Field of Science

Relishing Osmosis

Tomorrow is a day for iconic cooking. Turkey. Stuffing. And of course, cranberry sauce. At a dinner a few years ago, a friend produced an odd silver implement and asked the gathered group of foodies just what we all thought it might be. Would you believe a jellied cranberry server - just the right size, she pointed out, to cut the canned jelly! Turns out that serving pieces for jellied sauces, like tomatoes and cranberries pre-date the Ocean Spray cans, but it was a fun puzzle regardless.

For me, the whole question of canned or homemade sauce is moot, since I prefer cranberry relish. I make it by running a bag of cranberries and a whole orange through the food processor, then adding sugar to taste. Since it's best made ahead, so the flavors can blend, I made a batch yesterday afternoon when I went home for lunch between office hours. Straight from the food processor the relish is whitish, dry and pretty bitter. Stir in the sugar and not only does it become sweet, but a ruby syrup begins to appear.

This is a (literally) beautiful example of osmosis in action. The high concentration of sugar outside the cell walls of the finely chopped orange and cranberry mixture encourages the water within the cells to pass through the cell membrane to bring the concentrations inside and out into equilibrium. The sugar and cellular contents are too big to cross the membrane, so the best the poor cells can do is to dump their water out creating that lovely syrup. The process intensifies the flavors of the berry and orange bits as well, since they are essentially "dried".

After a day of cooking and now kitchen scrubbing, my fingers are wrinkled. [This is an osmotic process as well, in this case, the water is crossing the cell membranes into my cells, causing the out layer of skin to get larger, and wrinkle.] Or perhaps, not! As David Bradley points out in the comments to this post, the wrinkling of the skin on your fingers after prolonged immersion in water is not particularly well understood. My reading of the literature suggests that osmosis plays at most a small role.


  1. As I understood it, skin wrinkling on prolonged exposure to water is not fully understood and is now known to be due at least partly to vasoconstriction which alters the underlying volume of the digit. I seem to remember from biology class, a long, long time ago, that the malpighian layer, the deepest layer of the dermis is somehow involved.

    But, more to the point you don't see wrinkling at points where the autonomic nervous system is interrupted, which implies that it is some kind of reflex reaction rather than a simple osmotic over-hydration of the skin.


  2. David,

    Thanks..your comment led me to much interesting reading. You are, of course, correct - wrinkling of the skin upon immersion appears to be incredibly complicated, and at most osmosis plays a liminal role.


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