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

Tions, induces the formation or toxic free radical

atmosphere. Such spores generally have thick walls inereoses microbial death rates, and is used as

that retain moisture within the cell. Many fungal sterilization method.

spores can be transmitted over long distances

through the atmosphere; some spores even travel Sensitivities to ionizing radiation vary. Nonrq

from one continent to another. The transmission of ducing (dormant) stages of microorganisms tem

fungal spores through the air is a serious problem in be more resistant to radiation than growing on;

agriculture because it permits the spread of fungal isms. For example, endospores are more resis

diseases of plants from one field to another. than the vegetative cells of many bacterial spei

In the 1690s, witch hunts were a regular practice in Salem and surrounding regions of New England. People accused of being witches were burned at the stake. In most cases, these people were accused of casting spells or performing supernatural acts. Re­cent scientific evidence suggests that the accusors were suffering from ergotism, a form of food poisoning that is characterized by hallucinations. The dis­ease occurs when fungi grow on grains and produce chemicals called ergot alkaloids. Apparently, excessive rain wet the grains and without other means of food preservation, the fungi were able to grow on them. The spoilage of grains was widespread, which explains the occurrence of mass hallucinations. The accusors were hallucinating, which is why they thought they saw witches.

FIG. 11 -7 Ionizing radiation effectively kills microorganisms, including viruses.

318 CHAPTER 11 CONTROL OF MICROBIAL GROWTH AND DEATH Г

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 disrupts phase. This process generally eliminates damage to DNA molecules. Exposure to such radiation may microbial cells from the expansion of ice crystals. cause mutations, many of which are lethal. Exposure

to gamma radiation (short wavelengths of 10 ³ to

Lack of water prevents microbial growth but does Ю"¹ nanometers), X-radiation (wavelengths of 10 ³

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

possible preservation by drying. violet light with wavelengths of 100 to 400 nanome­ ters) increases the death rate of microorganisms and

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

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

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 (ions),