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Salt and sugar are effective preservatives as they absorb water through osmosis, preventing microbial growth and reproduction. They may also disrupt enzymes and DNA structure. However, they can impart a noticeable taste and may not be effective against molds and yeasts. Food spoilage can be dangerous as microbial by-products can be toxic.
The microorganisms that cause food spoilage require water to survive and reproduce, so anything that dehydrates food or removes water from microbes will help prevent their growth. Both foods and microorganisms contain water and their water content can be reduced by bringing them into contact with soluble substances which absorb water through a phenomenon known as osmosis. Salt and sugar are common and relatively harmless substances that dissolve easily in water, and have therefore long been used to preserve various types of food. Food that has been treated with salt or sugar to preserve it will usually not have enough water available on the surface to allow any microbes that settle on it to multiply. Microbes can, in fact, die from dehydration, as water is removed from them.
Food spoilage
Food spoils when certain microorganisms begin to reproduce and digest its surface, producing a variety of by-products from their metabolic processes. This may or may not be easily detectable. Often the food will look moldy or mushy, or it may smell bad, but sometimes it’s not obvious and can therefore be more dangerous, as the microbial by-products can be very toxic and the microbes themselves can sometimes cause very serious infections. Therefore, to preserve food, it must be ensured that it is difficult or impossible for microorganisms to reproduce or survive in it. One of the oldest methods of making it is preservation with salt or sugar.
Osmosis
If strong and weak salt solutions are separated by a porous membrane, the concentrations will adjust until they are the same for both solutions: salt will move across the membrane from stronger to weaker solution and water will behave in the opposite way. Both food and microorganisms are made up of cells with membranes, so osmosis can operate through these membranes to extract water. 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, the water will flow out or into the cells.
For example, if a red blood cell is placed in pure water, it will swell and eventually explode. This is due to the cell absorbing water from its environment. When placed in water with a salt concentration equal to that of blood, red blood cells do not undergo osmosis and remain constant in size. When placed in highly salty water, the environment around the red blood cell will begin to suck the fluids into the cell through osmosis, causing it to shrivel up and die. When food is preserved with salt or sugar, microbes cannot absorb water from it through their membranes, preventing growth and reproduction, and they may lose water and die.
Other effects
These preservatives may have other useful effects. It is thought that at high concentrations inside cells they can undermine the structure of DNA and disturb the functioning of some enzymes. It is known that some of the relatively weak bonds that maintain the shape of enzyme molecules can be broken by salts in high concentrations. The skin is not affected by contact with salt and sugar solutions because it is covered in a layer of dead cells that protect living tissue from drying out.
Advantages and disadvantages
Salt and sugar are very effective against most bacteria that can cause disease. Generally, these microorganisms cannot tolerate a salt concentration higher than 10%. Molds, however, will often survive 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 the foods they have been used to preserve. This may be acceptable in some foods but not in others.
