While water is essential to the chemical reactions on which living things and many foods depend, it is also important for the life of microorganisms such as bacteria, molds and yeasts. The actions of these microorganisms on food cause deterioration and decay. Atmospheric humidity alone increases the likelihood of foods degenerating. For example, a relative humidity of 75 percent or more, especially if combined with warm temperatures, encourages the growth of microorganisms. Thus removing water from fruits, vegetables, meats, and herbs was among the earliest forms of food preservation. Without water, microorganisms cannot survive, so limiting the amount of water available to them inhibits their growth. Conversely, water in a cool environment helps preserve the freshness of fruits and vegetables by preventing dehydration, hence those artificial “rain” showers we see in supermarket produce displays.
Removing dirt and other debris from fruits and vegetables by rinsing them in water or even washing them with detergent eliminates many microorganisms. Detergents lower the surface tension of water, which improves its ability to act as a cleansing agent.
A food’s water activity or water availability determines its perish ability. Foods high in water, such as milk, meat, vegetables, and fruits, are much more prone to microbial spoilage than drier foods such as grains, nuts, dried milk, dried beans, or dried fruits. Moreover, once deterioration sets in, the putrefying food itself releases water, which fuels the further growth of microorganisms. Pure water has a water activity of 1.0. Adding any solute to it decreases its water activity to below 1.00. Water molecules orient themselves around any added solute, making them unavailable for microbial growth.
Solutes such as sugar and salt added to jams and cured meats inhibit microbial growth by lowering water activity. The food industry makes water unavailable to microorganisms by using solutes such as salt, sugars, glycerol, propylene glycol, and modified corn syrups.
Salting has been used as a method of preserving foods for thousands of years because salt draws water out of foods and to itself. Of course, ancient peoples did not understand the process of osmosis, which causes water to be drawn to solutes; all they knew was that salting kept their food edible for long periods of time. Water passes through membranes freely, but most solutes do not. The side of the membrane with more solute has more osmotic pressure and draws the necessary water to that side to dilute its solute concentration. Any bacteria contacting heavily salted food lose their water by the same process and die by dehydration. Beef jerky is the result of the combined processes of salting, smoking. and drying of meats. The high sugar concentration of jams and jellies acts to preserve them in the same way as salt on meats.