Fundamental Food Microbiology, Third Edition - Fuad Fathir

CONTROL BY REDUCED A W
D. Foam-Drying
The foam-drying method consists of whipping a product to produce a stable foam
to increase the surface area. The foam is then dried by warm air. Liquid products,
such as egg white, fruit purees, and tomato paste, are dried in this manner. The
method itself has very little lethal effect on microbial cells and spores. However, a
concentration method before foaming, the pH of the products, and low A w cause
both lethal and reversible damages to microbial cells.
E. Smoking
Many meat and fish products are exposed to low heat and smoke for cooking and
depositing smoke on the surface at the same time. The heating process removes
water from the products, thereby lowering their A w. Many low-heat-processed meat
products (dry and semidry sausages) and smoked fishes are produced this way. Heat
kills many microorganisms. The growth of the survivors is controlled by low A w as
well as the many types of antimicrobial substances present in the smoke.
F. Intermediate Moisture Foods
Intermediate moisture foods (IMF) that have A w values of ca. 0.70 to 0.90 (with
moisture contents of ca. 10 to 40%). They can be eaten without rehydration, but are
shelf-stable for a relatively long period of time without refrigeration and considered
microbiologically safe. Some of the traditional IMFs includes salami, liverwurst,
semidry and dry sausages, dried fruits, jams and jellies, and honey. However, in
recent years, many other products have been developed, such as pop tarts, slim jims,
ready-to-spread frosting, breakfast squares, soft candies, fruit rolls, food sticks, and
soft granola bars. The low A w value and relatively high moisture is obtained by
adding water-binding solutes and hydrophilic colloids. Microorganisms can survive
in the products, but because of low A w, bacteria cannot grow. However, yeasts and
molds can grow in some. To inhibit their growth, specific preservatives, such as
sorbate and propionate, are added. 4,5
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VI. CONCLUSION
The A w of a food is directly proportional to the level of free water it contains. As
microbial growth is adversely affected even with a slight reduction in A w, it has been
used, where possible, to retard and prevent growth mainly of spoilage bacteria in
food. It is achieved either by removing or binding the free water. At lower A w,
microbial cells can lose viability and become injured with time. However, this alone
cannot be used to ensure safety of food from pathogens. Although as a single method
it has limited application in food preservation, along with other methods, such as
low temperature or low pH, it can be used to extend the shelf life of food very
effectively. The effectiveness of low pH, either alone or with other methods, in
controlling microorganisms in food is discussed in Chapter 35.
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