Pasteurization

Pasteurization is a process that uses relatively brie exposures to moderately high temperatures to reduce the numbers of viable microorganisms and tieliminate human pathogens (FIG. 11-2). Such procedures prolong shelf life and ensure safety of the foocas long as human pathogens are eliminated. A pateurized food, however, retains viable microorganisms, which means that additional preservation methods are needed to extend the shelf life of the product. These other preservation methods, such a refrigeration, are used to reduce the growth rates a the surviving microorganisms. Pasteurization ol milk, for example, is required by law in the U.S. ti eliminate pathogenic bacteria, namely Brucella sp Coxiella burnetii, and Mycobacterium tuberculosa. These bacteria are associated with the transmission of disease via contaminated milk. They are relativeli sensitive to elevated temperatures, and pasteurizaj

CONTROL OF MICROORGANISMS BY PHYSICAL ENVIRONMENTAL FACTORS 313

FIG. 11-2 Pasteurization reduces the numbers of viable microorganisms and eliminates human pathogens. The same reduction of viable microorganisms is achieved by heating to 63° С for 30 minutes as is accomplished by ex­posure to 71.5° С for only 0.33 minutes. The long time hold (LTH) pasteurization process uses 63° С and the high tem­perature-short time (HTST) pasteurization process uses 71.5° С

tion is, therefore, normally achieved by exposure of milk to 62.8° С for 30 minutes (low temperature-hold or LTH process) or 71.7° С for 15 seconds (high tem­perature-short time or HTST process). Milk pro­duced in the United States is normally preserved by pasteurization and requires refrigeration to extend its shelf life.

Pasteurization uses brief exposures to moderately high temperatures to reduce numbers of viable mi­croorganisms and eliminate human pathogens but does not eliminate all viable microorganisms.

Sterilization

Sterilization is the complete elimination of living or­ganisms. In many microbiological procedures, high temperatures are used to kill all viable microorgan­isms, that is, to sterilize materials. As long as there are no endospore-forming bacteria, boiling at 100° С for 10 minutes is adequate to eliminate microorganisms from water. When the potential for the contamination of water supplies with enteric pathogens exists, boiling ensures the bacteriological safety of the water. In many cases, higher temperatures are needed to ensure that all microorganisms, including endospore producers, are killed.

Sterilization eliminates all living organisms.

Excessive heat destroys the quality of milk, for ex­ample, boiling alters the smell and taste of milk, but brief exposure to high temperatures kills all the mi-

croorganisms in milk without destroying its quality. In the ultra high temperature or UHT process, exposure to 141° С for 2 seconds is used to sterilize milk. Sterilized milk is currently marketed in several European countries and has been introduced in the United States. It has an indefinite shelf life, provided the container remains sealed.

The heat used in sterilization can be moist or dry heat. Dry heat kills by oxidation. This is, for example, what happens when paper slowly chars in a heated oven when the temperature is below the ignition point of paper. Moist heat kills microorganisms more quickly because the water hastens the breaking of hydrogen bonds that hold proteins in their three-dimensional structure.

Dry heat sterilization requires higher tempera­tures for much longer exposure periods to kill all the microorganisms in a sample. Exposure in an oven for 2 hours at 170° С (328° F) is generally used for the dry heat sterilization of glassware and other items. The longer period of time and higher temperature (relative to moist heat) are required because heat in water is more readily transferred to a cool body than heat in air. Heat sterilization is very important in medical microbiology where sterile instruments are required for surgical procedures and where contaminated materials must be sterilized before they can be reused or safely discarded.

We routinely sterilize transfer loops by flaming them red hot before aseptically transferring a culture from one site to another. In this case we use a very high temperature for a short time. A similar principle is used in incineration, an effective way to sterilize and dispose of contaminated paper cups, bags, and dressings. Medical wastes are safely disposed by using incineration to kill any viable microorganisms.

Dry heat sterilization uses higher temperatures and often takes longer than moist heat sterilization.

Moist heat is far more penetrating than dry heat and, hence, more effective for killing microorganisms. Steam under pressure is frequently used in sterilization procedures. Steam at 121° С for 15 minutes is at least as effective as a dry oven treatment at 170° С for 2 hours and kills all microorganisms, including endospores (FIG. 11-3). Culture media in bacteriological laboratories are normally prepared by heat sterilization in an autoclave, an instrument that permits exposure to steam under pressure. The autoclave is basically a chamber that can withstand pressures of greater than two atmospheres. The materials to be sterilized are placed in a chamber, and the chamber is sealed. Steam then is transferred from a jacket into the chamber, forcing out all the air. The steam is held in the chamber for the necessary time and then vented from the chamber. In the normal heat steril-

314 CHAPTER 11 CONTROL OF MICROBIAL GROWTH AND DEATH

FIG. 11-3 An autoclave is used to sterilize materials by exposure to steam under pressure.

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