STUDY MATERIAL
COURSE
: II B.Sc (CS&HM)
SEMESTER
: IV
SUBJECT
: FOOD SAFETY AND MICROBIOLOGY
UNIT
: III
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Food Contamination in Meat:
The healthy inner flesh of meats has been reported to contain few or no microorganisms,
although they have been found in lymph nodes, bone marrow, and even flesh. Staphylococci,
streptococci, Clostridium, and Salmonella have been isolated from the lymph nodes of red-meat
animals. Normal slaughtering practices would remove the lymph nodes from edible parts. The
important contamination, however, comes from external sources during bleeding, handling and
processing. During, bleeding, skinning and cutting, the main sources of microorganisms are the
exterior of the animal (hide, hooves, and hair) and the intestinal tract. Recently approved
“humane” methods of slaughter- mechanical, chemical and electrical—have little effect on
contamination as with the older methods of use of knife on hogs and poultry, any contaminating
bacteria on the knife soon will be found in meat in various parts of the carcass, carried there by
blood lymph. The exterior of the animal harbors large numbers and many kinds of
microorganisms from soil, water, feed and manure, as well as its natural surface flora, and the
intestinal contents contain the intestinal organisms. Knives, cloths, air, and hands and clothing of
the workers can serve as intermediate sources of contaminants. During the handling of the meat
thereafter, contamination can come from carts, boxes, or other containers; other contaminated
meat; air; and personnel. Especially undesirable is the addition of psychotropic bacteria from any
source, e.g., from other meats that have been chilling storage. Special equipment such as grinders,
sausage stuffers and casings, and ingredients in special products, e.g., fillers and spices, may add
undesirable organisms in appreciable numbers. Growth of microorganisms on surfaces touching
the meats and on the meats themselves increases their numbers.
Certain important species of bacteria can grow at chilling temperatures. There also is the
possibility of the contamination of meat and meat products with human pathogens, especially
those of the intestinal type.
In retail market and in the home additional contamination usually takes place. In the market
knives, saws, cleavers, slicers, grinders, chopping blocks, scales, sawdust, and containers as well as
the market operators may be sources of organisms. In the home refrigerator containers used
previously to store meats can serve as sources of spoilage organisms.
Food Contamination in Poultry and game:
The discussion of poultry is concerned mostly with chicken meat, but the principles apply to meat
of other fowl, such as turkey, goose, duck and squab.
Contamination
Sources of contamination discussed for meats apply to poultry as well. The skin of live birds may
contain numbers of bacteria averaging 1,500 per square centimeter. These numbers probably
reflect the natural flora of the skin plus other organisms that could be derived from feet, feathers,
and feces. Contamination of the skin and the lining of the body cavity occur during washing,
plucking, and evisceration. Chickens are currently processed by a fully automated conveyor or
track line with vacuum evisceration.
Bacterial numbers vary considerably on the surface of chickens. This variations, however, is
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greater between birds than it is between different areas of the same bird. The incidence of
salmonellae on poultry carcasses and the role of poultry processing in transmitting salmonellosis
have received considerable attention. The incidence of salmonella-positive birds has been
reported to range from 0 to 50 percent. There is also a high incidence of Campylobacter jejuni in
poultry processing plants and on the processed bird.
Food Contamination in Cereals:
Cereal products discussed in this chapter include the grains themselves, meals, flours, alimentary
pastes, and breads, cakes, and other bakery products.
Contamination
The exteriors of harvested grains retain some of the microorganisms they had wile growing plus
contamination form soil, insects, and other sources. Freshly harvested grains contain a few
thousand to millions of bacteria per gram and from none to several hundred thousand mold
spores. Bacteria are mostly in the families Pseudomonadaceae, Micrococcaceae, Lactobacillaceae,
and bacillaceae. Scouring and washing the grains remove some of the microorganism, but most
of the microorganisms are removed with the outer portions of the grain during milling. The
milling processes, especially bleaching, reduce numbers of organisms, but there then is a
possibility of contamination during other procedures, such as blending and conditioning.
Bacteria in wheat flour include spores of Bacillus, coliform bacteria, Flavobacterium, Sarcina,
Micrococcus, Alcaligenes, and Serratia. Mold spores are chiefly those of aspergilli and penicillia,
with also some of Alternaria, Cladosporium, and other genra. Numbers of bacteria vary widely
from a few hundred per gram to millions. Most Samples of white wheat flour from the retail
market contain a few hundred to a few thousand bacteria per gram, twenty to thirty bacillus
spores per gram and 50 to 100 mold spores per gram, twenty to thirty bacillus spores per gram
and 50 to 100 mold spores per gram. Patent flours usually give lower counts than straight or clear,
and numbers decrease with storage of the flour. Higher counts usually are obtained from prepared
flours and still higher on graham and whole wheat flours which contain also the outer parts of the
wheat kernel and are not bleached. Cornmeal and flour contain several hundred to several
thousand bacteria and molds per gram. Species of Fusarium and Pencillium are the dominant
molds. Because of incubation in a moist condition malts contain high numbers of bacteria usually
in the millions per gram.
The surface of a freshly baked loaf of bread is practically free of viable microorganisms but is
subject to contamination by mold spores from the air during cooling and before wrapping.
During slicing, contamination may take place from microorganisms in the air, on the knives, or on
the wrapper. Cakes are similarly subject to contamination. Spores of bacteria able to cause
ropiness in bread will survive the baking process.
From a public health aspect, the contamination of grains and cereal products with molds has
become a significant concern because of the possible presence of mycotoxins. The need to reduce
contamination by mold and to avoid conditions which allow their growth is emphasized by the
frequent isolation of Aspergillus flavus and A.parasiticus, which can produce aflatoxin. Other
commonly isolated molds such as fusaria and penicillia are undesirable since certain species of
these genera are also capable of producing mycotoxins.
Food Contamination in Dairy Products
Dairy products include market milk and cream, butter, frozen desserts, cheese, fermented milks
and condensed and dried milk products.
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Contamination
Milk contains relatively few bacteria when it leaves the udder of a healthy cow, and generally these
bacteria do not grow in milk under the usual conditions of handling. However, micrococci and
streptococci have been recovered from aseptically drawn milk. During the normal milking
operation, however milk is subject to contamination from the animal especially the exterior of the
udder and adjacent areas. Bacteria found in manure, soil and water may enter from this source.
Such contamination is reduced by clipping the cow especially the flanks and udder, grooming the
cow, and washing the udder with water or a germicidal solution before milking. Contamination of
the cow with soil, water and manure is reduced by paving and draining barnyards, keeping cows
from stagnant pools and cleaning manure form the barns or milking parlors.
Probably the two most significant sources of contamination are dairy utensils and milk – contact
surfaces, including the milk pail or milking machines as the case may be strainers, milk cans or
pipelines, and the bulk-milk cooler. If dairy utensils or the milk-contact surfaces are inadequately
cleaned, sanitized and dried, bacteria may develop in large numbers in the dilute milk like residue
and enter the next milk to touch these surfaces. Undesirable bacteria from these sources include
lactic streptococci, coliform bacteria, psychrotrophic gram-negative rods and thermodurics those
which survive pasteurization, e.g. micrococci, enterococci, bacilli, and brevibacteria. In general,
these bacteria grow well in milk and hence endanger its keeping quality. When they are cleaned
and sanitized properly utensils and milk-contact surfaces add few bacteria per milliliter of milk but
under very poor conditions these sources may increase the bacterial content of milk by millions
per milliliter. Application of quaternary ammonium compounds as sanitizing agents tends to
increase the percentage of gram-negative rods on the utensils whereas hypochlorides favor grampositive bacteria. Modern dairy utensils and milk-contact surfaces particularly milking machines,
pipelines and bulk-milk coolers are designed to provide easy access for cleaning, sanitizing and
drying. Farm bulk-milk coolers are also equipped with excellent refrigeration capacity and
agitation to ensure proper cooling of the milk.
Other possible sources of contamination are the hands and the arms of the milker or dairy
workers, the air of the barn or milking parlor and flies. Normally these sources would contribute
very few bacteria but they might be a source of pathogens or spoilage microorganisms. The
quality of the farm water supply used in the milking parlor for cleaning, rinsing etc will have some
effect on the quality of the milk.
The numbers of bacteria per milliliter of milk added from the various sources depends on the care
taken to avoid contamination. For example, the exterior of the cow contributes comparatively few
organisms if precautions are taken and a milking machine is used, but under very poor conditions
thousands per milliliter could enter the milk.
Food Contamination in Fish – Shellfishes
Sea foods include fresh, frozen, dried, pickled and salted fish as well as various shellfish.
Freshwater fish are also considered.
Contamination
The flora of living fish depends on the microbial content of the waters in which they live. The
slime that covers the outer surface of fish has been found to contain bacteria of the genera
Pseudomonas, Acinetobacter, Moraxella, Alcaligenes, Micrococcus, Flavobacterium,
Corynebacterium, Sarcina, Serratia, Vibrio and Bacillus. The bacteria on fish from northern waters
are mostly psychrophiles whereas fish from tropical waters carry more mesophiles. Freshwater
fish carry freshwater bacteria which include members of most genera found in salt water plus
species of Aeromonas, Lactobacillus, Brevibacterium, Alcaligenes and Streptococcus. In the
intestines of fish from both sources are found bacteria of the genera Alcaligenes, Pseudomonas,
Flavobacterium, Vibrio, Bacillus, Clostridium and Escherichia. Boats, boxes, bins, fish houses and
fishers soon become heavily contaminated with these bacteria and transfer them to the fish during
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cleaning. The numbers of bacteria in slime and on the skin of newly caught ocean fish may be as
low as 100 and as high as several million per square centimeter and intestinal fluid may contain
from 1000 to 100 million per milliliter. Gill tissue may harbor 1000 to 1 million per gram.
Washing reduces the surface count. Oysters and other shellfish that pass large amounts of water
through their bodies pick up soil and water microorganisms in this way, including pathogens if
they are present. Alcaligenes, Flavobacterium, Moraxella, Acinetobacter and some gram- positive
bacteria will be found.
Shrimps, crabs, lobsters and similar seafood have a bacteria-laden slime on their surfaces that
probably resembles that of fish. Species of Bacillus, Micrococcus, Pseudomonas, Acinetobacter,
Moraxella, Flavobacterium, Alcaligenes and Proteus have been found on shrimp.
The numbers of microorganisms on the skin of fish can be influenced by the method of catching.
For example, trawling fish nets along the bottom for long period results in exposure of the fish to
high bacterial counts in the disturbed bottom sediment and this can be reflected in the initial
microbial load on the fish.
Fish cakes and fish sticks or similar products represent a large percentage of the consumed
seafood in the United States. Products of this type have additional sources of contamination. In
the manufacture of fish cakes various other products including potatoes, spices and flavorings are
mixed with the fish and then the product is molded, coated with batter and bread crumbs, packed
and usually frozen if not used immediately. Fish sticks are mechanically sliced from frozen blocks
of fish coated with batter and bread crumbs, packed and frozen for distribution. Many fish-stick
items are precooked in hot oil at temperatures of 204 to 232 C. The cooking process is short and
the inside of the product remains frozen. The microbial content of these products would of
course, be quite different from fresh fish as a result of contamination from the ingredients,
increased handling, machinery contact and packaging.
Destroying Micro-organisms in Food
Asepsis
In nature there are numerous examples of asepsis or keeping out microorganisms as a
preservative factor. The inner tissues of healthy plants and animals usually are free from
microorganisms and if any microorganisms are present they are unlikely to initiate spoilage. If
there is a protective covering about the food, microbial decomposition is delayed or prevented.
Examples of such coverings are the shells of nuts, the skins of fruits and vegetables, the husks of
ear corn, the shells of eggs and the skin, membranes or fat on meat or fish. It is only when the
protective has been damaged or decomposition has spread from the outer surface that the inner
tissues are subject to decomposition by microorganisms.
In the food industries an increasing amount of attention is being given to the prevention of the
contamination of foods, from the raw material to the finished product. The food technologist is
concerned with the “bioburden” of microorganisms on or in a food and considers both kinds and
numbers of organisms present. The kinds are important in that they may include spoilage
organisms, those desirable in a food fermentation or even pathogenic microorganisms. The
numbers of microorganisms are important because the more spoilage organisms there are the
more likely food spoilage will be, the more difficult will be the preservation of food and the more
likely will be the presence of pathogens. The bioburden may be the result of contamination,
growth of organisms or both. The quality of many kinds of foods is judged partly by the numbers
of microorganisms present. Following are some examples of the importance of aseptic methods
in food industries.
Packaging of foods is a widely used application of asepsis. The covering may range from a loose
carton or wrapping which prevents primarily contamination during handling to the hermetically
sealed container of canned foods which if tight protects the processed contents from
contamination by microorganisms.
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In the dairy industry, contamination with microorganisms is avoided as much as is practicable in
the production and handling of market milk and milk for other purposes and the quality of the
milk is judged by its bacterial content.
In the canning industry the bioburden or load of microorganisms determines the heat process
necessary for the preservation of a food, especially if the contamination introduces heat-resistant
spoilage organisms such as spore-forming thermophiles that may come from equipment and the
sealed can prevent recontamination after the heat treatment.
In the meat-packing industry sanitary methods of slaughter, handling and processing reduce the
load and thus improve the keeping quality of the meat or meat products.
In industries involving controlled food fermentation e.g. in cheese making, the fewer the
competing organisms in the fermenting material, the more likely the success of the fermentation.
Removal of Microorganisms
For the most part the removal of microorganisms is not very effective in food preservation, but
under special conditions it may be helpful. Removal may be accomplished by means of filtration,
centrifugation, washing or trimming.
Filtration is the only successful method for the complete removal of organisms and its use is
limited to clear liquids. The liquid if filtered through a previously sterilized “bacteria proof” filter
made of sintered glass, diatomaceous earth, unglazed porcelain, membrane pads, or similar
material and the liquid is forced through by positive or negative pressure. This method has been
used successfully with fruit juices, beer, soft drinks, wine and water.
Centrifugation or sedimentation generally is not very effective in that some but not all of the
microorganisms are removed. Sedimentation is used in the treatment of drinking water but is
insufficient by itself. When centrifugation is applied to milk, the main purpose is not to remove
bacteria but to take out other suspended materials although centrifugation at high speeds removes
most of the spores.
Washing raw foods can be helpful in their preservation but may be harmful under some
conditions. Washing cabbage heads or cucumbers before their fermentation into sauerkraut and
pickles respectively removes most of the soil microorganisms on the surface and in this way
increases the proportion of desirable lactic acid bacteria in the total flora. Washing fresh fruits and
vegetables removes soil organisms that may be resistant to the heat process during the canning of
these foods. Obviously the removal of organisms and of food for them from equipment coming
into contact with foods, followed by a germicidal treatment of the apparatus is an essential and
effective procedure during the handling of all kinds of foods. Washing foods may be dangerous if
the water adds spoilage organisms or increases the moisture so that growth of spoilage organisms
is encouraged.
Trimming away spoiled portions of a food or discarding spoiled samples is important from the
standpoint of food loss and may be helpful in food preservation. Although large numbers of
spoilage organisms are removed in this way, heavy contamination of remaining food may take
place. Trimming the outer leaves of the cabbage heads is recommended for the sauverkraut.
PRESERVATION BY USE OF HIGH TEMPERATURE
The killing of microorganism by heat is suppose to be caused by the denaturation of the
proteins and especially by the inactivation of enzymes required for metabolism. The heat
treatment necessary to kill organisms are the spores varies with the kind of organism,it state and
the environment during heating. Depending on the heat treatment employed, only some of the
vegetative cells, most are all of the cells part of the bacterial spores, are all of them may be killed.
The heat treatment selected will depend on the type of organism to be killed, other preservative
methods to be employed and the effect of heat on the food.
Certain factors are known to affect the heat resistance of cells or spores and must be kept in
mind when microorganisms are compared and when heat treatments for the destruction of an
organism are considered.
The chief known factors are as follows:
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1.The temperature time relationship.
2.Initial concentration of spores.
3.Previous history of the vegetative cells or spores,
A: cultural medium.
B: temperature of incubation
C: phase of growth or age
D: desiccation
4.Composition of the substrate in which cells or spore is heated.
A: moisture.
B: pH
I. Low-acid food
II. Medium acid foods
III. Acid foods
IV. High acid foods
5.Other constituents of the substrate.
PRESERVATION BY USE OF LOW TETPERATURE
Low temperatures are used to retard chemical reactions and action of food enzymes and to
slow down or stop the growth and activity of microorganisms in food. The lower the
temperature, the slower will be chemical reaction, enzyme action and growth; a low enough
temperature will prevent the growth of any microorganisms.
Any raw plant or animal food may be assumed to contain a variety of bacteria, yeasts, and
molds which need only conditions for growth to bring about undesirable changes in the food.
Each microorganism present has an optimal, or best, temperature for growth and a minimal
temperature below, which it cannot multiply. As the temperature drops from this optimal
temperature toward the minimal, the rate of growth of the organism decreases and is slowest at
the minimal temperature. Cooler temperature will prevent growth, but slow metabolic activity
may continue. Therefore, the cooling of a food from ordinary temperatures has a different effect
on the various organisms present. A drop of 10 degree may shop the growth of some organisms
and slow the growth of others but to an extent that would vary with the kind of organism. A
further decrease of 10 deg in temperature would stop the growth of more organisms and make
still slower the growth of the others. Low-temperature storage can therefore act as a significant
environmental factor influencing the type of spoilage flora to predominate. The growth and
metabolic reactions of microorganisms depend on enzymes, and the rate of enzyme reactions is
directly affected by temperature. The most important aspect of this temperature effect is
reflected in a decrease in the rate of growth of a microorganism when the temperature is lowered.
Growth of microorganisms at low temperatures
In general, freezing prevents the growth of most food borne microorganisms and refrigeration
temperatures slow growth rates. Commercial refrigeration temperatures, i.e., lower than 5 to 7.2
C, effectively retard the growth of many food borne pathogens. One notable exception is
Clostridium botulinum type E, which has a minimum temperature for growth of about 3.3. Yersinia
enterocolitica can survive and grow at temperatures as low as 0 to 3 C. An earlier reference
suggested a minimum temperature of 1 to 7 C. although concern has been expressed about the
low temperature growth limits of Salmonella, Mossel et al. examined the growth potential of
numerous strains and found that only one strain. Other bacterial food borne pathogens have a
minimum temperature for growth below 7.2 C, and refrigeration therefore may not be depended
on to prevent significant growth indefinitely.
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A few examples reported by various workers, of low-temperature growth of microorganisms are
of interest. Of the molds, Cladosporium and Sporotrichum have been found growing on foods at –
6.7 C and Penicillium and Monilia at –4 C. Growth by one yeast has taken place at –34C, and two
others grew at –18 C. Bacteria have been reported growing at temperatures as low as –5 C on
meats, -10 C on cured meats, -11 C on fish, -12.2 C on vegetables (peas), and –10 C in ice cream;
yeast at –5 C on meats and –17.8 C on oysters; and molds at –7.8 C on meats and vegetables and
–6.7 C on berries.
Drying
Preservation of foods by drying has been practiced for centuries. Some foods, e.g., grains, are
sufficiently dry as harvested or with a little drying remain unspoiled for long periods under proper
storage conditions. Most foods, however, contain enough moisture to permit action by their own
enzymes and by microorganisms, so that to preserve them by dryness the removal or binding of
moisture is necessary.
Drying usually is accomplished by the removal of water, but any method that reduces the amount
of available moisture, i.e., lowers the aw, in a food is form a drying. Thus, for example, dried fish
may be heavily salted so that moisture is drawn from the flesh and bound by the solute and hence
is unavailable to microorganisms. Sugar may be added, as in sweetened condensed milk, to
reduce the amount of available moisture.
Moisture may be removed from foods by any of a number of methods, from the ancient practice
of drying by the sun’s rays to the modern artificial ones. Many of the terms used in connection
with the drying of foods are rather in exact. A sun-dried food has had moisture removed by
exposure to the sun’s rays without any artificially produced heat and without controlled
temperatures, relative humidity, or air velocities. A dehydrated or desiccated food has been
dried by artificially produced heat under controlled conditions of temperature, relative humidity,
and airflow. Condensed usually implies that moisture has been removed from a liquid food, and
evaporated may have a similar meaning or may be used synonymously with the term dehydrated.
METHODS OF DRYING
Solar Drying, Drying by Mechanical Dryers, Freeze Drying, Drying during Smoking, Electronic
heating, Foam-mat drying, Tower drying.
ADDITIVES
A food additive is a substance or mixture of substances, other than the basic foodstuff, which is
present in food as a result of any aspect of production, processing, storage or packing. The term
does not include chance contamination. This definition emphasizes one interpretation of a food
additive; i.e., it is an intentional additive. Those food additives which are specifically added to
prevent the deterioration or decomposition of a food have been referred to as chemical
preservatives. These deteriorations may be caused by microorganisms, by food enzymes, or by
purely chemical reactions. The inhibition of the growth and activity of microorganisms is one
of the main purposes of the use of chemical preservatives. Preservatives may inhibit
microorganisms by interfering with their cell membranes, their enzyme activity, or their genetic
mechanisms. Other preservatives may be used as antioxidants to hinder the oxidation of
unsaturated fats, as neutralizers of acidity, as stabilizers to prevent physical changes, as firming
agents, and as coatings or wrappers to keep out microorganisms, prevent loss of water, or hinder
undesirable microbial, enzymatic, and chemical reactions.
In addition to the chemicals intentionally added to foods or put on them or around them to help
preserve them, there are many chemicals that get on or into foods during production, processing,
or packaging. Residues of pesticides, herbicides, and fungicides on fruits and vegetables; residues
of detergents used in washing foods; and residues of detergents and sanitizers used on utensils
and equipment are likely to carry over into foods.
Factors that influence the effectiveness of chemical preservatives in killing microorganisms or
inhibiting their growth and activity are similar to the effectiveness of heating: (1) concentration of
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the chemical, (2) kind, number, age, and previous history of the organism, (3) temperature, (4)
time, and (5) the chemical and physical characteristics of the substrate in which the organism is
found (moisture content, pH, kinds and amounts of solutes, surface tension, and colloids and
other protective substances). A chemical agent may be bactericidal at the certain concentration,
only inhibitory at a lower level, and ineffective at still greater dilutions.
Organic Acids and their salts
Lactic, acetic, prop ionic, and critic acids or their salts may be added to or developed in foods.
Their development in foods during fermentation will be discussed in a following section. Citric
acid is used in syrups, drinks, jams, and jellies as a substitute for fruit flavors and for preservation.
Lactic and acetic acids are added to brines of various kinds, green olives, etc.
Additives used in various food products.
Propiontes (sodium or calcium)- butter, jams, jellies, figs, apple slices etc.,
Benzoates (sodium)- jams, jellies, margarine, carbonated beverages, fruit salads, pickles, fruit
juices, etc.
Sorbates ( Sorbic acid)- chesses, cheese products, baked goods, beverages, syrups, fruit cocktails,
dried fruits, pickles, and margarine.
Acetates (Derivatives of acetic acid in the form of vinegar)- mayonnaise, pickles, pickled
sausages,etc.,
Sugar, salt, Alcohol, Spices and Other Condiments are used as natural preservatives.
RADIATION
In their search for new, improved methods of food preservation, investigators have paid special
attention to the possible utilization of radiations of various frequencies, ranging from lowfrequency electrical current to high-frequency gamma rays. Much of this work has focused on the
use of ultraviolet radiation, ionizing radiation and microwave heating.
It is common to group the entire spectrum of radiation into two categories, one on each side of
visible light. Low frequency, long-wavelength, low-quantum energy radiation ranges from radio
waves to infrared. The effect of these radiations on microorganisms is related to their thermal
agitation of the food. Conversely, the high frequency, shorter-wavelength radiations have high
quantum energies and actually excite or destroy organic compounds and microorganisms without
heating the product. Microbial destruction without the generation of high temperatures suggested
the term “cold sterilization”.
When applied to the food industry, shorter-wavelength radiation can be further divided into two
groups. Lower-frequency and lower-energy radiation, for example, the ultraviolet part of the
spectrum, has sufficient energy only to excite molecules. This area of the spectrum is employed
in the food industry and is covered in the section on ultraviolet irradiation. Radiations of higher
frequencies have high-energy contents and are capable of actually breaking individual molecules
into ions, hence the term ionizing irradiation.
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