Transcript Control by Physical Removal

Food Biotechnology
Dr. Kamal E. M. Elkahlout
Food Microbiology 2
Control of Microorganisms in
Food
1
• Definitions
• Controlling access of
microorganisms
• Control By Physical Removal
• Centrifugation
• Filtration
• Trimming
• Washing
• Control By Heat
• Low-heat processing or
pasteurization.
• High-heat processing
• Microwave Heating
•
•
•
•
•
•
Control By Low Temperature
Ice Chilling
Refrigeration
Freezing
CONTROL BY REDUCED Aw
Control by low pH and Organic
acids
• Control by Modified
Atmospheric (O-R potential)
• Control by Irradiation
• Control by antimicrobial
preservatives
2
Definitions
• Sterilization: The process by which all the living cells,
viable spores, viruses, and viroids are either destroyed
or removed from an object or habitat.
• Disinfection: Is the killing, inhibition or removal of
microorganisms that may cause disease. Disinfectants
are usually chemical agents, and are normally used only
on inanimate objects.
• Sanitization: Microbial population is reduced to levels
that are considered safe by public health standards.
• Antiseptics: Chemical agents applied to tissue to prevent
infection by killing or inhibiting pathogen growth.
• Bactericide: A disinfectant/ antiseptic against bacteria.
Cidal: A suffix meaning that “the agent kills.”
For
example, a bacteriocidal agent kills bacteria.
• Bacteriostatic: Do not kill, but prevent growth of bacteria.
Static: A suffix that means “the agent inhibits growth.”
For example, a fungistatic agent inhibits the growth of
fungi, but doesn’t necessarily kill it.
• Some microorganisms are desirable
– for the production of bioprocessed foods
• Many are undesirable due to their ability to
cause food spoilage and food borne diseases
• Several methods (individually or in
combination) are used to achieve control
• These are:
• Controlling access of the microorganisms
present in foods.
• Control by physical removal.
• Control by heat.
• Control by low temperature.
• Control by reduced Aw
• Control by low pH and Organic acids
• Control by Modified Atmospheric (O-R
potential)
• Control by Irradiation
• Control by antimicrobial preservative
Controlling access of
microorganisms (Cleaning and
Sanitation)
7
Controlling access of microorganisms
(Cleaning and Sanitation)
To minimize the access of microorganisms in foods:
• the microbial quality of the environment to which a food is
exposed (food contact surfaces) should be good.
• The ingredients added to the food should be of good
microbial quality.
• Sanitation minimizes the access of microorganisms in food
from various sources at all stages of food handling.
• Proper sanitation helps to produce food that have a long
shelf life.
Plant Design
• When designing a food processing plant, an efficient sanitary
program must be integrated in order to provide maximum
protection against microbial contamination of foods.
• This includes both the outside and the inside of the plant.
- floor plan and approved materials used in construction.
- adequate light, air ventilation, direction of air flow.
separation of processing areas of the raw and finished products.
- sufficient space for movement and operations.
- water supply and sewage disposal system, waste
treatment facilities, drainage and surrounding
environment.
Quality of Water, Ice, Brine and Curing
Solution
• Water is the most important element in food
manufacturing operations.
• Water is used as an ingredient in many foods
• also used in some products after heat treatment.
• Eg: ready-to-eat types, should not only be
free from pathogens (like drinking water),
but also be low (if not free) in spoilage
bacteria, such as Pseudomonas spp.
• Important for foods that are kept at low temperature
for extended shelf life.
– Eg., ice used for chilling unpackaged foods should
also not contaminate a food with pathogenic and
spoilage bacteria.
• Brine and curing solutions used in products such as
cured beef can be a source of contamination hence
should be made fresh daily to be used for processing.
Quality of Air
• Food processing operations, such as spray
drying of nonfat dry milk, require large
volumes of air that come into direct
contact with the food.
• Important to install air inlets to obtain dry
air with least amount of dust and filtration
of air.
Training of Personnel
A processing plant should:
• Have an active program to teach the plant personnel the
importance of sanitation and personal hygiene in-order to
ensure product safety and stability.
• Also monitor the implementation of such program.
• People with an illness and infection should be kept away
from handling the food products.
Equipment
• design of food processing equipment should
protect a food from microbial contamination.
• Protection is achieved if the equipment
– does not contain dead spots where
microorganisms harbor and grow and cannot be
easily and readily cleaned in place or by
disassembling.
• Some of the equipments such as meat grinders,
choppers, slicers and several types of conveyor
systems not properly sanitized can be a source of
contamination.
• Equipment sanitizing is important for products that
come in contact with equipment surfaces after
treatment and before packaging.
Control
by
Physical Removal
16
Centrifugation
• A process used to separate or concentrate materials
suspended in a liquid medium.
• The technique is based on the effect of gravity on
particles in suspension. Two particles of different
masses will settle in a tube at different rates in
response to gravity.
• Centrifugation - used in some liquid foods, such as
milk, fruit juices and syrup,to remove suspended
undesirable particles (dust, leukocytes and food
particles).
•
17
18
• Under high forces, as much as 90% of the microbial
population can be removed.
• Following centrifugation, a food will have fewer
thermoduric microorganism (bacterial spores) that
otherwise would have survived mild heat treatment
(e.g. milk pasteurization).
19
Filtration
• Filtration - used in some liquid foods,
– such as soft drinks,
– fruit juices, beer,
– wine and water
• to remove undesirable solids and
microorganisms and to give a sparkling clear
appearance.
20
• As heating is avoided (or given only at minimum levels,)
the natural flavor of the products and heat- sensitive
nutrients (e.g. vitamin C in citrus juices) are retained to
give the products natural characteristics.
• Coarse filters are initially used to remove the large
component, followed by ultra-filtration to remove small
particles.
21
• Filtration of air
– used in food processing operations.
– such as spray drying milk; to remove dust from air
used for drying.
– The process removes microorganisms with dust and
they reduce the microbial level in food from source
(air)
22
Trimming
• Fruits and vegetables
showing damage and spoilage
are generally trimmed.
• Areas heavily contaminated with microorganisms are
removed.
• Trimming of outside leaves in cabbage helps reduce
microorganisms coming from soil.
23
• Trimming is also used to remove visible mold growth
from hard cheeses, fermented sausages, bread and
some low pH products.
• If a mold strain is a mycotoxin producer, trimming will
not ensure removal of toxins from the remaining food.
• Trimming is also used to remove fecal stain marks,
unusual growths and abscesses or small infected areas
from carcasses of food animals and birds.
• Trimming allows complete removal of the causative
microorganisms.
24
Washing
• Washing equipment and work areas is discussed
under cleaning and sanitation.
• Fruit and vegetables are washed to reduce
temperature (that helps to reduce metabolic rate of
a produce and microbial growth) and remove soil.
• Washing removes the microorganisms present,
especially from the soil. It is also used for shell eggs
to remove fecal materials and dirt.
25
CONTROL BY HEAT
26
• The desirable effect of heat (fire) on the taste of
foods of animal and plant origin, especially seeds,
tubers and roots, was probably accidentally
discovered by our ancestors .
– They also possibly recognized that heated foods
did not spoil as fast as raw foods.
27
• The main objective (microbiological) of heating food
is to destroy vegetative cells and spores of
microorganisms that include molds, yeasts, bacteria
and viruses.
• Drastic heat treatment (sterilization) can be used to
kill all the microorganisms, which is present in a
food.
• Most foods are heated to destroy – pathogenic and
spoilage microorganisms
28
Antimicrobial Action of Heat


Depending upon the temperature and time
of heating, microbial cells and spores can
be sub-lethally injured or dead.
Death occurs from damages in vital
functional and structural components.
29
Factors Affecting Heat killing of
microbial cells

The effectiveness of heat in killing
microbial cells and spores is dependent on
factors:

related to the inherent nature of the foods

on both the nature of microorganisms and the
nature of processing.
30
1.
Nature of Food

Composition (amount of carbohydrates, proteins,
lipids and solutes),
Aw (moisture),
pH, and anti-microbial content (natural or added)
greatly influence microbial destruction



Microorganisms in liquid food and food containing
small-sized particles suspended in a liquid are
more susceptible to heat destruction than in a solid
food or in a food with large chunks.
31
2. Nature of Microorganisms

Factors that influence microbial sensitivity to heat is
inherent resistance, stage of growth, previous
exposure to heat and initial load.

In general vegetative cells (moulds, yeasts and
bacteria) are more sensitive than spores

thermoduric and thermophilic bacterial cells
(important in foods) are destroyed in 5 to 10
minutes at 75 to 80°C
32



Yeast and most mould spores are destroyed
at 65 to 70°C in a few minutes,
Spores of some moulds can survive as high as
90°C for 4 to 5 h.
Bacterial spores varies greatly



Heating at 80 to 85°C for few minutes does
not kill.
But destroyed at 100°C in 30 min however
some can withstand this
Destroyed at 121°C in 15min (sterilization
Temp /Time)
33



Cells at exponential stage of growth are more
susceptible to heat than the resting cells
(stationary phase)
Cells previously exposed to low heat become
relatively resistant to subsequent heat
temperature.
The higher the initial microbial load in a food –
the longer time at a given temperature it takes
to reduce the population.
34
3. Nature of Process

Microbial destruction in food by heat ( inverse
relationship). Higher the temperature, the shorter
the period of time required for destroying the
microorganisms provided other factors are kept
constant.

As a food is heated by conduction (molecule-tomolecule energy transfer) and convection
(movement of heated molecules), a liquid food is
heated more rapidly than a solid food and a
container with high conduction (metal) is better. 35

Food in a small container is heated more
rapidly than in a large container

Heating a food at a given temperature for a
specific time means that every particle of
that food should be heated to the specified
temperature and stay at that temperature for
the specified time –”holding time”.
36
Methods using heat
Low-heat processing or pasteurization.
• temperature used is below 100oC.
• Process aims to destroy all vegetative cells of
pathogens and microorganism which cause food
spoilage.
• Pasteurization of milk has been used for a long time –
heating at 62.8oC for 30 mins or 71.7 oC for 15 secs.
37
Methods using heat
High-heat processing



Process involves heating food at or above 100oC.
Temperature and time of heating are selected on
the basis of product characteristics and specific
microorganisms to be destroyed.
Most products are given a commercially sterile
(sterilization) treatment to destroy to destroy
microorganism growing in a product under
normal storage conditions.
38
• High-heat treated products are either first packed in
containers and then heated or heated first and then
packed in sterile containers while still hot (hot pack).
• Commercial sterility is also obtained by heating a
food at very high temperatures for a short time
(process called ultrahigh temperature (UHT)
processing .
39
Methods using heat
Microwave heating

Heating of foods by microwave (quite common at home).

Frozen foods can be thawed and heated rapidly in a few
minutes depending upon the size of the product.

Microwave treatment is lethal to microorganisms and
destruction is caused by high temperature.

If the food is not heated uniformly, some areas can remain
cold and if food harbors pathogens, there is chance of
their survival.

40
CONTROL BY LOW
TEMPERATURE
41
• Effectiveness of low temperature, especially freezing
in food preservation was probably recognized by our
ancestors in the last Ice Age.
• The major drawback of refrigerated goods is their
relatively short shelf life.
• But in recent years, several technological
improvements have helped in increasing the shelf
life.
42
Mechanisms of microbial control
• Metabolic activities, enzymatic reactions and growth
rates of microorganisms are maximum at optimum
growth temperatures.
• When temperature is lowered, microbial activities
associated with growth slow down.
• Rate of catalytic activity of enzymes decreases with
reduced temperature.
• As the temperature in a food drops to about – 2 oC, free
water in the food starts freezing and forming ice crystals,
hence Aw is also reduced.
43
• Foods are stored at low temperature in
different ways in order to extend their shelf life.
• Many fresh fruits and vegetables are kept at
temperatures between 10oC and 20oC or lower
to reduce their metabolic rates.
• Highly perishable products are generally stored
at low temperature below 7 oC often in
combination with other preservation methods.
44
Methods using heat
Ice chilling

Usually used Retail stores where foods are kept
over ice.

The surface is in contact with ice, temperature
can reach between 0oC – 1oC.

Temperature fluctuation, duration of storage and
cross contamination can cause microbiological
problems – food borne pathogens.
45
Methods using heat
Refrigeration

The temperature specification for refrigeration of
foods has changed from time to time.

From 7oC, technological improvements have
made it economical to have domestic
refrigeration units at 4 to 5 oC.

For perishable products, ≤ 4.4oC is considered
desirable refrigeration temperature.
46
• Commercial food processors may use as low as 1 oC for
refrigeration of perishable foods such as fresh meat and
fish.
• Refrigerated products are often combined with
additional preservation methods with lowest
temperature possible for long shelf life.
• As the products are non-sterile, even a very low initial
microorganism population is capable of growing under
the storage conditions.
47
Methods using heat
Freezing

Minimum temperature used in home freezers is –
20 oC, a temperature at which most of the free
water in a food remains in a frozen state.

Dry ice ( -78oC) and liquid nitrogen (- 196oC) can
also be used for instant rapid freezing, but not for
food.

After freezing, the temperature of the food is
maintained around -20oC to -30oC.
48
• Microbial cells will die upon during frozen
storage, but survivors can multiply in the frozen
state.
• Accidental thawing or slow thawing can facilitate
growth of survivors.
• Enzymes released by dead microbial cells can
reduce the acceptance quality of food.
49
CONTROL BY REDUCED
Aw
• The main objective of reducing Aw in food are
to prevent or reduce the growth of vegetative
cells and germination and outgrowth of
spores of microorganisms
• Microorganisms need water for the transport
of nutrients, nutrient metabolism and removal
of cellular wastes.
• In a food, the total water (moisture) is present
as free water and bound water.
• Recall:
Bound water: not available for biological functions.
• Only the free water (related to Aw) is important for
microbial growth.
• If free water in the environment is reduced either by
removing water or by adding solutes and hydrophilic
colloids, the free water from the cells flow outside in an
effort to establish equilibrium. (osmosis).
• The loss of water will cause osmotic shock and
plasmolysis during which the cells do not grow.
• Water loss can be considerable even with a
slight change in Aw
E.g 0.005 reduction in Aw from 0.955 to 0.950
in the environment reduced the intracellular
water content by 50% in Staphylococcus
aureus and reduces the cell volume by 44% in
Sal. typhimurium.
Methods Used
• Water activity of foods can be reduced by
using one or more of three basic principles:
– removing water by dehydration
– removing water by crystallization
– by adding solutes.
Natural Dehydration
• is a low-cost method in which water is removed by
the heat of the sun.
• Used for dry grains as well as for dry some fruits
(raisins) vegetables, fish, meat, milk and curd
especially in warmer countries.
• The process is slow and depends upon the conditions
used, spoilage and pathogenic bacteria as well as
yeasts and molds (including toxigenic types) can
grow during drying.
Mechanical Drying
•
is a controlled process and drying is achieved in a few seconds to
a few hours.
– Tunnel drying in which a food travels through a tunnel against
the flow of hot air and the water is removed.
– Roller Drying in which a liquid is dried by applying in a thin
layer on the surface of a roller drum heated from inside.
– Spray Drying, liquid is sprayed in small droplets, which then
come in contact with hot air that dries the droplets instantly.
Used for vegetables, fruits, fruit juices, milk, coffee, tea and meat.
• Depending on Temp & time of exposure, some microbial cells die
during drying, while some other cells can be sublethally injured.
Freeze-Drying
• Freeze-drying involves freezing the food rapidly at a
low temperature and then exposing the frozen food
to a relatively high vacuum environment.
• The water molecules are removed from the food by
sublimation without affecting its shape or size.
• Microbial cells are exposed to two stresses
– freezing and drying that reduces some viability as
well as induces some sub-lethal injury.
Foam Drying
•
The foam drying method consists of whipping a product to
produce a stable foam and increase to the surface.
• The foam is then dried by means of warm air.
•
Liquid products, such as egg white, fruit purees and tomato
paste are dried in this manner.
• The method itself has very little lethal effect on microbial cells
and spores.
• However, a concentration method prior to foaming,
– the pH of the products and low Aw will cause both lethal
and reversible damages to microbial cells.
Smoking
• Many meat and fish products are exposed to low heat and
smoke for cooking and depositing smoke on the surface at the
same time.
• The heating process removes water from the products
lowering their Aw.
• Low heat processed meat products (dry and semidry
sausages) and smoked fish are produced this way.
• Heat kills many microorganisms.
• Growth of survivors is controlled by low Aw as well as
antimicrobial substances present in the smoke.
Intermediate Moisture Foods (IMF)
• These are foods that have an Aw value of 0.70 to 0.90
(moisture content, ∼10 to 40%).
• Can be eaten without rehydration, are shelf-stable for a
relatively long period of time without refrigeration and
are considered microbiologically safe.
• Traditional IMF includes semidry and dry sausages, dried
fruits jam and jellies and honey.
• Low Aw value is obtained by adding water-binding
solutes and hydrophilic colloids.
• Microorganisms can survive in the products but due to
low Aw bacteria cannot grow.
• Yeasts and molds can grow, to inhibit their growth
specific preservatives such as sorbate and propionate
are added.
CONTROL BY LOW pH AND
ORGANIC ACIDS
62
• The major antimicrobial objective of using weak organic
acids is to reduce the pH of food in order to control
microbial growth.
• As the pH drops below 5.0, some bacteria die.
• A pH lower than the minimum growth pH prevents
microbial growth by affecting energy production, enzymatic
activity, transportation of nutrients and others.
• In addition to controlling growth, microbial cells are
sublethally injured or killed by low pH.
• Many food-grade organic acids along with other methods
are used to control microbial growth in foods.
• Some acids which are used include:
63
• Acetic Acid
• Acetic acid is usually used as vinegar (5 to 10% acetic acid)
or as salts of sodium and calcium at 25% or higher levels. It
is more effective against bacteria than against yeast and
molds. Those bacteria that grow better above pH 6.0 are
more inhibited.
• The inhibitory action of acetic acid is produced through
neutralizing the electrochemical gradient of the cell
membrane as well as denaturing proteins inside the cells.
• Besides its use in food, acetic acid has been recommended
for use (1 to 2% levels) in carcass wash to reduce bacterial
level.
64
– Propionic Acid
• Propionic acid is used as salts of calcium and
sodium at a level of 1000 to 2000 ppm (0.1 to
0.2%). It is effective against molds and bacteria and
almost ineffective against yeast at concentrations
used in foods.
• Antimicrobial action is produced through the
acidification of cytoplasm as well as destabilization
of membrane proton gradients.
65
– Lactic Acid
• Lactic acid is used as acid or the sodium salt up to 2%.
• It is less effective than acetic, propionic, benzoic or sorbic
acids but more effective than citric acid.
• Effective against bacteria yet quite ineffective against yeast
and molds.
• Produces an inhibitory effect mainly by neutralizing the
membrane proton gradient.
• Sodium salt of lactic acid may also reduce Aw.
• Recommended at 1 to 2% levels to wash carcasses of food
animals to reduce microbial load
66
– Citric Acid
• Citric acid is used at 1% level. It is less effective
than lactic acid against bacteria as well as yeast and
molds.
• It produces an antibacterial effect probably by
different mechanisms than the lipophilic acids. The
antibacterial effect is partially due to its ability to
chelate divalent cations. However it must have
sufficient divalent cations to neutralize this effect.
67
– Sorbic Acid
• Sorbic acid is an unsaturated acid and used either as acid
or as salt of sodium, potassium or calcium.
• Vary between 500 and 2000 ppm (0.05 to 0.2%).
• More effective against molds and yeast than against
bacteria.
– Benzoic Acid
• Benzoic acid is used as an acid or a sodium salt at a
concentration of 500 to 2000 ppm (0.05 to 0.2%) in many
low pH products.
• It is more effective against yeasts and molds than against
bacteria.
68
• Antimicrobial effectiveness
• Acetic acid > propionic acid > lactic acid > citric acid.
• Low pH is also used to prevent germination of
bacterial spores in food.
• Although many gram-negative pathogenic bacteria
are very sensitive to low pH, it cannot be used to
eliminate these pathogens during storage of food.
• Some strains of bacteria can become acid resistant.
69
CONTROL BY MODIFIED
ATMOSPHERE (OR REDUCING O-R
POTENTIAL).
70
• The use of modified atmosphere to reduce O-R
potential of the environment has been a very
widely used method to control growth mainly of
aerobic microorganisms in food.
• Modified atmosphere packaging (MAP) controls or
reduces the growth of undesirable microorganisms
in food by retarding the enzymatic and respiratory
activities.
• The growth of aerobes (molds, yeast and aerobic
bacteria) are prevented in products that are either
vacuum-packaged or flushed with – 100% CO2, –
100% N2, or a mixture of CO2 and N2.
71
• In these conditions, anaerobic and facultative
anaerobic bacteria can grow unless other
techniques are used to control their growth.
• The anti-microbial action in modified atmosphere
packaging (MAP) foods can be produced by the
changes in redox potential (Eh) and CO2
concentrations based on the methods used.
• When CO2 is used in high concentration (20 to
100%) alone or in combination with N2 and/or O2
the shelf life of MAP foods is also extended.
• Several mechanisms been proposed:
72
• a. Rapid cellular penetration of CO2 and alteration
in cell permeability
• b. Solubilization of CO2 to carbonic acid (H2 CO3 ) in
the cell with the reduction of the pH inside the cells
• c. and interference of CO2 with several enzymatic
and biochemical pathways, which in turn slow the
microbial growth rate.
73
• Vacuum Packaging
• Vacuum packaging is predominantly used in retail
packs in many fresh and ready-to-eat meat
products.
• Red meat, due to change in color to purple
(reduced myoglobin) is not very popular with
consumers. The refrigerated storage life of these
products varies greatly:
• - in fresh meat, about 3 to 4 weeks;
• - while in processed meats, as long as 8 weeks or
more longer.
74
• Gas Flushing
• Gas flushing method has been used in both bulk
and retail packs to increase the shelf life of many
refrigerated foods.
• Gases usually used a CO2 and N2 mixture, with
some O2 for packaging red meats.
• Raw Meats, – a composition of 75% CO2, 15% N2
and 10% O2 was found to effectively prevent
growth of Pseudomonas fragi flushed with CO2
alone were found to be dominated by lactic acid
bacteria, principally Leuconostoc spp and
Lactobacillus spp.
75
CONTROL BY IRRADIATION
76
• Irradiation is the process of exposing food to
ionizing radiation to destroy microorganisms,
bacteria, viruses, or insects that might be present in
the food.
• Irradiation like heat kills microbial cells and destroys
their spores at a predictable rate that is basically
dependent on dose level, exposure time and
microbial type.
• A food is irradiated because of the destructive
power of the microorganisms it harbors.
• Depending on the method used, it can either
completely or partially destroy molds, yeast,
bacterial cells and spores and viruses.
77
• Irradiation is capable of destroying worms, insects
and larvae in food, prevents sprouting of some
foods such as potatoes and onions.
• Irradiation cannot destroy toxins or undesirable
enzymes in food and in that respect it differs from
heat treatment.
• Irradiation is a cold sterilization process in as much
as the temperature of a food does not increase
during treatment and thus the irradiated foods do
not show some of the damaging effects of heat on
food quality.
78
• However, irradiation can cause oxidation of lipids
and denature food proteins, especially when used
at higher doses.
• Some commonly used irradiation mechanisms
include:
• γ-rays
• When exposed to high-energy γ-rays (10-1 to 10-2
nm), the energy is absorbed by thousands of atoms
and molecules in a fraction of second, which strip
electrons from them.
• This produces negative and positive ion pairs.
79
• The released electrons can be highly energized and
thus can remove electrons from other atoms and
convert them into ions.
• This energization and ionization can adversely affect
the normal characteristics of biological systems.
• The ionizing radiation produces both direct and
indirect effects on microorganisms
• Direct effect includes DNA damage and breakdown
of other important molecules.
• Indirect effect includes death of microbial cells by
ionizing radiation similar as in heat treatment.
80
• UV Radiation
• When microorganisms are exposed to UV radiation
(≈260 nm), the energy is absorbed by the
nucleotide bases in the DNA. The bases can react
with each other to form dimers (e.g. thymine
dimers) and cause DNA strand breaks.
• Microbial death and injury are mainly associated
with DNA damage.
• Dosage
• The current unit is Gray (Gy) .
• The relative sensitivity of microorganisms to
irradiation dose is a function of their size and water
content.
81
• – Approximate lethal dose levels for insects and
different microorganisms have been suggested as
follows:
• – Insects, ≤1 kGy;
• – Molds, yeast, bacterial cells, 0.5 to 10 kGy;
• Viruses, 10 to 200 lGy
• Recommended level of 10 kGy, Clostridium
botulinum spores are not destroyed in foods (need
about 30 to 60 kGy) although cells of pathogenic
spoilage bacteria are destroyed.
82
• Some Specific Terms
• Radurization- Radiation pasteurization is intended
to destroy spoilage bacteria in high pH-high Aw
foods, especially Gram negative psychrotophs in
meat and fish and yeast and molds in low ph-low
Aw foods. The treatment is generally milder.
Products should be packaged and chilled to prevent
growth of pathogens, which were previously
thought to be mesophiles.
• Radicidation- Radiation of foods to destroy
vegetative food borne pathogens. The dose level
used is about 2.5 to 5.0 kGy.
83
• Although it is effective against pathogenic
vegetative bacterial cells and molds, spores of the
pathogens will not be destroyed.
• Radappertization- This method involves the
radiation of food at high doses (≈30 kGy) to destroy
Clo. botulinum spores in order to get safety similar
to 12D heat treatment. However, this is not
recommended for use in food.
Figure 1. Logo for irradiated foods used to
show a food has been treated with ionizing
radiation.
84
CONTROL BY ANTIMICROBIAL
PRESERVATIVES
85
• Antimicrobial chemicals are used in food in
relatively small doses either to kill undesirable
m/organisms or to prevent or retard their growth in
food.
• They differ greatly in their ability to act against
different m/organisms.
• Those that are capable of killing m/organisms are
designated as germicides (kill all types), fungicides,
bactericides, sporicides and viricides depending
upon their specificity of killing actions against
specific group.
86
• They Inhibit or retard microbial growth and are
classified as fungistatic or bacteriostatic.
• Foods contain antimicrobial preservatives in three
ways:
• – Present naturally
• – Formed during processing
• - Added as ingredients
– These are to be approved by the regulatory agencies
87
• Examples of Antimicrobial preservatives.
• Nitrite (NaNO2 and KNO2)
• - Curing agents contains nitrite with NaCl, sugar,
spices, ascorbate and erythobate are permitted for
use in heat-processed meat, poultry and fish
product, to control growth and toxin production by
Clostridium botulinum.
• - Antibacterial action of nitrites include reaction
with enzymes in the vegetative cells and
germinating spores, restriction of the bacterial use
of iron, interference with membrane permeability
limiting transport across membranes.
88
• - Antibacterial effects enhanced at low pH (5.0-6.0),
in presence of reducing agents (ascorbate,
erythorbate and cysteine), by addition of sorbate,
by reducing Aw, low Eh.
• Sulfur dioxide (SO2) & Sulfites (SO3)
• - Sulfur dioxide, sodium sulfite (Na2SO3), sodium
bisulfide (NaHSO3) and sodium metabisulfite
(N2S2O5) are used to control m/organisms in soft
drinks, beverages, wines, sausages, fresh shrimps
• - They are more effective against molds and yeasts
than bacteria.
89
• - Antimicrobial action is produced by sulfurous acid which
enters the cell and reacts with thiol group in structural
protein, enzymes and cofactors and other cell components.
• Fungicidial effect – more pronounced at low pH (pH < 4.5)
and low Aw.
• In bacteria it is effective at high pH (> 5), low concentration
– are bacteriostatic and high concentration – are
bactericidal.
• Recommended level – 200-300ppm
• Sulfur dioxide and sulfites are also used as antioxidants.
• People with respiratory problems can be mildly to severely
allergic. The products needs to be labeled to show
presence of sulfites.
90
• H2O2
• Solution of H2O2 (0.05-0.1%) recommended in raw
milk to be used in cheese processing, liquid egg
(destruction of Salmonella) + low heat
pasteurization, packaging materials in aseptic
packaging of foods & food processing equipments.
• Epoxides
• Ethylene oxide and propylene oxide are used as
fumigants (destroy microorganisms + insects)
• Germicidal and effective against cells, spores and
viruses
91
– Antioxidants
• Includes – butylated hydroxyansiol (BHA), butylated
hydroxytoluene (BHT), t-butylhydroquinone (TBHQ) used at
200ppm or less to delay oxidation of unsaturated lipids.
• At a Concentration Range from 50-400ppm, BHA – inhibits
growth of Gram positive and Gram negative bacteria.
• used effectively to prevent growth & toxin production by
molds and growth of yeasts.
• Antimicrobial Action – has adverse effect on the cell
membrane and enzymes. Antimicrobial effectiveness
increases in the presence of sorbate and decreases in foods
with high lipids and low temp.
92
• Chitosan
• A polycationic polymer that is obtained by Alkaline
hydrolysis of chitin from the shells of crustaceae. It
is used preservation due t its antimicrobial
capability
• Antimicrobial action - Causes destabilization of the
cell wall and cell membrane functions.
• Effective against bacteria, yeast and molds.
93
• Ethylenediaminetetraacetate (EDTA)
• Sodium & Ca salts of EDTA are approved for use in foods to
chelate trace metals in order to prevent their adverse effect
on food quality.
• EDTA may not have much antimicrobial effect but due to its
ability to chelate divalent cations it can destabilize the
barrier functions of the outer membrane of the Gram
positive and cell wall of Gram negative bacteria.
• It enhances the antibacterial action of other chemicals
those that can act on cell membranes.
• EDTA, also inhibitory for germination and outgrowth of
spores of Clo.botulinum.
94
• Lysozyme
• Enzyme lysozyme (a muramidase) present in egg
white, shellfish (oyster & clams) & in small amounts
in milk and some plants
• Antimicrobial action- disrupts the cell cellwall of
Gram positive and Gram negative bacteria
• Antimicrobial effect is manifested by the lysis of
cells.
• Lysozyme is most effective at pH 6.0-7.0 and at
concentration of about 0.01 to 0.1%.
• Used in wine (sake) to prevent the growth of
undesirable lactic acid bacteria.
95
• Antibiotics
• Tetracyclines – 10ppm as recommended by FDA to extend
the refrigerated shelf life of seafoods and poultry. However
due to the possible increase in antibiotic resistant bacteria
the use of this antibiotic was banned.
• Natamycin – a microlid produced by Streptomyces
natalensis – an antifungal agent. Used as a dip or spray to
prevent growth of molds and formation of mycotoxins on
the surface of cheese & sausages.
• Tylosin – a microlid that inhibits protein synthesis. This
bacterial antibiotic that is more effective against Gram
positive bacteria than Gram negative bacteria.
96