Chapter 20-Microbial Growth Control

Chapter 7The Control of Microbial GrowthLearning Objectives1. Define the following key terms related to microbial control: sterilization, disinfection, antisepsis,germicide, bacteriostasis, asepsis, degerming, and sanitation.2. Describe the patterns of microbial death caused by treatments with microbial control agents.3. Describe the effects of microbial control agents on cellular structures.4. Know about the efficacy of moist heat (boiling, autoclaving, and pasteurization) and dry heat.5. Know how filtration, low temperature, desiccation, and osmotic pressure suppress microbial growth.6. Know how radiation kills cells.7. Know how microbial growth is affected by the type of microbe and the environmental conditions.8. List the factors related to effective disinfection.9. Identify the methods of action and preferred uses of chemical disinfectants.10. Know the advantage of glutaraldehyde over other chemical disinfectants.11. Identify the method of sterilizing plastic labware.12. Explain how microbial control is affected by the type of microbeThe Terminology of Microbial Control(Table 7.1)1. The control of microbial growth can prevent infections and food spoilage.2. Sterilization is the process of destroying all microbial life on an object.3. Commercial sterilization is heat treatment of canned foods to destroy C. botulinum endospores.4. Disinfection is the process of reducing or inhibiting microbial growth on a nonliving surface.5. Antisepsis is the process of reducing or inhibiting microorganisms on living tissue.6. The suffix -cide means to kill; the suffix - stat means to inhibit.7. Bacterial contamination is sepsis.The Rate of Microbial Death1. Bacterial populations subjected to heat or antimicrobial chemicals usually die at a constant rate. This isgraphed as a death curve.2. Such a death curve, when plotted logarithmically, shows this constant death rate as a straight line (Fig.7.1).3. The time to kill a microbial population is proportional to the number of microbes.4. Factors affecting microbial control by physical and chemical methods include:A. Microbial species (their characteristics) and life cycle phases (i.e., endospores) have differentsusceptibilities to physical and chemical controls.B. Organic matter may interfere with heat treatments and chemical control agents.C. Longer exposure to lower heat can produce the same effect as shorter time at higher heat.D. Type and concentration of agent used also affect the efficacy of microbial control. Some agents,for example the preservative sodium benzoate, do not work at physiological pH. The pH needsto be lowered for them to be effective.E. Number of microbes present affect the efficacy of antimicrobial process or agent. A highmicrobial load may require more time of exposure and/or a greater concentration ofantimicrobial agent.Actions of Microbial Control Agents (Ways of killing or inhibiting microbes) Alteration of Membrane Permeability1. The susceptibility of the plasma membrane is due to its lipid and protein components.2. Certain chemical control agents damage the plasma membrane by altering its permeability. Damage to Proteins and Nucleic Acids1. Some microbial control agents damage cellular proteins by breaking hydrogen and covalent bonds.2. Other agents interfere with DNA and RNA replication and protein synthesis.Physical Methods of Microbial ControlHeat1. Heat is frequently used to eliminate microorganisms.2. Moist heat kills microbes by denaturing enzymes.3. Thermal death point (TDP) is the lowest temperature at which all bacteria in a liquid culture will bekilled in 10 minutes.4. Thermal death time (TDT) is the length of time required to kill all bacteria in a liquid culture at a giventemperature.5. Decimal reduction time (DRT) is the length of time in which 90% of a bacterial population will bekilled at a given temperature.6. Boiling (100o C) kills many vegetative cells and viruses within 10 minutes. At higher altitudes, time ofheating must be increased to achieve 100o C.7. Autoclaving (steam under pressure) is the most effective method of moist heat sterilization. The steammust directly contact the material for it to be sterilized. Steam, at a temperature of 121o C under 15 psi for 15 min is enough to sterilize at sea level. At higher altitudes the pressure must be increased, which concomitantly increases the temperature of the steam (Boyle's law). See Text, Figure 7.28. In HTST pasteurization, a high temperature is used for a short time (72o C for 15 seconds) to destroypathogens without altering the flavor of the food. Ultra-high- temperature (UHT) treatment (140o C for3 sec.) is used to sterilize dairy productsr Additional Readings: "Why such a mystery over a Mycobacterium" ?, 2000. B. Dixon. ASMNews 66 (7): 384-385. Copyrighted by ASM and reproduced with permission of the ASM.9. Methods of dry heat sterilization include direct flaming, incineration, and hot-air sterilization. Dry heatkills by oxidation. Hot-air sterilization means that an object is heated at 170o C for 2 h.10. Different methods that produce the same effect (reduction in microbial growth) are called equivalenttreatments.Filtration (Text, Figure 7.4)1. Filtration is the passage of a liquid or gas through a filter with pores small enough to retain microbesFiltration is normally used to decontaminate liquid substances sensitive to heat treatment (i.e., enzyme solutions).2. Microbes can be removed from air by high-efficiency particulate air filters (pore size = 0.3 µm).3. Membrane filters composed of cellulose esters are commonly used to filter out bacteria, viruses, andeven large proteins (smallest pore size = 0.01 µM, although 0.2 µM will filter out almost all bacteria). Low Temperature1. The effectiveness of low temperatures depends on the particular microorganism and the intensity of theapplication.2. Most microorganisms do not replicate at ordinary refrigerator temperatures (0o-7o C).3. Many microbes survive (but do not grow) at the subzero temperatures used to store foods. Desiccation1. In the absence of water, microorganisms cannot grow but can remain viable.2. Viruses and endospores can resist desiccation.3. Lyophilization (freeze-drying) is often used as a preservative of foods and microorganisms. This is aclassical example of a paradox! How can lyophilization both prevent microbial growth and preserve it? Osmotic Pressure1. Microorganisms in high concentrations of salts and sugars undergo plasmolysis.2. Molds and yeasts are more capable than bacteria of growing in materials with low moisture or highosmotic pressure (i.e., moldy jam).Radiation1. The effects of radiation depend on its wavelength, intensity, and duration.2. Ionizing radiation (gamma rays, X-rays, and high- energy electron beams, see Text, Fig. 7.5) has a highdegree of penetration and exerts its effect primarily by ionizing water and forming highly reactivehydroxyl radicals3. Ultraviolet (UV) radiation, a form of nonionizing radiation, has a low degree of penetration and causescell damage by making thymine dimers in DNA that interfere with DNA replication; the most effective germicidal wavelength is 260 nm.4. Longer wavelengths of electromagnetic energy, such as microwaves, are not very effective in killingmicrobes. However, microwaves can kill microbes indirectly as materials get hot.Summary of Physical Methods of Control1. For a summary of the physical methods of microbial control and their modes of action, see Text,Table7.5.Chemical Methods of Microbial Control1. Chemical agents may be used on living tissue (as antiseptics) or on inanimate objects (as disinfectants).2. Few chemical agents achieve sterility.Principles of Effective Disinfection1. Careful attention should be paid to the properties and concentration of the disinfectant to be used.2. The presence of organic matter, degree of contact with microorganisms, and temperature should also beconsidered.Evaluating a Disinfectant1. In the use-dilution test, bacterial (S. choleraesuis, S. aureus, and P. aeruginosa) survival in a selecteddilution of a disinfectant is determined.2. Viruses, endospore-forming bacteria, mycobacteria, and fungi can also be used in the use-dilution test.3. In the filter paper method, a disk of filter paper is soaked with a chemical and placed on an inoculatedagar plate; a clear zone of inhibition indicates effectiveness. You cannot determine MICs with thismethod.Types of DisinfectantsPhenol and Phenolics1. Phenolics exert their action by injuring plasma membranes, inactivating enzymes, and denaturingproteins.2. Common phenolics are cresols.Bisphenols1. Bisphenols such as triclosan (over the counter) and hexachlorophene (prescription) are widely used inhousehold products.Biguanides1. Chlorhexidine damages the plasma membrane of vegetative cells.Halogens1. Some halogens (iodine and chlorine) are used alone or as components of inorganic or organic solutions.2. Iodine inactivate enzymes and other cellular proteins by combining with the amino acid tyrosinecausing it to lose its function.3. Iodine is available as a tincture (in solution with alcohol) or as an iodophor (combined with an organicmolecule).4. The germicidal action of chlorine is based on the formation of hypochlorous acid when chlorine isadded to water.5. Chlorine is used as a disinfectant in gaseous form (Cl2) or in the form of a compound, such as calciumhypochlorite, sodium hypochlorite, or chloramines.Alcohols1. Alcohols exert their action by denaturing proteins and dissolving lipids.2. In tinctures, they enhance the effectiveness of other antimicrobial chemicals.3. Aqueous ethanol (60-90%) and isopropanol are used as disinfectants.Heavy Metals and Their Compounds1. Silver, mercury, copper, and zinc are used as germicides.2. They exert their antimicrobial action through oligodynamic action. When heavy metal ions combinewith sulfhydryl (-SH) groups, proteins are denatured.Surface-Active Agents1. Surface-active agents decrease the tension between molecules that lie on the surface of a liquid; soapsand detergents are examples.2. Soaps have limited germicidal action but assist in the removal of microorganisms through scrubbing.3. Acid-anionic detergents are used to clean dairy instrumentation.4. Does bacterial resistance to antibacterial cleaning and hygiene products arise during routine householduse?r Additional Readings:"Antibacterial Cleaning Products and Drug Resistance", Allison E.Aiello, Bonnie Marshall, Stuart B. Levy, Phyllis Della-Latta, Susan X. Lin, and Elaine Larson.2005. Emerging Infectious Diseases. 11 (10): 1565-1570. Copyrighted by CDC and reproducedwith permission from CDC.Quaternary Ammonium Compounds (Quats)1. Quats are cationic detergents attached to NH4+.2. They are also surface-active agents.3. By disrupting plasma membranes, they allow cytoplasmic constituents to leak out of the cell.4. Quats are most effective against gram-positive bacteria.Chemical Food Preservatives1. Sulfur dioxide, sorbic acid, benzoic acid, and propionic acid inhibit fungal metabolism and are used asfood preservatives.2. Nitrate and nitrite salts prevent germination of Clostridium botulinum endospores in meats.3. However, nitrates may cause cancer.Antibiotics1. Nisin and natamycin are antibiotics used to preserve foods, especially cheese.Aldehydes1. Aldehydes, such as formaldehyde and glutaraldehyde, exert their antimicrobial effect by inactivatingproteins.2. They are among the most effective chemical disinfectants.Gaseous Chemosterilizers1. Ethylene oxide is the gas most frequently used for sterilization.2. It penetrates most materials and kills all microorganisms by protein denaturation.Peroxygens (Oxidizing Agents)1. Ozone, peroxide, and peracetic acid are used as antimicrobial agents.2. They exert their effect by oxidizing molecules inside cells.Microbial Characteristics and Microbial Control1. Gram-negative bacteria are generally more resistant than gram-positive bacteria to disinfectants andantiseptics.2. Endospores, protozoan cysts and oocysts, and mycobacteria are very resistant to disinfectants andantiseptics.3. Nonenveloped viruses are generally more resistant than enveloped viruses to disinfectants andantiseptics.4. Prions are resistant to disinfection and autoclaving.Summary1. For a summary of the chemical methods of microbial control and their modes action, see Text,Table 7.82. Summary of efficacy, see Text, Table 7.7.Additional Readings Appendix1. "Why such a mystery over a Mycobacterium?", 2000. B. Dixon. ASM News 66 (7): 384-385.Copyrighted by ASM and reproduced with permission of the ASM.2. "Antibacterial Cleaning Products and Drug Resistance", Allison E. Aiello, Bonnie Marshall, Stuart B.Levy, Phyllis Della-Latta, Susan X. Lin, and Elaine Larson. 2005. Emerging Infectious Diseases. 11(10): 1565-1570. Copyrighted by CDC and reproduced with permission from CDC.。

低温贮藏的定义低温贮藏是一种有效的食品安全保障技术，总的来说就是将食品保存在较低的温度条件下，以达到食品的安全保质，使食品的质量和营养价值更佳的目的。

主要应用于肉制品、水果蔬菜海产品、奶制品等食品。

低温贮藏的目的，一是降低灭菌时需要的温度，来保护活体的营养物质，达到营养成分保持和色泽华丽，使食品保持最佳质量；二是降低微生物繁殖的环境，从而有效地减少微生物破坏，避免腐败和失水。

从功能上来说，低温贮藏主要有以下四大作用：一是抑制食品中微生物的繁殖。

由于微生物的活动受温度条件限制，减少温度会抑制微生物的繁殖，延缓剩余期。

二是保存食材本身的原味。

低温贮藏可以有效防止风味物质的结合，从而达到保持食材的原始鲜美口感的目的。

三是以少量冰醋酸钠调节食品质量。

由于醋酸钠蒸发温度低，可以将温度控制在一定范围内，避免食材变形，从而促进食品质量的稳定。

四是保持食品的理化性质和营养成分。

由于低温贮藏抑制了蛋白质的水解，可以有效保留食品的营养成分和内在的性质，使食品保持相对稳定。

低温贮藏技术的应用受到广泛的重视，它可以有效的抗击食品安全问题，保障我们的食品安全，为食品提供更好的保质、控制腐败以及阻止微生物增殖，降低食品的制作和流通成本，保持食品更长的期限，使产品能够更有效地流通，受到消费者广泛欢迎。

总之，低温贮藏可以有效地增加食品的保存时间，保护食品的本质属性，确保食物安全。

它在食品安全领域发挥着重要的作用，但也存在一定的风险，所以应该加强相关法规的制定和执行，优化低温贮藏设备的使用，以提高我国的食品安全水平。

Low temperature storage is an effective food safety assurance technology, generally speaking, it is to preserve food under lower temperature conditions to achieve foodsafety and quality and nutritional value of the best purpose. Mainly applied to meat products, fruits and vegetables,aquatic products, dairy products and other foods.The purpose of low-temperature storage is firstly toreduce the temperature required for sterilization to protect the nutrients of the living body, achieve nutrient retention and gorgeous color, so as to maintain the best quality of food, and secondly to reduce the microorganism's breeding environment, so as to effectively reduce microorganism's destruction, avoid corruption and dehydration.Functionally, low-temperature storage mainly has the following four functions:First, inhibit the proliferation of microorganisms in food. Because the activity of microorganisms is restricted by temperature conditions, reducing temperature will inhibit the proliferation of microorganisms and prolong the shelf life.Secondly, preserve the original flavor of food ingredients. Low-temperature storage can effectively prevent the combination of flavor substances, thus achieving the purposeof maintaining the original freshness of food ingredients.Thirdly, adjusting the quality of food with a small amount of sodium acetate. Due to the low evaporation temperature of sodium acetate, the temperature can be controlled within acertain range, so as to prevent the food from deformation, which can promote the stability of food quality.Fourthly, maintain the physical and chemical properties and nutritional components of food. Due to the low temperature storage can inhibit the hydrolysis of protein, so as to effectively retain the nutritional components and inherent properties of food, so that food can maintain relatively stable.The application of low-temperature storage technology has been widely valued, it can effectively combat food safety problems, ensure our food safety, provide better quality control and control of corruption and prevention of microbial growth, reduce food production and circulation costs, retain food for longer periods of time, make products circulate more efficiently and be welcomed by consumers.In conclusion, low-temperature storage can effectively increase the storage time of food, protect the essential properties of food, and ensure food safety. It plays an important role in food safety, but there are also certain risks, therefore, we should strengthen the formulation and enforcement of relevant laws and regulations, optimize the use of low-temperature storage equipment, so as to improve the food safety level of our country.。

黑曲霉生产柠檬酸的发酵工艺流程1.首先，选择适当的黑曲霉菌种作为发酵的起始种。

First, choose the appropriate Aspergillus Niger strain as the starting culture for fermentation.2.将黑曲霉菌种接种到含有适量碳源和氮源的发酵基质中。

Inoculate the Aspergillus Niger strain into a fermentation medium containing suitable amounts of carbon and nitrogen sources.3.确保发酵基质的pH值在合适的范围内，通常为3.0至6.0。

Ensure that the pH of the fermentation medium is within the appropriate range, typically between 3.0 and 6.0.4.控制发酵温度在25°C至35°C之间，提供适宜的温度条件。

Control the fermentation temperature between 25°C and 35°C to provide suitable conditions for growth.5.确保发酵过程中的通气充足，以促进微生物的生长和代谢活动。

Ensure adequate aeration during the fermentation processto promote microbial growth and metabolic activity.6.在发酵过程中定期监测黑曲霉的生长情况和产酸量。

Regularly monitor the growth of Aspergillus Niger and the production of citric acid during the fermentation process.7.当黑曲霉的生长达到高峰并且产酸量稳定时，进行收获。

PRETREATMENT OPTIONS预处理选项4. PRETREATMENT OPTIONS 预处理选项4.1 INTRODUCTION 简介Pretreatment is all process steps or unit operations prior to the last (final) water treatment step. Pretreatment is a series of unit operations to modify the feed water quality so that it will be of adequate quality to be fed to a final treatment step. This final step may be Reverse Osmosis, Ultrafiltration, Multi or Mixed Bed Deionization or Distillation. These final steps are discussed in Chapters 5 and 6.预处理是指最后（最终）水处理步骤之前的所有加工步骤或单元操作。

预处理是一系列的单元操作，用来改善给水水质以使其水质足够好而进入终处理步骤。

最终步骤可以是反渗透、超滤、多或混合床去离子或蒸馏。

第5、6章中将详细讨论这些最终步骤。

Reverse Osmosis is unique since it can be a pretreatment step, in addition to being a final treatment step.Reverse Osmosis applications in pretreatment are discussed in this chapter and Chapter 11, but Reverse Osmosis as a technology is discussed in Chapters 5 and 6.反渗透很特别，因为它即可以是一个预处理步骤，另外还能是一个终处理步骤。