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Strawberries and Sugar
Name: Audrey M.
Status: student
Age: 40s
Location: N/A
Country: N/A
Date: 6/30/2004
Question:
Why do strawberries get juicy when you sprinkle with sugar?
Replies:
This is due to osmosis or the passage of water across a semipermeable
membrane to equalize the water on both sides. In other words, as my mother always
used to say "water seeks its own level". So if the water inside the
strawberry and outside the strawberry are equal, there will be no net movement of
water. But if there is more water on one side or the other, the water will move
to the side with the least water until there is equal water on both sides.
In the case of sugar (or salt), these molecules take up space. When sugar
molecules are present, there is not as much room for water. If you sprinkle
dry sugar on the outside of strawberries, there is no water on the outside.
The water inside the strawberry will begin to leave the cells until there is
equal water on both sides of the cell membrane. So the strawberry juice comes
out of the strawberry and mixes with the sugar and makes them "juicy".
Actually the strawberry should now be smaller and mushier than before because it
has lost some of its water.
Van Hoeck
Audrey,
The "juice" does not come out of nowhere -- most of it comes from inside the
strawberries. What is happening is an excellent example of osmosis -- a biological
process wherein atoms, ions, and/or molecules diffuse across semi-permeable membranes
in an attempt to reach an equal concentration on both sides of the membrane. In other
words, diffusion across the membrane barrier occurs so as to make the concentrated side
more dilute.
The sugar begins to dissolve in whatever water is on or near the surface of the
strawberries. This concentrated sugar solution is then diluted by moisture diffusing
out of the strawberries and into the sugar solution. Gradually, the strawberry tissue
is depleted of water as the solution outside becomes more dilute. Depending on the
amount of sugar added and the water-content of the strawberries, the result is a very
sweet juice (syrup) surrounding the pieces of strawberry.
If you look closely at the strawberries after they have rested in the solution, you will
note that the pieces have become rather shriveled and soft because their internal
moisture has been depleted. This is most apparent in store-bought containers of
pre-sweetened frozen strawberries. Their residence time in the syrup is so long that
the strawberries are almost transformed into a formless mush.
Regards,
ProfHoff 865
The dissolved sugar creates an osmotic pressure across the water permeable
membrane of the strawberry. This is not unique to strawberries, but is more
evident in cases where the fruit membranes allow the passage of water more
easily.
Vince Calder
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Update: June 2012
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