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Introduction Early civilizations practiced salting, smoking, pickling, drying, and exposure of food and clothing to sunlight to control microbial growth. Use of spices in cooking was to mask taste of spoiled food. Some spices prevented spoilage. Aseptic Packaging System is the most dependent process of packing pre-sterilized food products in a sterile condition.

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Definition Sterilization: Killing or removing all forms of microbial life (including endospores) in a material or an object. Heating is the most commonly used method of sterilization. Commercial Sterilization: Heat treatment that kills endospores of Clostridium botulinum the causative agent of botulism, in canned food. Does not kill endospores of thermophiles, which are not pathogens and may grow at temperatures above 45oC.

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Terminologies: Sterilant: When sterilization is achieved by a chemical agent, the chemical is called a sterilant. Disinfection: Killing, inhibition, or removal of microorganisms that may cause disease. Disinfectant: Applied to inanimate objects. Antiseptic: Applied to living tissue (antisepsis). Asepsis: Absence of significant contamination. Aseptic techniques are used to prevent contamination of surgical instruments, medical personnel, and the patient during surgery. Aseptic techniques are also used to prevent bacterial contamination in food industry.

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Bacteriostatic Agent: An agent that inhibits the growth of bacteria,but does not necessarily kill them. Bactericide: An agent that kills bacteria. Most do not kill endospores. Viricide: An agent that inactivates viruses. Fungicide: An agent that kills fungi. Sporicide: An agent that kills bacterial endospores of fungal spores.

Common methods of Sterilization : 

Common methods of Sterilization The two common methods of sterilization are: 1. Sterilising in containers 2. Sterilisation of foods before placing in the container Sterilisation of foods in containers can be done by following methods: Indirect heating by saturated steam: It is a common method. Low acid foods are sterilised at temperatures above 100˚ C with the help of pressurized sterilisers. Forced Convection of hot air. By direct flame contact.

Batch Sterilizers : 

Batch Sterilizers In batch and non-agitating type of sterilisers the labour requirement is high but still they are quite popular. The batch sterilisers are flexible and can accommodate containers of different sizes. The batch sterilizers are also suitable for different type of processes. In batch sterilisers the steam pressures are in the range of 350-415 KN/m2

Continuous pressure steriliser : 

Continuous pressure steriliser There are three types of continuous sterilisers: The Continuous pressure cooker-cooler. The Continuous rotary sterilisers The Hydrostatic steriliser. 1. The Continuous pressure cooker-cooler sterilisers are of non-agitating types. The containers which are to be sterilised are carried on a roller track or a chain conveyer through the pre-heating, sterilizing and cooling sections.

2. Continuous Rotary sterilisers : 

2. Continuous Rotary sterilisers There are 3 cylinders. The inner walls of these cylinders have helical track. The containers are carried in the track by the flanges at the periphery of cylinder. The helical track causes agitation of containers by the combined rolling and sliding action. Rotary sterilisers have large capacity.

Hydrostatic sterilisers : 

Hydrostatic sterilisers In the Hydrostatic sterilisers, hot water columns are used to generate pressure. The temperature of water in the preheated zone and air cooling section is between 107 to 108˚ C. The water seal temperatures in the steam chamber are between 114 to 127˚C. The containers are carried on a twin roller-chain conveyor operating at slow speed of around 2m/min. The containers are usually carried with their long horizontal axes. This helps the convective heat transfer within the container.

Pattern of Microbial Death : 

Pattern of Microbial Death A microbial population is not killed instantly when exposed to a lethal agent. Population death, like population growth, is generally exponential or logarithmic—that is, the population will be reduced by the same fraction at constant intervals If the logarithm of the population number remaining is plotted against the time of exposure of the microorganism to the agent, a straight line plot will result. When the population has been greatly reduced, the rate of killing may slow due to the survival of a more resistant strain of the microorganism.

Conditions Influencing the Effectiveness of Antimicrobial Agent Activity: : 

Conditions Influencing the Effectiveness of Antimicrobial Agent Activity: Population size: Larger population requires a longer time to die than a smaller one. Population composition: Bacterial endospores are much more resistant to most antimicrobial agents than are vegetative forms, and younger cells are usually more readily destroyed than mature organisms. Concentration or intensity of an antimicrobial agent: The more concentrated a chemical agent or intense a physical agent, the more rapidly Microorganisms are destroyed. However, agent effectiveness usually is not directly related to concentration or intensity. Over a short range a small increase in concentration leads to an exponential rise in effectiveness; beyond a certain point, increases may not raise the killing rate much at all.

Contd… : 

Contd… Duration of exposure: The longer a population is exposed to a microbicidal agent, the more organisms are killed. To achieve sterilization, exposure duration sufficient to reduce the probability of survival to 10–6 or less should be used. Temperature: An increase in the temperature at which a chemical acts often enhances its activity. Frequently a lower concentration of disinfectant or sterilizing agent can be used at a higher temperature. Local environment: The population to be controlled is not isolated but surrounded by environmental factors that may either offer protection or aid in its destruction. For example, because heat kills more readily at an acid pH, acid foods and beverages such as fruits and tomatoes are easier to pasteurize than foods with higher pH’s like milk.

Physical Methods of Microbial Control : 

1.Heat: Kills microorganisms by denaturing their enzymes and other proteins. Heat resistance varies widely among microbes. Thermal Death Point (TDP): Lowest temperature at which all of the microbes in a liquid suspension will be killed in ten minutes. Thermal Death Time (TDT): Minimal length of time in which all bacteria will be killed at a given temperature. Decimal Reduction Time (DRT): Time in minutes at which 90% of bacteria at a given temperature will be killed. Used in canning industry. Physical Methods of Microbial Control

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i) Moist Heat: Kills microorganisms by coagulating their proteins. In general, moist heat is much more effective than dry heat. Reliable sterilization with moist heat requires temperatures above that of boiling water. (a) Boiling: Heat to 100oC or more at sea level. Kills vegetative forms of bacterial pathogens, almost all viruses, and fungi and their spores within 10 minutes or less. Endospores and some viruses are not destroyed this quickly. However brief boiling will kill most pathogens. Hepatitis virus: Can survive up to 30 minutes of boiling. Endospores: Can survive up to 20 hours or more of boiling. Canning of food is the process of moist heat preservation

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(b) Autoclave: Chamber which is filled with hot steam under pressure. Preferred method of sterilization, unless material is damaged by heat, moisture, or high pressure. Temperature of steam reaches 121oC at twice atmospheric pressure. Most effective when organisms contact steam directly or are contained in a small volume of liquid. All organisms and endospores are killed within 15 minutes. Require more time to reach center of solid or large volumes of liquid.

Autoclave: Closed Chamber with High Temperature and Pressure : 

Autoclave: Closed Chamber with High Temperature and Pressure

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(c) Pasteurization: Developed by Louis Pasteur to prevent the spoilage of beverages. Used to reduce microbes responsible for spoilage of beer, milk, wine, juices, etc. Classic Method of Pasteurization: Milk was exposed to 65oC for 30 minutes. High Temperature Short Time Pasteurization (HTST): Used today. Milk is exposed to 72oC for 15 seconds. Ultra High Temperature Pasteurization (UHT): Milk is treated at 140oC for 3 seconds and then cooled very quickly in a vacuum chamber. Advantage: Milk can be stored at room temperature for several months.

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ii) Dry Heat: Kills by oxidation effects. a) Direct Flaming: Used to sterilize inoculating loops and needles. Heat metal until it has a red glow. b) Incineration: Effective way to sterilize disposable items (paper cups, dressings) and biological waste. c) Hot Air 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 Advantages: Dry heat transfers heat less effectively to a cool body, 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 of this procedure used in the bakery industry. Disadvantage: Dry heat sterilization is slow and not suitable for heat sensitive materials

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2. Low Temperature: Effect depends on microbe and treatment applied. (a) Refrigeration: Temperatures from 0 to 7oC. Bacteriostatic effect-Reduces metabolic rate of most microbes so they cannot reproduce or produce toxins. (b) Freezing: Temperatures below 0oC. Used in preservation of milk product, ice cream, and many aqueous foods. (c) Flash Freezing: Does not kill most microbes. (d) Slow Freezing: More harmful because ice crystals disrupt cell structure. Over a third of vegetative bacteria may survive 1 year. Most parasites are killed by a few days of freezing.

3. Filtration: : 

3. Filtration: Removal of microbes by passage of a liquid or gas through a screen like material with small pores. It 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. This is mainly used in food industry for the removal of contamination of liquid food material such as fruit juice and edible oil.

4. Radiation: : 

4. Radiation: Three types of radiation kill microbes: Ionizing Radiation: Gamma rays, X rays, electron beams, or higher energy rays. Have short wavelengths (less than 1 nanometer). Dislodge electrons from atoms and form ions. Cause mutations in DNA and produce peroxides. Used to sterilize pharmaceuticals and disposable medical supplies. Food industry is interested in using ionizing radiation. Disadvantages: Penetrates human tissues. May cause genetic mutations in humans.

(b) Non-ionizing Radiation (Ultraviolet light): : 

(b) Non-ionizing Radiation (Ultraviolet light): Wavelength is longer than 1 nanometer. Damages DNA by producing thymine dimers, which cause mutations. Used to disinfect operating rooms, microbiology and biotech labs in laminar air flow to carry out research work. Disadvantages: Damages skin, eyes. Doesn’t penetrate paper, glass, and cloth.

(c) Microwave Radiation: : 

(c) Microwave Radiation: Wavelength ranges from 1 millimeter to 1 meter. Heat is absorbed by water molecules. May kill vegetative cells in moist foods. Bacterial Endospores, which do not contain water, are not damaged by microwave radiation. Solid foods are unevenly penetrated by microwaves.

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5. Dessication: In the absence of water, microbes cannot grow or reproduce, but some may remain viable for years. After water becomes available, they start growing again. Susceptibility to dessication varies widely: Neisseria gonnorrhea: Only survives about one hour. Mycobacterium tuberculosis: May survive several months. Viruses are fairly resistant to dessication. Clostridium sp. and Bacillus sp: May survive decades.

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6. Osmotic Pressure: The use of high concentrations of salts and sugars in foods is used to increase the osmotic pressure and create a hypertonic environment. 7. Plasmolysis: As water leaves the cell, plasma membrane shrinks away from cell wall. Cell may not die, but usually stops growing. Yeasts and molds: More resistant to high osmotic pressures. Staphylococci spp. that live on skin are fairly resistant to high osmotic pressure.

Chemical Methods of Microbial Control : 

Types of Disinfectants 1. Organic acid and their salt are used as anti-microbial agents in food processing. Propionate: Propionic acid is a short chain fatty acid which is effective against cell membrane of fungi and mould. Sodium and calcium propionate is use most extensively in the prevention of mould growth and rope deployment in the backed food and in cheese food. Benzoate: The sodium salt of benzoic acid has been use extensively as an antimicrobial agent in food. It has been incorporated in to jams jellies, fruit salad, pickles. Chemical Methods of Microbial Control

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Sorbate: Sorbic acid as the sodium, sodium, or potassium salt is used as a direct antimicrobial additives in food as a spay, dip, or coating on packing material. It is widely used in cheeses, backed product and in beverages. Acetates: Derivative of acetic acid (mono chloro acetic acid, paracetic acid, dehydro acetic acid is used as a mould inhibitor in pickle, jam, and in cheese. Nitrites and nitrates: Combination of these various salts has been used in curing solutions and curing mixture of meat.

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Sugar and salt: These compound decreases the water activity of food result in decrease in the microbial activity. Alcohol: Ethanol is a coagulant and denaturizes cell protein, is most germicidal in concentration between 70-90%. Ethylene and propylene oxide: Sterilants gases such as ethylene oxide and propylene oxide have been used in the past but their use has been severely limited due to substantial harmful residue formation (i.e., chlorohydrins) in the food products.

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Peroxygens (Oxidizing Agents): Oxidize cellular components of treated microbes. Also disrupt membranes and proteins. (a) Ozone: Used along with chlorine to disinfect water. Helps neutralize unpleasant tastes and odors. More effective killing agent than chlorine, but less stable and more expensive. Highly reactive form of oxygen. Made by exposing oxygen to electricity or UV light. (b) Hydrogen Peroxide: Used as an antiseptic. Effective in disinfection of inanimate objects. Sporicidal at higher temperatures.

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