Moist Heat Kills Microorganisms By An Irreversible Coagulation Of Proteins

5.2 Selection of an Antimicrobial Procedure

Type of Microorganism

1. One of the most critical considerations in selecting a method of destroying microorganisms and viruses is the type of microbial population present on or in the product.

Numbers of Microorganisms Initially Present

1. The amount of time it takes for heat or chemicals to kill a population of microorganisms is dictated in part by the number of organisms initially present.

2. Microbial death generally occurs at a constant rate.

3. In the commercial canning industry, the D value, or decimal reduction time, is defined as the time it takes to kill 90% of a population of bacteria under specific conditions. (Figure 5.2)

Environmental Conditions

1. Factors such as pH or presence of fats and other organic materials strongly influence microbial death rates.

Potential Risk of Infection

1. To guide medical biosafety personnel in their selection of germicidal procedures, items are categorized as critical,

126 Chapter 5 Control of Microbial Growth semicritical, and non-critical according to their potential risk of transmitting infectious agents.

5.3 Using Heat to Destroy Microorganisms and Viruses

1. Heat can be used to destroy vegetative microorganisms and viruses, but temperatures above boiling are required to kill endospores.

Moist Heat

1. Moist heat, such as boiling water and pressurized steam, destroys microorganisms by causing irreversible coagulation of their proteins.

2. Pasteurization utilizes a brief heat treatment to destroy spoilage and disease-causing organisms, increasing the shelf life of products and protecting consumers.

3. Pressure cookers and autoclaves heat water in an enclosed vessel that causes the pressure in the vessel to increase beyond atmospheric pressure, increasing the temperature of steam, which kills endospores. (Figure 5.3)

4. The most important aspect of the commercial canning process is to ensure that endospores of Clostridium botulinum are destroyed. (Figure 5.5)

Dry Heat

1. Direct flame and ovens generate dry heat, which destroys microorganisms by oxidizing cells to ashes or irreversibly denaturing their proteins.

5.4 Using Chemicals to Destroy Microorganisms and Viruses

1. Germicidal chemicals can be used to disinfect and, in some cases, sterilize, but they are less reliable than heat.

2. Most chemical germicides react irreversibly with vital enzymes and other proteins, the cytoplasmic membrane, or viral envelopes. (Figure 5.6)

Potency of Germicidal Chemical Formulations

1. Germicides registered with either the FDA or EPA are grouped according to their potency as sterilants, high-level disinfectants, intermediate-level disinfectants, or low-level disinfectants. (Table 5.3)

Selecting the Appropriate Germicidal Chemical

1. Factors that must be included in the selection of an appropriate germicidal chemical include toxicity, residue, activity in the presence of organic matter, compatibility with the material being treated, cost and availability, storage and stability, and ease of disposal.

Classes of Germicidal Chemicals (Table 5.4)

1. Solutions of 60% to 80% ethyl or isopropyl alcohol in water rapidly kill vegetative bacteria and fungi by coagulating enzymes and other essential proteins, and by damaging lipid membranes.

2. Glutaraldehyde and formaldehyde destroy microorganisms and viruses by inactivating proteins and nucleic acids.

3. Chlorhexidine is a biguanide extensively used in antiseptic products.

4. Ethylene oxide is a gaseous sterilizing agent that penetrates well and destroys microorganisms and viruses by reacting with proteins.

5. Sodium hypochlorite (liquid bleach) is one of the least expensive and most readily available forms of chlorine. Chlorine dioxide is used as a sterilant and disinfectant. Iodophores are iodine-releasing compounds used as antiseptics.

6. Metals interfere with protein function. Silver-containing compounds are used to prevent wound infections.

7. Ozone is used as an alternative to chlorine in the disinfection of drinking water and wastewater.

8. Peroxide and peracetic acid are both strong oxidizing agents that can be used alone or in combination as sterilants.

9. Phenolic compounds destroy cytoplasmic membranes and denature proteins. Triclosan is used in lotions and deodorant soaps.

10. Quaternary ammonium compounds are cationic detergents; they are non-toxic enough to be used to disinfect food preparation surfaces.

5.5 Removal of Microorganisms by Filtration

Filtration of Fluids (Figure 5.7)

1. Depth filters have complex, tortuous passages that retain microorganisms while letting the suspending fluid pass through the small holes.

2. Membrane filters are produced with graded pore sizes extending below the dimensions of the smallest known viruses.

Filtration of Air

1. High-efficiency particulate air (HEPA) filters remove nearly all microorganisms.

5.6 Using Radiation to Destroy Microorganisms and Viruses

Gamma Irradiation

1. Gamma rays cause biological damage by producing superoxide and hydroxyl free radicals; gamma rays can be used to sterilize heat-sensitive materials and to decrease the numbers of microorganisms in foods.

Ultraviolet Irradiation

1. Ultraviolet light damages the structure and function of nucleic acids by causing the formation of covalent bonds between adjacent thymine molecules in DNA, creating thymine dimers; it is used to disinfect surfaces.

Microwaves

1. Microwaves do not affect microorganisms directly, but they can kill microorganisms by the heat they generate in a product.

5.7 Preservation of Perishable Products

1. Preservation techniques slow or halt the growth of microorganisms to delay spoilage.

Chemical Preservatives

1. Benzoic, sorbic, and propionic acids are organic acids sometimes added to foods to prevent microbial growth.

2. Nitrate and nitrite are added to some foods to inhibit the germination and subsequent growth of Clostridium botulinum endospores. They also react with myoglobin to form a stable pigment that gives a pink color associated with fresh meat.

Low-Temperature Storage

1. Low temperatures above freezing inhibit microbial growth because many enzymatic reactions are rendered slow or nonexistent.

2. Freezing essentially stops all microbial growth.

Reducing the Available Water

1. Sugar and salt draws water out of cells, preventing the growth of microorganisms.

2. Lyophilization is used for preserving food. The food is first frozen and then dried in a vacuum.

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