The microorganisms that cause food to spoil require water to survive and reproduce, and so anything that dehydrates food or removes water from microbes will help to prevent them from growing. Food and microorganisms both contain water, and their water content can be reduced by bringing them into contact with soluble substances that absorb the water through a phenomenon known as osmosis. Salt and sugar are common, and relatively harmless, substances that dissolve easily in water, and so have been used for preserving various kinds of food for some time. Food that has been treated with salt or sugar to preserve it will not usually have enough water available at the surface to allow the multiplication of any microbes that land on it. The microbes may, in fact, die from dehydration, as water is removed from them.
Food is spoilt when certain microorganisms begin to reproduce on, and digest, its surface, producing a variety of byproducts from their metabolic processes. This may or may not be easily detectable. Often, the food will appear moldy or mushy, or it may smell bad, but sometimes it is not obvious, and may therefore be more dangerous, as microbial byproducts can be very toxic, and the microbes themselves can sometimes cause very serious infections. Therefore, to preserve food, it is necessary to ensure that it is difficult or impossible for microorganisms to reproduce or survive in it. One of the oldest methods of doing this is preservation using salt or sugar.
If strong and weak solutions of salt are separated by a porous membrane, the concentrations will adjust until they are the same for both solutions — the salt will move across the membrane from the stronger to the weaker solution, and water will behave in the opposite way. Food and microorganisms both consist of cells with membranes, and so osmosis can operate across these membranes to draw water out. Cells contain water with a certain amount of dissolved salts and sugars, but if they are placed in an environment with a very different concentration of dissolved substances, water will flow either out of or into the cells.
As an example, if a red blood cell is placed in pure water, it will swell and eventually burst. This is due to the cell absorbing water from its environment. When placed in water with a salt concentration equal to that of the blood, the red blood cell experiences no osmosis, and stays a constant size. When placed in highly salty water, the environment around the red blood cell will start sucking out the fluids inside the cell through osmosis, causing it to shrivel and die. When food is preserved with salt or sugar, microbes cannot absorb water from it through their membranes, preventing growth and reproduction, and may lose water and die.
These preservatives may have some other useful effects. It is thought that at high concentrations inside cells they can undermine the structure of DNA and disrupt the functioning of some enzymes. It is known that some of the relatively weak bonds that maintain the shapes of enzyme molecules can be broken by salts at high concentrations. Skin is unaffected by contact with salt and sugar solutions because it is covered with a layer of dead cells that protect living tissue from dehydration.
Advantages and Disadvantages
Salt and sugar are very effective against most of the bacteria that can cause disease. Generally, these microorganisms cannot tolerate a salt concentration of more than 10%. Molds, however will often survive at much higher concentrations, and yeasts may be able to grow in highly concentrated sugar solutions. The use of salt and sugar is also limited by the fact that they impart a very noticeable taste to foods they have been used to preserve. This may be acceptable in some foods, but not in others.