Because water is required for microbial growth, many foods can be preserved by desiccation.

Although lack of available water prevents microbial growth, it does not necessarily accelerate the death rate of microorganisms. Some microorganisms, therefore, can be preserved by drying. Active dried yeast is used for baking purposes. After the addition of water, the yeasts begin to carry out active metabolism. Freeze-drying (lyophilization) is a common means of removing water that can be used for pre­serving microbial cultures (FIG. 11-6). During freeze-

FIG. 11 -6 Lyophilization, or freeze-drying, is used to preserve microbial cultures. The in­strument used for this process uses a high vacuum and low temperature so that water sub­limes (goes from the solid frozen state directly to a gas). This removes water from the spec­imen without disrupting cellular structures, allowing viability to be maintained.

318 CHAPTER 11 CONTROL OF MICROBIAL GROWTH AND DEATH

Radiation

drying, water is removed by sublimation, that is, wa
ter is converted directly from the solid to the gas High-energy, short-wavelength radiation disr
phase. This process generally eliminates damage to DNA molecules. Exposure to such radiation
microbial cells from the expansion of ice crystals. cause mutations, many of which are lethal. Expo

to gamma radiation (short wavelengths of 10

Lack of water prevents microbial growth but does 10 ^{- 1} nanometers), X-radiation (wavelengths of

not accelerate the microbial death rate, making to 10 ² nanometers), and ultraviolet radiation (u

possible preservation by drying. violet light with wavelengths of 100 to 400 nano

ters) increases the death rate of microorganisms

Whereas some microorganisms are relatively resis- is used in various sterilization procedures to kill

tant to drying, other microorganisms are unable to croorganisms. Gamma and X-radiation have 1

survive desiccating conditions for even a short pe penetrating power and are able to kill microorgan

riod of time. The ability to withstand drying can have isms by inducing or forming toxic free radicals (it

important pathogenic implications. Mycobacterium Therefore gamma and X-radiations are referred t

tuberculosis is a classic example of an organism capa- ionizing radiations. Free radicals are highly reac

ble of withstanding severe desiccation and still re- chemical species that can lead to polymerization

maining infective. In contrast, Treponema pallidum, the other chemical reactions disruptive to tFte biochi

bacterium that causes syphilis, is extremely sensitive cal organization of microorganisms. Viruses

to drying and dies almost instantly in the air or on a other microorganisms are inactivated by exposui

dry surface. ionizing radiation (FIG. 11-7).

Some microorganisms produce specialized spores
that can withstand the desiccating conditions of the Exposure to radiation may cause microbial mute

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