1. Precipitation of protein with an acid (vinegar)
Procedure:
1. Weigh and record the weight of 120 mL of milk in the beaker.
2. Place the beaker with the milk on a hot plate. Heat the milk to 21oC. Turn off
the hot plate and remove the beaker.
3. Add 11 mL of vinegar to the warm milk and stir for 2 minutes, then allow the milk to sit
for 5 minutes. The casein will precipitate into heavy white curds.
4. Cut out a piece (2−3 layers) of cheesecloth large enough to cover the top and 2 inches
down the sides of a beaker. Using the rubber band, fasten the cheesecloth over the
top of the beaker. Pour the curdled milk into the beaker, collecting the curds (casein)
in the cheesecloth and allowing the vinegar and whey to drain off into the bottom of
the beaker.
5. Gather up the cheesecloth with the casein and rinse in cool water by dipping into
another beaker containing water.
6. Squeeze the casein until almost dry, then spread out the cheesecloth to let the casein
dry for 5 minutes.
7. Weigh the precipitate. (Do not weigh the cheesecloth with the precipitate).
2. Precipitation of protein with rennet
Procedure:
1. Weigh and record the weight of 120 mL of milk in the beaker.
2. Place the beaker with the milk on a hot plate. Heat the milk to 43o
C. Pour
the hot milk over the rennet powder (1 g), stir for 2 minutes, and allow the milk to sit
for 5 minutes.
3. Follow step 4-7 in experiment 1.
3. Precipitation of protein with salt
Procedure:
1. Weigh and record the weight of 120 mL of milk / soymilk in the beaker.
2. Place the beaker with the milk on a hot plate. Heat the milk to boil and turn off the
heat. Monitor this step closely.
3. Add 1.6 grams of Epsom salt (magnesium sulfate, a mineral salt) to the hot milk and stir.
4. Wait until the curds are floating in an almost clear liquid.
5. Fasten a piece (2−3 layers) of cheesecloth over the top of a beaker with a rubber
band. Pour the curds and liquid into the beaker, collecting the curds in the
cheesecloth and allowing the liquid to drain into the bottom of the beaker.
7. Gather up the cheesecloth and squeeze out as much water as possible. Spread out the
cheesecloth to allow the curds to dry for 5 minutes.
8. Weigh the curds and record the results