some sauce to flow all the way down to the bottom of the glass. Did this sauce run faster or slower than the sauce with all ingredients? Note the only difference between the two sauces is that you omitted the tablespoon of flour in the second sauce. Are you surprised a relatively small amount of flour can have this effect?
• Save the sauce left in the pan in a clean container for later. Wash and dry your saucepan, whisk, measuring cup and spoon and get ready to try your final sauce. (This version will be easier and quicker to make.)
• Follow the instructions to create your initial sauce (including the flour) with one single change: Do not add any cheese. Does this sauce feel different when you stir it in the pot? Does it run differently as you let it flow from the spoon?
• Note that you eliminated only one ingredient each time. Why do you think you do this?
• Perform the procedure, as described above, to measure how this sauce runs down the third glass of macaroni. How does the time compare with the previous two measurements?
• Now that you have all your data, can your measurements help you conclude which ingredients help make this sauce thicker or more viscous?
• Now it's time to taste your three versions of macaroni and cheese! Divide them up among different plates if you are sharing with your helpers or friends. Which one is your favorite? If others sampled them as well, is there a group favorite?
• Extra: In this procedure you did not test what happens when you omit the butter from the recipe. Perform the test to find out.
• Extra: This sauce was thickened with cheese as well as a flour–butter mixture called a roux (pronounced “roo,” in cooking). What do you think would happen if you reduced the amount of cheese and increased the amount of roux? If it is hard to imagine, try it with extremes. What would a sauce taste like if it was only thickened with butter and flour? Can you change the recipe so you get exactly the viscosity and cheesy taste you prefer?
• Extra: Look at the ingredients of some of your favorite sauces and find the items that thicken the sauce.

Observations and results
Was your first sauce the most viscous? Did it take the most time to reach the bottom of the glass? Both the flour and the cheese help thicken the sauce.

Next you omitted the flour. You had about one cup of cheese for a little more than two cups of sauce (one cup of cheese, one cup of milk and one eighth cup of butter). About half the volume in your final sauce is taken up by suspended droplets or particles. This is the ratio at which a sauce starts to clearly thicken. Any addition of particles will dramatically increase the viscosity (or thickness) of the sauce.

Finally, you omitted the cheese. You had one tablespoon of flour to thicken the sauce. Flour is mainly a starch and starches expand when heated in a liquid. Your tablespoon of flour took up about half a cup of space in your final sauce! That is a lot compared to the tiny amount of flour added, although not enough to thicken one cup of milk.

With both the cheese and the flour, you had 60 to 70 percent of the volume of the sauce taken up by suspended droplets or particles, yielding a nice, thick sauce.

Cleanup
Clean the kitchen and wash all your equipment with soapy water. Eat your test batches now, share them—or save them for later!

More to explore
Marble Race in Liquids, from Scientific American
Fake Blood Made Scientific, from Scientific American
What Is Viscosity?, from Princeton University
Processed Cheese, Please! Get Your Grilled Cheese Oooey Gooey Instead of Oily and Gloppy, from Science Buddies

This activity brought to you in partnership with Science Buddies