Separation of Milk Particles
Goal: To demonstrate that milk is made of particles suspended in water
Items Needed:
skim milk
vinegar or citric acid
coffee filter
funnel
hot plate (optional)
What to do:
1. Pour out about 50 mL of skim milk into a beaker.
2. Add about 2 grams of citric acid or 10 mL of vinegar to the milk and stir.
3. Filter the mixture through the coffee filter.
Or with the hot plate 1. Pour out about 50 mL of skim milk into a beaker.
2. Add about 2 grams of citric acid or 10 mL of vinegar to the milk and stir.
3. Heat milk on the hot plate until it is warm for 3 minutes - do not let it boil!
4. Filter the mixture through the coffee filter.
What should be observed:
With the addition of the citric acid or vinegar, you should observe small white particles in the
milk. These are large enough to filter out. With heat, the particles should be larger and easier to
filter.
What is happening:
The added citric acid (or almost any acid, like acetic acid found in vinegar) acts the same way as
when bacteria convert the lactose to lactic acid in the cultured dairy products. Lactic acid, or in
this case citric acid, promotes coagulation of the casein particles. These are the visible white
clumps that can be filtered out. The addition of heat actually helps promote polymerization of the
protein. The differences between the heated and the unheated particles should be larger particles
for the heated milk mixture.
Other things to try: This experiment can also be performed with whole milk. The clumps in this
case may be slightly bigger than with skim milk. More noticeably, though, the whole-milk
particles will feel smoother and creamier, resembling something like softened cream cheese. This
is due to the higher fat content in the whole milk.
Determination of Fat Content in Milk
Goal: To demonstrate that the amount/concentration of fat in milk can be determined
through means other than taste.
Items Needed:





whole milk
skim milk
half-and-half or cream
three shallow pans or bowls
food coloring
What to do:
Pour a small amount of each kind of milk into separate bowls. When the milk is steady and not
moving, add one drop of food coloring to each bowl - the bowl and milk must be perfectly still.
Watch how the color spreads.
What should be observed:
The food coloring in the skim milk should spread quickly and become faint in color. The
coloring in the cream will not spread as much or as fast as in the skim milk. The whole milk
should behave somewhere in between.
What is happening:
The food coloring is water-based and will travel and diffuse better through the aqueous (water)
medium than through the fat. So in skim milk, which has a very low fat content and is
predominantly water, the food coloring spreads rapidly, whereas in cream the food coloring will
take longer to disperse since there is less water for it to travel through.
Other things to try: After the food coloring is added, dab a cotton swab with soap or detergent
and dab the cotton swab in the center of the food coloring. This should cause the food coloring to
disperse rapidly into the milk. The soap is a surfactant that changes the water/fat interactions and
allows for rapid diffusion of the food coloring into the milk.