Key concepts
Physical Science

Whoever told you oil and water don't mix might not have considered the term "emulsion." It is possible for tiny particles of two seemingly unmixable substances to suspend in one another—like oil and water! This super scientific experiment demonstrates the magic of the invisible globule while bringing a yummy result to the table.

Milk is mostly water with about 5 to 10 percent protein and fat globules. Cream is milk that contains closer to 15 to 25 percent fat globules. What's a "globule"? A globule is a super tiny membrane filled with fat molecules—think of a microscopic water balloon. Because these globules are so small and fat is lighter than water, it floats! This forms a "stable suspension," a colloid! The bigger the globules, the slower it moves—and the thicker the milk or cream.

When shaken, the globules' membranes smash against each other and break apart like bursting water balloons. The fat then spills out and clumps together with the contents of other burst globules, which causes the freed fat to separate from the water. As this process continues, two new substances are formed: a solid (butter) and the remaining liquid (buttermilk)!

•    Jar or other airtight container
•    Heavy cream (at room temperature)

•    Make sure you are working with a clean jar or airtight container.
•    It is important that your heavy cream or whipping cream be room temperature.

•    Pour the cream into the jar.
•    Screw the lid onto the jar securely.
•    Now, hold on tight to your jar and "shake with force." No wimpy shakes here! Use your arms to make firm, vigorous strokes. Do this for between five and 20 minutes. You should start to see results in about 10 minutes. Can you see changes happening inside the container? Does it start to feel different when you are shaking it? The butter is done when it has completely separated from the liquid and forms a solid, single clump.

Observations and results
You will have created two new substances—butter and buttermilk! The butter is the result of the globules having broken apart and their fatty contents adhering together. The buttermilk, then, is the liquid that is left over. How are the two substances different? Are they both different from the heavy cream you started out with? How so?

What different things can you do with butter and with buttermilk? Can you think of other mixtures that are emulsions? (Hint: think of other oil and water substances.)

•    If you intend to use them later, keep the butter and buttermilk in the refrigerator to prevent them from going bad—that's a whole other transformation, and one with results that are not quite so yummy.

More to explore
"How to Make Butter out of Cream, and Why It Works" from Crazy Aunt Lindsey
"Salad Dressing Science Mixes Up Researchers" from Scientific American
"Ice Cream Science" Scientific American
"Making Butter at Home" from Boston Children's Museum

This activity brought to you in partnership with