What’s the Difference Between Osmosis and Diffusion? Ask Paul

Like many people, back in junior high school biology, I did a project about osmosis in living cells, and never intended to think about it again. But no sooner do I get into cooking than it turns out that osmosis, and its cousin diffusion, are unexpectedly relevant.

Both osmosis and diffusion are forms of passive transport, meaning that they move stuff (like water or salt) from inside an organism’s cell to outside the cell, or from outside to inside, without the organism having to actively make it happen. That passivity comes in handy in the kitchen, because the meats and vegetables we work with are usually no longer part of a living organism, and hence not in a condition to do much of anything actively.

Osmosis and diffusion both happen when there’s more of a substance in one place, and less of it in an adjacent place. The movement is caused by the natural tendency of things to spread out evenly until the concentrations are equalized. And the greater the difference in concentrations, the faster the movement.

One of the most common ways we take advantage of passive transport in cooking is when we salt meat. Sprinkle salt on the surface of a steak and watch closely: Water emerges from the meat in little beads and pools on the surface. Osmosis in action! Where salt touched the surface and dissolved, it created a highly concentrated salt solution, much saltier than the steak’s interior. Water flows out of the meat’s muscle cells to dilute the solution and rectify that imbalance.

Any time we pull water out of cells by adding salt (or sugar, which works in the same way), we’re harnessing osmosis. Add enough salt, and we can draw out almost all the water, as in dry-as-a-bone salt cod. And the phenomenon can happen in liquid, not just in air, as in firmed-up egg yolks that have been soaked in salty soy sauce, which partially dries them out even though they’ve been submerged in liquid the whole time.

If you want to see osmosis working in both directions, make salmon caviar. It’s simple: Put raw salmon eggs in a seasoned brine. The eggs (which are in fact single cells) are very sensitive to the salt concentration of the brine: Make it overly salty and they shrivel up before your eyes; dilute the brine with extra water and they plump up. Put the eggs in fresh, clean water and some of them will swell up so much that they burst!

While osmosis refers to water moving toward a region where there’s a high concentration of something it can dilute, diffusion is when the soluble substance, salt for instance, moves from an area of high concentration to an area of lower concentration.

This is what’s happening when we drop a raw chicken in a bath of salt water, wait a few hours, and pull out a delicious, ready-to-grill, salt-infused bird.

The brine is full of sodium and chlorine ions (also known as dissolved salt)—much more of those ions than are in the chicken. The salt ions move from the area of more concentration (the brine) to the area of less concentration (inside the chicken).

Most molecules, even when dissolved in water, are too large to cross cell membranes, but salt does it nicely, diffusing into the meat.

If we leave the meat in the brine long enough—say, two weeks—diffusion will distribute the salt evenly throughout the whole bird, so the chicken and the brine will be equally salty. In most cases, we don’t want to wait weeks, so we make a brine that’s heavily salted, and pull the chicken out when just a small amount of salt has permeated in.

That’s diffusion across cell membranes, but the term diffusion is also used to describe how many other types of things spread out from where there’s more of them to where there’s less. When a droplet of red food coloring spreads gradually through a glass of water until all of the water is equally pastel pink, that’s diffusion; likewise when you bake a pie and the neighbor kid comes knocking, the aroma molecules have diffused out from the oven, to the kitchen, through the house, and out the window. Even heat moves by diffusion, from the hottest area to the coldest. That’s why piping-hot food cools off faster than merely warm food: The greater the difference in temperature between the food and its surroundings, the faster the movement of heat from one area to the other.