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So it's Thanksgiving here in the states and an odd combination of things are on my mind. In the past day, I've

  • Brined a turkey whole, skin on
  • Taken a long epsom salt bath

(Same thing, right? What a turkey! Haha.)

Anyways, it got me wondering, can a long salt water bath increase water retention? It's basically the same process as brining meat:

Salt is added to cold water in a container, where the meat is soaked usually six to twelve hours [making] cooked meat moister by hydrating the cells of its muscle tissue before cooking, via the process of osmosis

I'm sure we've all experienced dry, shriveled or even cracking skin (called xeroderma when chronic), which can apparently be improved by warm baths or showers. And here is where I'm in danger of leaving the bounds of rigorous science: can you cause or worsen water retention by soaking in a salt water bath? How much water can the skin take on? If you weighed yourself regularly and rigorously, would you be able to perceive a difference in weight before and after such a bath?

If it doesn't occur, what makes human skin different in this regard from poultry skin? Would brining not be effective if the body cavity of the bird weren't opened and vacated? (Brining and marinating of course works perfectly well on skinless cuts of meat.)

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Sorry for the dearth of tags; I'm surprised there isn't a tag for osmosis. –  Patrick M Nov 28 '13 at 18:37
    
but now there is! –  Emilio Pisanty Nov 28 '13 at 19:04
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I would naïvely expect soaking in salt water to dehydrate cells. In the case of brining it's only through the combination of high concentration of other non-permeating solutes, and then protein denaturalization, that humidity is increased. You would not want to sit in brine for long enough that the absorbed salt started to denature your proteins! (Not that it would - active mechanisms in your (live) cells would get rid of the excess salt before that.) However, the cell salt content could increase temporarily, providing inward osmosis. –  Emilio Pisanty Nov 28 '13 at 19:17
    
difference of man bathing to poultry is life, the total system is gone from the poultry. Actually I swim in the sea for 50 or 60 minutes in the summer and I have noticed the opposite of water retention: increased water expulsion (the normal way :) ) as if I had drunk two or three teacups tea. –  anna v Nov 28 '13 at 19:45
    
Of course, reading up on osmosis has made it clear that I had it backwards: water would tend to move from the less salty side to the more salty side. But that raises the question of whether my bath is saltier than the water in my cells... The bath certainly was less salty than the ocean. It also raises the question whether osmosis can occur when one side of the membrane (skin) is lacking any solute (as in the case of a regular bath). –  Patrick M Nov 28 '13 at 19:51

1 Answer 1

up vote 1 down vote accepted

solutes can also move across concentration gradients. that's why brining meat cannot be explained by osmosis alone (osmosis only involves solvent molecules)--there is diffusion of sodium ions into interstitial space.

as anna v alluded to, the difference between brining thanksgiving turkey and brining yourself is that it is a lot harder to perform the latter because you actually have a functioning renal system which dumps excess salt from your circulatory fluids into your bladder for later disposal. this is a diuretic process, i.e, it dehydrates you instead of making you more juicy. if i read right the paper i linked below, then the skin itself can function as a salt storage which gives some measure against your kidneys even having to deal with short exposure to salt water.

more on brining: net salt absorption from brines through your skin cannot be stopped because it is a diffusion process through the pores. what affects the rate is thickness of the skin, brine concentration, temperature, etc. (brining birds skin on is probably not as consequential as brining beef or pork skin on). the actual muscle cells have membranes ("lipid bilayer") which resist automatic ion diffusion and ion pumps to control ion movement in either direction. what brining actually does is increase the extracellular fluid volume--the amount of liquid trapped between cells rather than intracellular volume. the denaturing effect of salt on proteins undoubtedly increases the trapping effect even during the cooking process, leading to juicier meat. the idea of brining thus seems to be getting as much liquid in the meat before you cook it, and getting most of it to stay there after its done.

http://ndt.oxfordjournals.org/content/24/11/3282.full

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I meant to accept this a long time ago. Thank you, gregsan. –  Patrick M Jan 9 at 16:56

protected by Qmechanic Jan 5 at 12:08

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