Fundamental Food Microbiology, Third Edition - Fuad Fathir

476 FUNDAMENTAL FOOD MICROBIOLOGY
Since the beginning, reduced A w has been used throughout human civilization
in many ways not only to preserve foods and stabilize the food supply but also to
develop different types of shelf-stable foods. Some of these include salted fish and
meats; semidry and dry fermented sausages; dried fish, meat, vegetables, and fruits;
evaporated and sweetened condensed milk; and dry milk, cheeses, bread and bakery
products, flour, cereals, molasses, jams and jellies, chocolate, noodles, crackers,
dried potatoes, dried eggs, and confectioneries. In more recent years, new technologies
have helped produce foods with low A w by freeze-drying, puffed-drying, freezeconcentration,
and osmotic-concentration methods. In addition, a better understanding
of the relationship and influence of moisture and A w on microbial growth has
been instrumental in producing many types of intermediate-moisture ready-to-eat
foods. Efforts are also in progress to use low A w along with other microbial control
parameters, such as low pH, vacuum packaging, and low heat, to produce ready-toeat
meat, fish, and dairy products that can be stored at ambient temperature for
relatively long periods of time. Because of their convenience, these foods are popular;
the trend shows that the popularity will continue.
II. OBJECTIVES
The main objectives of reducing A w in food are to prevent or reduce the growth of
vegetative cells and germination and outgrowth of spores of microorganisms. 1–3
Prevention of toxin production by toxigenic molds and bacteria is also an important
consideration. Microbial cells (not spores) also suffer reversible injury and death in
foods with low A w, although not in a predictable manner as in heat treatment. Finally,
reduced A w is also used to retain viability of starter-culture bacteria for use in food
bioprocessing, which is discussed in Chapter 13. In this chapter, preservation of
food by controlling microbial growth at low A w is described.
III. MECHANISM OF ANTIMICROBIAL ACTION
Microorganisms need water for transport of nutrients, nutrient metabolism, and
removal of cellular wastes. In a food, the total water (moisture) is present as free
water and bound water; the latter remains bound to hydrophilic colloids and solutes
(it can also remain as capillary water or in a frozen state as ice crystals) and is not
available for biological functions. Thus, only the free water (which is related to A w)
is important for microbial growth. Microorganisms also retain a slightly lower A w
inside the cells than the external environment to maintain turgor pressure, and this
is important for cell growth. If the free water in the environment is reduced either
by removing water or by adding solutes and hydrophilic colloids, which cannot
readily enter the cells, the free water from the cells flows outside in an effort to
establish equilibrium. The loss of water causes an osmotic shock and plasmolysis,
during which the cells do not grow. The water loss can be quite considerable even
with a slight reduction in A w. A 0.005 reduction in A w from 0.955 to 0.950 in the
environment reduces the intracellular water content by 50% in Staphylococcus