Control by Reduced Water Activity and Drying ◾ 473
are rehydrated before consumption. Microbial cells are exposed to two stresses—freezing and drying—that reduce some viability as well as inducing sublethal injury. During storage, especially at
a high storage temperature and in the presence of oxygen, cells die rapidly initially and then more
slowly. Spores are not affected by the process.
Foam Drying
The foam-drying method consists of whipping a product to produce stable foam to increase the
surface area. The foam is then dried by warm air. Liquid products, such as egg whites, 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.
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.
Intermediate Moisture Foods
Intermediate moisture foods (IMF) have A W values of approximately 0.70–0.90 (with moisture
contents of ca. 10%–40%). They can be eaten without rehydration but are shelf stable for a relatively long period of time without refrigeration and are considered microbiologically safe. Some
of the traditional IMFs include 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 or irradiated. 10,11
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 36.
are rehydrated before consumption. Microbial cells are exposed to two stresses—freezing and drying—that reduce some viability as well as inducing sublethal injury. During storage, especially at
a high storage temperature and in the presence of oxygen, cells die rapidly initially and then more
slowly. Spores are not affected by the process.
Foam Drying
The foam-drying method consists of whipping a product to produce stable foam to increase the
surface area. The foam is then dried by warm air. Liquid products, such as egg whites, 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.
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.
Intermediate Moisture Foods
Intermediate moisture foods (IMF) have A W values of approximately 0.70–0.90 (with moisture
contents of ca. 10%–40%). They can be eaten without rehydration but are shelf stable for a relatively long period of time without refrigeration and are considered microbiologically safe. Some
of the traditional IMFs include 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 or irradiated. 10,11
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 36.