.Coatroll Table 7.3 D Values and z Values for Some Food-Borne Pathogens Organism Substrate D Value (C)in Minutes 2Value(C) ) 99 05%Nacl 0=2025 1079.A %g (b) (a)Am dem.au sy o
Prescott−Harley−Klein: Microbiology, Fifth Edition II. Microbial Nutrition, Growth, and Control 7. Control of Microorganisms by Physical and Chemical Agents © The McGraw−Hill Companies, 2002 7.4 The Use of Physical Methods in Control 141 Table 7.3 D Values and z Values for Some Food-Borne Pathogens Organism Substrate D Value (°C) in Minutes z Value (°C) Clostridium botulinum Phosphate buffer D121 = 0.204 10 Clostridium perfringens Culture media D90 = 3–5 6–8 (heat-resistant strain) Salmonella Chicken à la king D60 = 0.39–0.40 4.9–5.1 Staphylococcus aureus Chicken à la king D60 = 5.17–5.37 5.2–5.8 Turkey stuffing D60 = 15.4 6.8 0.5% NaCl D60 = 2.0–2.5 5.6 Values taken from F. L. Bryan, 1979, “Processes That Affect Survival and Growth of Microorganisms,” Time-Temperature Control of Foodborne Pathogens, 1979. Atlanta: Centers for Disease Control and Prevention, Atlanta, GA. Recorder Pressure regulator Safety valve Exhaust to atmosphere Steam from jacket to chamber or exhaust from chamber Baffle Steam jacket Steam supply Condensate to waste Steam trap Trap Temperature sensing bulb Steam supply valve Discharge Door gasket Control handle Steam to jacket Steam from jacket to chamber Strainer Jacket condensate return (a) (b) Figure 7.3 The Autoclave or Steam Sterilizer. (a) A modern, automatically controlled autoclave or sterilizer. (b) Longitudinal cross section of a typical autoclave showing some of its parts and the pathway of steam. (b) From John J. Perkins, Principles and Methods of Sterilization in Health Science, 2nd edition, 1969. Courtesy of Charles C. Thomas, Publisher, Springfield, Illinois
7.Control of 42 Chapter 7 Control of Microorganisms by Physical and Chemical Age e of 15 pounds.The chamber should not be packed to definite advantages.Dry heat does not Bacterial endospores vill be terilize powders ils,andsimilar item Most laboratories ste 12 ize glas es and pipettes with dry heat.D will be ne because it will take longer for the ce of th sensitive matera like many plastic and rubber items. Low Temperatures com e 679. era days.If This 0.F 2Cann6od has and the en spe with the appears tape ting microbes.n h approachesare co and many labor as the of ba age at 00r use fro milk ed with ptly after thawing in order to avoid s wboiling. a pn :In the pas Frenc wine industry was pl d by the but do ur examine for actic at tem of ch dro that a b ef heating ater is present hnique onl ng p ods.In 1886 the Ger man ed inte Filtration and many Filtation isan way toreduce the microbial population in milk is held at 63C for 30 minutes.Large quantities of milk ar that have bee bonded intoa thick layer twisting iohigh-mr for 15 seconds. then rapid cooli The ed by ysical screening or entrap ent and also by high-t on to the face o Depth 50C for 1 to 3 seconds.UHT-pro sed milk lain (Chamberain ilters).asbestos,or other si ters have rep lace often are prepared using UHT ster- .mm thick.made cellulos acetate,cellulose nitrate,poly objectsare best the absence of water by dry heat ed are placed in a with pore in diameter are used toremove most veg The mem ve than mos d by depth
Prescott−Harley−Klein: Microbiology, Fifth Edition II. Microbial Nutrition, Growth, and Control 7. Control of Microorganisms by Physical and Chemical Agents © The McGraw−Hill Companies, 2002 pressure of 15 pounds. The chamber should not be packed too tightly because the steam needs to circulate freely and contact everything in the autoclave. Bacterial endospores will be killed only if they are kept at 121°C for 10 to 12 minutes. When a large volume of liquid must be sterilized, an extended sterilization time will be needed because it will take longer for the center of the liquid to reach 121°C; 5 liters of liquid may require about 70 minutes. In view of these potential difficulties, a biological indicator is often autoclaved along with other material. This indicator commonly consists of a culture tube containing a sterile ampule of medium and a paper strip covered with spores of Bacillus stearothermophilus or Clostridium PA3679. After autoclaving, the ampule is aseptically broken and the culture incubated for several days. If the test bacterium does not grow in the medium, the sterilization run has been successful. Sometimes either special tape that spells out the word sterile or a paper indicator strip that changes color upon sufficient heating is autoclaved with a load of material. If the word appears on the tape or if the color changes after autoclaving, the material is supposed to be sterile. These approaches are convenient and save time but are not as reliable as the use of bacterial endospores. Many substances, such as milk, are treated with controlled heating at temperatures well below boiling, a process known as pasteurization in honor of its developer Louis Pasteur. In the 1860s the French wine industry was plagued by the problem of wine spoilage, which made wine storage and shipping difficult. Pasteur examined spoiled wine under the microscope and detected microorganisms that looked like the bacteria responsible for lactic acid and acetic acid fermentations. He then discovered that a brief heating at 55 to 60°C would destroy these microorganisms and preserve wine for long periods. In 1886 the German chemists V. H. and F. Soxhlet adapted the technique for preserving milk and reducing milktransmissible diseases. Milk pasteurization was introduced into the United States in 1889. Milk, beer, and many other beverages are now pasteurized. Pasteurization does not sterilize a beverage, but it does kill any pathogens present and drastically slows spoilage by reducing the level of nonpathogenic spoilage microorganisms. Milk can be pasteurized in two ways. In the older method the milk is held at 63°C for 30 minutes. Large quantities of milk are now usually subjected to flash pasteurization or high-temperature short-term (HTST) pasteurization, which consists of quick heating to about 72°C for 15 seconds, then rapid cooling. The dairy industry also sometimes uses ultrahigh-temperature (UHT) sterilization. Milk and milk products are heated at 140 to 150°C for 1 to 3 seconds. UHT-processed milk does not require refrigeration and can be stored at room temperature for about 2 months without flavor changes. The small coffee creamer portions provided by restaurants often are prepared using UHT sterilization. Pasteurization and the dairy industry (pp. 970–71) Many objects are best sterilized in the absence of water by dry heat sterilization. The items to be sterilized are placed in an oven at 160 to 170°C for 2 to 3 hours. Microbial death apparently results from the oxidation of cell constituents and denaturation of proteins. Although dry air heat is less effective than moist heat— Clostridium botulinum spores are killed in 5 minutes at 121°C by moist heat but only after 2 hours at 160°C with dry heat—it has some definite advantages. Dry heat does not corrode glassware and metal instruments as moist heat does, and it can be used to sterilize powders, oils, and similar items. Most laboratories sterilize glass petri dishes and pipettes with dry heat. Despite these advantages, dry heat sterilization is slow and not suitable for heatsensitive materials like many plastic and rubber items. Low Temperatures Although our emphasis is on the destruction of microorganisms, often the most convenient control technique is to inhibit their growth and reproduction by the use of either freezing or refrigeration. This approach is particularly important in food microbiology (see p. 970). Freezing items at 20°C or lower stops microbial growth because of the low temperature and the absence of liquid water. Some microorganisms will be killed by ice crystal disruption of cell membranes, but freezing does not destroy contaminating microbes. In fact, freezing is a very good method for long-term storage of microbial samples when carried out properly, and many laboratories have a low-temperature freezer for culture storage at 30 or 70°C. Because frozen food can contain many microorganisms, it should be prepared and consumed promptly after thawing in order to avoid spoilage and pathogen growth. Effect of temperature on microbial growth (pp. 125–27) Refrigeration greatly slows microbial growth and reproduction, but does not halt it completely. Fortunately most pathogens are mesophilic and do not grow well at temperatures around 4°C. Refrigerated items may be ruined by growth of psychrophilic and psychrotrophic microorganisms, particularly if water is present. Thus refrigeration is a good technique only for shorter-term storage of food and other items. Filtration Filtration is an excellent way to reduce the microbial population in solutions of heat-sensitive material, and sometimes it can be used to sterilize solutions. Rather than directly destroying contaminating microorganisms, the filter simply removes them. There are two types of filters. Depth filters consist of fibrous or granular materials that have been bonded into a thick layer filled with twisting channels of small diameter. The solution containing microorganisms is sucked through this layer under vacuum, and microbial cells are removed by physical screening or entrapment and also by adsorption to the surface of the filter material. Depth filters are made of diatomaceous earth (Berkefield filters), unglazed porcelain (Chamberlain filters), asbestos, or other similar materials. Membrane filters have replaced depth filters for many purposes. These circular filters are porous membranes, a little over 0.1 mm thick, made of cellulose acetate, cellulose nitrate, polycarbonate, polyvinylidene fluoride, or other synthetic materials. Although a wide variety of pore sizes are available, membranes with pores about 0.2 m in diameter are used to remove most vegetative cells, but not viruses, from solutions ranging in volume from 1 ml to many liters. The membranes are held in special holders (figure 7.4) and often preceded by depth filters made of glass fibers to remove larger particles that might clog the membrane filter. The solution is pulled or forced through the filter with a vacuum 142 Chapter 7 Control of Microorganisms by Physical and Chemical Agents