Stop Slumping: What Makes Foams Stand Up Straight
AbstractHere's a riddle for you: What would a latté be without a froth of bubbly milk on top? Answer: Black coffee! And how about a pumpkin pie without the whipped cream? Answer: Sad. Delicious, edible foams are everywhere, from sodas, meringues, and soufflés to mousses and whipped creams. They provide a delicious, spongy contrast to the foods they accompany, and their airiness releases aromas that enhance the eating experience. So, what makes a good foam? One with high volume and lots of staying power, that doesn't collapse in just a minute or two? Try this food science fair project to find out!
Kristin Strong, Science Buddies
To determine which foods make good foams that have high volume and longevity.
A scoop of vanilla ice cream in a tall glass. A splash of earthy root beer. Put them together and you have that classic treat—a root beer float. The ice cream bobs to the top of the soda and floats in a halo of creamy bubbles so high and thick they threaten to spill over the glass. You can hear them softly hissing and popping, and if they aren't slurped up quickly enough, they will slowly sink to the surface of the soda.
Figure 1. This photo shows a root beer float, topped with a thick layer of foam.
That delicious mass of bubbles is called an aqueous foam. It is formed when gas bubbles get trapped in a liquid (like the carbon dioxide bubbles in root beer) and prevent the molecules in that liquid from flowing freely. The resulting foam is a spongy, springy matrix of bubbles.
How do the bubbles get into a liquid? There are several ways to form foams. You can:
- Shake a liquid up and down inside a container,
- Whip a liquid with a whisk,
- Whip a liquid with a hand-held blender,
- Shoot steam (a combination of water vapor and air) into a liquid with the wands on a special coffee machine, or
- Add pressurized carbon dioxide or nitrous oxide to the liquid.
Depending on the liquid, the foam formed may not last long though. Air inside the bubbles, and water inside the liquid, have very different densities. The air inside the bubbles tries to rise up, while the liquid forming the bubble walls is pulled down by gravity. This tugging on the bubbles in opposite directions eventually leads to weakened bubble walls. The bubbles pop and the foam collapses.
Foams can be made stiffer and longer-lasting, or obtain greater volume, by adding substances to the liquid that strengthen the bubble walls. Egg yolks, for example, contain an important emulsifier, called lecithin. One end of the lecithin molecule is water-soluble, (it can be dissolved in water) while the other end is water-insoluble (it cannot be dissolved in water). When lecithin is added to a liquid that is made into a foam, the water-soluble portion of the lecithin molecule is attracted to the bubble wall while the insoluble portion is attracted to the bubble air. This layer of emulsifier acts as a bridge between the liquid and gas, stabilizing the bubble walls and making the foam last longer.
In this food science fair project, you'll investigate what additives increase the volume and longevity of a foam. Do milk proteins create foams that are thicker and taller than Santa Claus' beard? Do egg yolks make foams stand up straight and stop slumping? Shake up some liquids to find out!
Terms and Concepts
- Aqueous foam
- Carbon dioxide
- Nitrous oxide
- Negative control
- Positive control
- What are some examples of aqueous foams?
- What two states of matter are required to form an aqueous foam?
- How are foams formed?
- Why do foams collapse?
- What are emulsifiers? What do they do?
These sources discuss what forms are and how they can be formed and stabilized:
- McGee, Harold. On Food and Cooking. New York, NY: Scribner, 2004.
- Barry, P.L. and Phillips, T. (2003, June 9). The Strange Physics of Foam. Retrieved May 20, 2009.
This source describes what emulsifiers, like the lecithin in egg yolk, do:
- Wikipedia Contributors. (2009, May 3). Emulsion. Wikipedia: The Free Encyclopedia. Retrieved May 15, 2009.
Materials and Equipment
- Giant test tubes with threaded caps, 25 mm X 135 mm; available at Home Science Tools.
- Test tube rack; available at Home Science Tools.
- Impact safety goggles; available at Home Science Tools.
- Small medicine cup or tablespoon
- Test liquids, all at the same temperature—either refrigerated or room temperature (1 cup of each)
- Tap water
- Nonfat milk
- 2% or whole milk
- Eggs, at same temperature as the test liquids (4)
- Small bowl
- Large bowl
- Liquid dish detergent, at same temperature as the test liquids
- Water-soluble marker
- Ruler, metric
- Optional: Magnifying glass
- Lab notebook
- Graph paper
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Note: Because the test tubes used in this experiment are not "food-grade" (designed for food preparation), and because raw eggs are not safe to eat, you should not eat any of the foams you make in your experiment.
Preparing for Testing
Create a data table for each of your test liquids:
- Tap water (This will serve as your negative control, meaning it will produce a negative result and will not make a foam.)
- Nonfat milk
- 2% or whole milk
- Egg white
- Tap water plus egg yolk
- Tap water plus liquid dish detergent (positive control, meaning it will produce a positive result and will make a lot of foam.)
Note: The times shown in the example data table are just suggestions. Feel free to adjust the times at which measurements are made, and for how long they are made.
Nonfat Milk Data Table
|Time (min)||Trial 1: Distance from the starting level to the top of the liquid(cm)||Trial 2: Distance from the starting level to the top of the liquid(cm)||Trial 3: Distance from the starting level to the top of the liquid(cm)||Average distance from the starting level to the top of the liquid(cm)||Trial 1: Distance from the top of the foam to the starting level (cm)||Trials 2: Distance from the top of the foam to the starting level (cm)||Trial 3: Distance from the top of the foam to the starting level (cm)||Average distance from the top of the foam to the starting level (cm)|
- Since time is an important variable in this experiment, read through the steps in the next section first, so that you are familiar with the testing procedure.
Testing Your Liquids
Fill the small medicine cup to the 3-teaspoon (tsp.) mark with one of the test liquids.
- As an alternative, you can use a tablespoon (Tbsp.) measuring spoon, since 1 Tbsp.=3 tsp., although it may be more difficult to pour the test liquid into the test tube using a tablespoon.
For the Tap water plus egg yolk test liquid, add 1/4 tsp. of egg yolk to the cup and then fill the rest with tap water to the 3-tsp. mark.
- To get only the egg yolk, crack an egg in half over a large bowl and carefully pour the yolk from one egg shell back to the other until the egg white runs off into the large bowl. Pour the egg yolk into the small bowl.
- For the Egg white test liquid, it may be necessary to stir the egg white a few times with a spoon to break it up before transferring part of it to the medicine cup, otherwise it will stick together into one big mass.
- For the Tap water plus liquid dish detergent test liquid, add 1/4 tsp. of dish detergent to the medicine cup and fill the rest with tap water to the 3-tsp. mark.
- Put on your safety goggles.
- Pour the test liquid into one of the test tubes as shown in Figure 2.
Figure 2. This photo shows a test liquid being poured from a small medicine cup into the test tube, in preparation for testing. Note, that in this image a different test tube with a rubber stopper was used.
- Screw the cap onto the test tube.
- Hold the test tube vertically, and mark the starting level with a water-soluble marker as shown in Figure 3.
- Reset and start the stopwatch.
Immediately begin shaking the test tube vigorously up and down for 40 seconds.
- Be sure to shake the test tube away from counters, tables, chairs, or other people. You don't want it to hit anything or crack.
- Hold on to it securely.
Figure 3. This photo shows how to mark the starting level, and how to hold the test tube when you are shaking it. Note, that in this image a different test tube with a rubber stopper was used.
- Immediately after shaking, stop, reset, and restart the stopwatch.
Immediately hold the test tube vertically and make two more marks, as shown in Figure 4:
- One mark at the top of the foam (if there is any).
A second mark at the surface of the liquid.
- If it is hard to see the surface of the liquid, then try turning it away from a light source.
Holding the test tube vertically, measure the distance from the starting level to the surface of the liquid and record your measurement in the appropriate data table.
- As an alternative, you can measure the distances after the trial is over by marking the changing levels of the liquid surface on the test tube. After the trial is over, pour out the liquid, lay the test tube flat, and record your measurements. Just be sure to keep track of the marks on the test tube, so that you know what mark corresponds to what time in the experiment.
Holding the test tube vertically, measure the distance from the top of the foam to the starting level and record your measurement in the appropriate data table.
- As an alternative, you can measure the distances after the trial is over by marking the changing levels of the top of the foam on the test tube. After the trial is over, pour out the liquid, lay the test tube flat, and record your measurements. Just be sure to keep track of the marks on the test tube, so that you know what mark corresponds to what time.
When milk is shaken in a tube there is a drop in the starting level of liquid and an increase in the level of foam present.
Figure 4. This photo shows a test tube after shaking it with a test liquid inside. The two measurements that you need to make with a ruler are marked by double red arrows. Note, that in this image a different test tube with a rubber stopper was used.
Set the test tube in the rack and wait for the next time increment.
- Do not re-shake the test tube.
- If desired, look at the bubbles with the magnifying glass and make note of their size and uniformity. Are they bigger at the top or bottom, or a uniform size throughout? Record your observations in your lab notebook.
Repeat steps 9–12 until the trial is over.
- A trial is over when there is no foam, or the amount of foam is immeasurable.
- A trial is also over when more than 30 minutes have passed.
- Wash, rinse, and dry the test tube and the cap, or use a fresh test tube.
- Repeat steps 1–14 two more times for the first test liquid.
Repeat steps 1–15 for all of the other test liquids, until you have a total of three trials for each.
- Feel free to test two liquids simultaneously by shaking one in each hand.
Analyzing Your Data Charts
- For each data table, calculate the average distance from the starting level to the top of the liquid for each time increment.
- For each data table, calculate the average distance from the top of the foam to the starting level for each time increment.
- For each data table, plot the time on the x-axis and the distance from the starting level to the top of the surface on the y-axis.
- For each data table, plot the time on the x-axis and the distance from the top of the foam to the starting level on the y-axis.
- Which test liquid produced the greatest volume of foam? Which test liquid produced a foam with greatest longevity? Did any of the test liquids have a better foam than the positive control? Compare the foams of nonfat milk and 2% or whole milk. If you were going to make a latte or a cup of chai for your parents, which milk would you choose to give the best and longest-lasting layer of foam? If you looked at bubble sizes, which test liquid produced the most uniform and smallest bubbles? Which one produced the largest and most varied bubbles? Were bubbles bigger at the top or bottom of the test tube?
Ask an Expert
- Using instant, nonfat, dry milk powder and tap water, investigate how changing the amount of milk protein affects the time it takes for the foam to collapse.
- Investigate what happens when you create a foam and then leave it exposed to the air and evaporation (remove the rubber stopper). How does this change the rate at which the foam collapses?
- Investigate if the rate of foam collapse is dependent upon how much foam surface area is exposed to the atmosphere.
- See if temperature affects foam volume or longevity.
- Experiment with other emulsifiers, like honey and dry yellow mustard.
- Try combining ingredients to see if foam volume or longevity is improved.
- Add fat, like cooking oil, to a test liquid to see how that impacts foam volume or longevity.
- Focus on egg whites and investigate how salt, water, sugar, egg yolk, fat, temperature, and egg age affect the foam volume and longevity.
- For each time increment in each data table, calculate what percentage of liquid was required to make the foam by dividing the distance from the starting level to the top of the liquid by the total foam distance and multiplying by 100. (The total foam distance is the distance from the top of the foam to the bottom of the foam.) Plot how this percentage changed with time. Which test liquids produced the most foam with the least amount of liquid?
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