AN ASSIGNMENT

ON

“STERILIZATION”

SUBMITTED BY:

NITIN RANA

M.TECH-FOOD & NUTRITIONAL

BIOTECHNOLOGY

SRM UNIVERSITY, CHENNAI

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 also prevents spoilage to some

extend.

Aseptic Packaging System is the most dependent process of packing pre-sterilized food

products in a sterile condition.

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.

Terminologies used in sterilization:

Sterilant -When sterilization is achieved by a chemical agent, the chemical is called

a sterilant.

Disinfection is the killing, inhibition, or removal of microorganisms that may cause

disease.

Disinfectants are agents usually chemical, used to carry out disinfection and are

normally used only on inanimate objects.

Antisepsis is the prevention of infection or sepsis and is accomplished with

antiseptics.

Antiseptics: These are chemical agents applied to tissue to prevent infection by

killing or inhibiting pathogen growth; they also reduce the total microbial

population. Because they must not destroy too much host tissue, antiseptics are

generally not as toxic as disinfectants.

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:

a) 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.

b) Forced Convection of hot air.

c) By direct flame contact.

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. But

in comparison to continuous sterilisers the steam and water consumption are higher.

Horizontal and side loading type batch sterilisers occupy more space then top loading and

vertical types. But the loading and discharge of containers are easier in case of horizontal

types. In batch sterilisers the steam pressures are in the range of 350-415 KN/m2

Batch Sterilizers

Continuous pressure steriliser

There are three types of continuous sterilisers:

1. The Continuous pressure cooker-cooler.

2. The Continuous rotary sterilisers

3. The Hydrostatic steriliser.

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.

In the 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.

Continuous Rotary sterilisers (Inside View)

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.

The 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:

1. Population size. Because an equal fraction of a microbial population is killed during

each interval, a larger population requires a longer time to die than a smaller one.

2. Population composition: The effectiveness of an agent varies greatly with the nature

of the organisms being treated because microorganisms differ markedly in

susceptibility. 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.

3. 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.

4. 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.

5. 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.

6. 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.

Methods of Microbial Control:

[A] Physical Method; and [B] Chemical Method

[A] Physical Methods:

[1] Heat: Kills microorganisms by denaturing their enzymes and other proteins. Heat

resistance varies widely among microbes.

[i] Moist Heat: Kills microorganisms by coagulating their proteins. In general, moist

heat is much more effective than dry heat.

(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.

Reliable sterilization with moist heat requires temperatures above that of boiling water.

Canning of food is the process of moist heat preservation.

(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

(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.

[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: 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

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.

[2] Low Temperature: The most convenient control technique to inhibit microbial

growth and reproduction by the use of either freezing or refrigeration. This approach is

particularly important in food microbiology. 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.

(a) Refrigeration: Temperatures from 0 to 7oC. 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: 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: Three types of radiation kill microbes:

(a) 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.

Forms of Radiation

.

(b) Ultraviolet light (Non-ionizing Radiation): 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. Do not penetrate paper, glass, and cloth.

(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.

[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.

[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 sp. that lives on skin is fairly resistant to high osmotic

pressure.

[B] Chemical Methods: Types of Disinfectants

i. 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.

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 (monochloro acetic acid, paracetic acid,

dehydro acetic acid is use as a mould inhibitor in pickle, jam, and in cheese.

ii. Nitrites and nitrates: Combination of these various salts has been used in curing

solutions and curing mixture of meat.

iii. Sugar and salt: These compound decreases the water activity of food result in

decrease in the microbial activity.

iv. Alcohol: Ethanol is a coagulant and denaturizes cell protein, is most germicidal in

concentration between 70-90%.

v. 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. Presently, because of this residue formation various Government

regulations such as those in the United States regarding food product sterilization

permit use of ethylene oxide solely for ungrounded spices, black walnut meats and

copra and the use of propylene oxide for treating cocoa, glace fruits, gums,

processed nut meats, dried prunes, processed spices and starch.

vi. 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.