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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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