Osmotic Pressure
Mode of Action:
The use of high concentrations of salts and sugars to preserve food is based on the effects of osmotic pressure. High concentrations of these substances create a hypertonic environment that causes water to leave the microbial cell, this effect is also called as plasmolysis. Loss of water severely interferes with cell function and eventually leads to cell death. This process resembles preservation by desiccation, in that both methods deny the cell the moisture it needs for growth.
Application of Osmotic Pressure:
The use of sugar jellies, jams, and syrups or salts solution in curing meat and making pickles plasmolysis most organisms present and prevents growth of new organisms. A few halophilic organisms, however, thrive in these conditions and cause spoilage, especially of pickles, and some fungi can live on the surface of jams.
Limitations:
As a general rule, molds and yeasts are much more capable than bacteria of growing in materials with low moisture or high osmotic pressures. This property of molds, sometimes combine with their ability to grow under acidic conditions, is the reason fruits and grains are spoiled by molds rather than by bacteria. It is also part of the reason molds are able to form mildew on a damp wall or a shower curtain.
High Pressure
Mode of Action:
High pressure applied to liquid suspensions in transferred instantly and evenly throughout the same. If the pressure is high enough, the molecular structures of proteins and carbohydrates are altered, resulting in rapid inactivation of vegetative bacterial cells.
Limitation and Solutions:
Endospores are relatively resistant to high pressure. They can however, be killed by other techniques, such as combining high pressure with elevated temperatures or by alternating pressure cycles that cause spore germination, followed by pressure-caused death of resulting vegetative cells. Fruit juices preserved by high pressure treatments have been marketed in Japan the United States. An advantage is that these treatments preserve the flavors, colors, nutrient values of the products.