4) DESSICATION:  It is effective for preventing microbial reproduction but don't kills the mos. Some mos required water for growth & bacteria will not generally grow below water activity below 0.9. Similarly, fungi will not grow at water activity below 0.05. So, maintenance of water activities below 0.65 prevents microbial growth.  Therefore, drying or dessication can be used  for food preservation. Eg In powder milk over 85% water is generally removed.

5) OSMOTIC PRESSURE:  The use of high concentrations of salts & sugars to preserve food is based on the effects of osmotic pressure . High concentrations of these substances create a hyper tonic environment that causes water to leave the microbial cell. This process resembles preservation by dessication; in both methods deny the cell moisture it needs growth. The principle of osmotic pressure is used in the preservation of foods. Eg; Concentrated salts solutions are used to cure meats & thick sugar solution are used to preserve fruits.
                  The molds & yeasts are much more capable than bacteria growing in materials with low moisture or high osmotic pressure. This property of molds, sometimes combined with their ability to grow under acidic condition is the reason fruits & grains are spoiled by molds rather than bacteria. It is also part of reason molds are able to form mildew on a damp wall or shower curtain.


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