Microcheck

Filters can be used to remove microorganisms and viruses from liquids and air.

■ What is the difference between the mechanism of a depth filter and that of a membrane filter?

■ Describe two uses of HEPA filters.

■ How could too much pressure overcome the filtering action of a depth filter?

5.6 Using Radiation to Destroy Microorganisms and Viruses

Electromagnetic radiation can be thought of as waves having energy but no mass. Examples include X rays, gamma rays, and ultraviolet and visible light rays (figure 5.8) The energy that electromagnetic rays possess is proportional to the frequency of the radiation (waves per second). Short-wavelength electromagnetic radiation, such as gamma rays, has much more killing power than long-wavelength electromagnetic radiation, such as visible light. Irradiation with gamma rays and ultraviolet light are valuable tools for microbial control. Microwaves do not kill microorganisms directly, but by the heat they generate.

Gamma Irradiation

Gamma rays are an example of ionizing radiation that causes biological damage by producing reactive molecules such as superoxide (O2:) and hydroxyl free radicals (OH*) when the rays transfer their energy to a microorganism. Gamma radiation from the radioisotope cobalt-60 is effective for controlling most microorganisms. Bacterial endospores are among the most radiation-resistant microbial forms, whereas the Gram-negative bacteria such as Salmonella and Pseudomonas are among the most susceptible. Radiation is used extensively to sterilize heat-sensitive materials including medical equipment, disposable items, and drugs such as penicillin. It provides an alternative for ethylene oxide sterilization and can even be carried out after packaging. ■ superoxide, p. 89

Foods can be either sterilized or pasteurized with radiation, depending on the doses employed. Treatments designed to sterilize food can cause undesirable flavor changes, however, which limits their usefulness. More commonly, food is irradiated as a method of pasteurization, eliminating pathogens and decreasing the numbers of spoilage organisms. For example, it can be used to kill pathogens such as Salmonella species in poultry with little or no change in taste of the product.

In the United States, irradiation has been used for many years to control microorganisms on spices and herbs. The Food and Drug Administration (FDA) has also approved irradiation of fruits, vegetables, and grains to control insects; pork to control the trichina parasite; and most recently, meats including poultry, beef, lamb, and pork to control pathogens such as Salmonella species and E. coli O157:H7.

Many consumers have been reluctant to accept irradiated products, even though the FDA and officials of the World

Energy

Gamma rays J_I_I

Microwaves Radar waves

Television waves Radio waves

Gamma rays J_I_I

Microwaves Radar waves

Television waves Radio waves

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Figure 5.8 The Electromagnetic Spectrum Visible wavelengths include the colors of the rainbow.

Ultraviolet nfrared

5.7 Preservation of Perishable Products 123

Health and the United Nations Food and Agriculture Organizations have endorsed the technique. Some people erroneously believe that irradiated products are radioactive, which they are not. Others have lingering doubts about the possibility of irradiation-induced toxins or carcinogens being present in food, even though available scientific evidence indicates that consumption of irradiated food is safe. Another argument raised against irradiation is that it will cause a relaxation of other prudent food-handling practices. Irradiation, however, is intended to complement, not replace, proper food-handling procedures by producers, processors, and consumers.

Ultraviolet Irradiation

Ultraviolet (UV) light in wavelengths of 220 to 300 nm kills microorganisms by damaging their DNA. The absorption of these wavelengths causes covalent bonds to form between adjacent thymine molecules in the DNA, creating thymine dimers. Actively multiplying organisms are the most easily killed; bacterial endospores are the most resistant. ■ thymine dimer, p. 195

Ultraviolet rays penetrate very poorly. A thin film of grease on the UV bulb or extraneous materials covering microorganisms may markedly reduce effective microbial killing. Most types of glass and plastic also screen out ultraviolet radiation, and so UV light is most effective when used at close range against exposed microorganisms in air or on clean surfaces. Caution should be used, however, because UV rays can also damage the skin and eyes and promote the development of skin cancers.

Microwaves

Microwaves do not affect microorganisms directly, but they can kill microorganisms by the heat they generate in an item. Organisms often survive microwave cooking, however, because the food heats unevenly.

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