Osmotic pressure is the pressure that must be applied to stop the net flow of solvent across a semipermeable membrane from the dilute side to the concentrated side. In food science it is the working principle behind some of the oldest preservation methods: surrounding food with sugar or salt creates a hypertonic environment that draws water out of microbial cells, dehydrating and inhibiting them. This is why jams, cured hams, salted fish, and brined vegetables keep. Osmosis also governs processes like reverse-osmosis concentration of fruit juices and whey, osmotic dehydration of fruit pieces, and the behaviour of dried foods on rehydration. In the body, osmotic pressure regulates fluid balance between blood and tissues, and oral rehydration salts exploit coupled solute transport to pull water into dehydrated patients.