Preservative 2

The reason food goes bad is bacteria, microscopically small living organisms. These putrefactive bacteria break down foods containing proteins (such as foods) into ammonia, hydrogen sulfide and numerous other, foul-smelling substances.
The bacteria that spoil our foods need water to stay alive. If these same bacteria come into contact with salt water, the salt removes water from the bacteria's cells, and they die. The salt also removes all moisture from the food. And without water, the bacteria cannot multiply. But this also kills off germs and pathogens. When we preserve vegetables in salt water (pickled cucumbers, olives etc.), this takes advantage of those germ-killing effects.


Sausage skin as a bacteria model

You'll need

  • animal intestine
  • 300g salt
  • 1 liter water
  • container

Experiment

  • Sausage skin made from animal intestine has the same characteristics as the protective layer of bacteria.
  • Ask your butcher for such natural sausage skin and fill it with water. The water won't run out, even though the skin is water-permeable, just like the outside of bacteria. Now firmly tie up the sausage skin.
  • Pour salt water (ca. 300g salt in 1 liter of water) into a glass container, then lay the water-filled sausage skin in the salt water. If you look carefully, you will see fine streaks, created by the water escaping through the sausage skin into the salt water.
  • After a few hours, the water will have almost completely escaped the sausage skin, with our model bacterium lying there limp in the salt water.

The reason is osmosis

Take a container and divide it with a solid barrier. If you now fill one side with pure water and the other with salt solution, as you might expect, nothing happens. If that barrier is removed, the pure water and the salt water mingle, without anyone having to stir the water. This process is known as diffusion, resulting from the motion of water particles. The warmer the water is, the faster the two will mix.
Now, if, instead of using a solid barrier, we use a material that is semipermeable (such as the natural sausage skin from before, the cell shell..), the water level in the half with the salt solution will begin to rise. The reason for this is that both fluids are trying their best to balance out the ion concentration, but the semipermeable membrane won't let the salt ions pass through. As a result, the water flows into the salt solution in order to dilute it. The interior of cells contains a relatively low-concentration salt solution. If you now lay this cell in salt water with a higher concentration, the water will be sucked out of the cell and therefore "dehydrated".