Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
7. Color
PROECT: COLOR BLOSSOMS
Materials
Small bowl |
Food coloring |
Whole milk |
Liquid dishwashing detergent |
My last chemistry book, Culinary Reactions, deals with chemistry from the viewpoint of the kitchen and discusses food chemistry in some detail.
One of the subjects in the book is how proteins and fats interact in water. A good example of this is how proteins form around droplets of butterfat in cream and whole milk, stabilizing the emulsion of fat in water.
You can destabilize the emulsion by adding something that combines with fat and water better than the proteins do. Soaps and detergents do exactly that, and you can demonstrate their effect with a very colorful display.
In the photo on the next page, I started off with a bowl of whole milk (nonfat milk won’t work), to which I added some drops of food coloring. Notice that the drops are just sitting there, not mixing much at all with the milk.
Next I added a drop of dishwashing detergent.
The colors exploded, racing away from the detergent. But they didn’t stop. They continued to mix and swirl around the bowl for more than a minute. They flowed and folded and moved around, quite unlike what they were doing before I added the detergent.
What happened is that the detergent interacted with the oil, the water, and the proteins in the milk. The detergent had molecules on which one end liked to stay in water and the other end liked to stay in fats and oils. Many of the proteins in the milk also had parts that were water-loving (hydrophilic) and other parts that were wateravoiding (hydrophobic).
See the video at http://youtu.be/SwsCQtipAus
The detergent moved in to replace the proteins at the interface between the butterfat and the water. But the detergent also attached to the proteins at their water-loving and water- avoiding parts, and this changed the shapes of the proteins and changed how the proteins attached to one another.
All of this rearranging can take some time—up to several minutes—to complete. As the molecules rearranged, they pushed the water and the food coloring around, causing them to stir up into beautiful blossoms of color.