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Chapter 23 Separation Techniques: Mechanical and Chemical Methods
water but not sugar to pass back and forth. Because the membrane
is semipermeable, the two solutions and the membrane are a
connected system.
The different concentrations of the two solutions cause the system
to be unstable. A pressure builds up on the side of the membrane
with the lower concentration. The pressure caused by two solutions
with different concentrations separated by a semipermeable membrane
is called osmotic pressure. This pressure will force water through
the membrane. Water will move from the solution with a low sugar
concentration toward the solution with a high sugar concentration.
Water will continue to move until osmotic pressure drops and the
mass percent of the two solutions is the same.
Osmosis in Food Products
Foods are composed of plant or animal tissue. The cells in this
tissue are surrounded by semipermeable walls or membranes. This
makes these tissues subject to the effects of osmosis after harvest
or slaughter. Therefore, food scientists need to understand how to
control osmosis in food mixtures.
To observe the osmotic movement of water through plant cell
walls, slice and salt an eggplant. Wait 30 minutes before examining
the salted eggplant. Water droplets will have formed on the surface.
The salt has pulled water out of the cells of the eggplant to equalize
salt concentrations.
Salting eggplant is the fi rst step in making eggplant parmesan.
Osmosis reduces the water content in the cells. This makes the
eggplant easier to fry and gives it a milder fl avor. See 23-8.
Similarly, a fi rst step in making sauerkraut is to put shredded
cabbage in brine. The salt in the brine draws water out of the cabbage
cells through the semipermeable cell walls. Cell walls
are complex systems that use protein molecules to
help transport solutes into and out of cells. The cell
walls allow sugar to be pulled out of the cabbage cells
with the water. This sugar serves as food for the bacteria
that ferment the cabbage and turn it into sauerkraut.
Reverse Osmosis
Pressure can be applied to the more highly
concentrated of two solutions separated by a
semipermeable membrane. This will cause water
to fl ow from an area of high concentration to an area
of low concentration. This type of movement is in the
opposite direction of osmosis. That is why it is called
reverse osmosis. Reverse osmosis involves pumping
liquids under pressure against a membrane. This
process will cause a concentrated solution to become
even more concentrated.
Janet Ward
23-8 Sprinkling salt on sliced eggplant pulls
water and bitter fl avor compounds from the slices.