Transcript Control by Physical Removal

Food Biotechnology
Dr. Kamal E. M. Elkahlout
Food Microbiology 2
Control of Microorganisms in
Food
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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
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Definitions
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Sterilisation: 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.
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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.
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Some microorganisms are desirable
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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
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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)
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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
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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.
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Important for foods that are kept at low
temperature for extended shelf life.
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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 ham, bacon, cured beef etc 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:
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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
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design of food processing equipment
should protect a food from microbial
contamination.
Protection is achieved if the equipment
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does not contain dead spots where
microorganisms harbor and grow and cannot
be easily and readily cleaned in place or by
disassembling.
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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
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Centrifugation
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A process used to separate or concentrate
materials suspended in a liquid medium.
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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.
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Centrifugation - used in some liquid foods, such as
milk, fruit juices and syrup,to remove suspended
undesirable particles (dust, leukocytes and food
particles).
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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).
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Filtration
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Filtration - used in some liquid foods,
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such as soft drinks,
fruit juices, beer,
wine and water
to remove undesirable solids and
microorganisms and to give a sparkling
clear appearance.
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As heating is avoided (or given only at minimum
levels,) the natural flavor of the products and heatsensitive 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.
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Filtration of air
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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)
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Trimming
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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.
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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.
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Washing
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Washing equipment and work areas is discussed
under cleaning and sanitation (last week).
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.
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CONTROL BY HEAT
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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 .
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They also possibly recognized that heated foods
did not spoil as fast as raw foods.
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The main objective (microbiological) of heating food
is to destroy vegetative cells and spores of
microorganisms that include molds, yeasts, bacteria
and viruses.
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Drastic heat treatment (sterilization) can be used to
kill all the microorganisms, which is present in a
food.
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Most foods are heated to destroy – pathogenic and
spoilage microorganisms
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Antimicrobial Action of Heat
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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.
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Factors Affecting Heat killing of
microbial cells
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The effectiveness of heat in killing
microbial cells and spores is dependent on
factors:
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related to the inherent nature of the foods
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on both the nature of microorganisms and the
nature of processing.
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1.
Nature of Food
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Composition (amount of carbohydrates, proteins,
lipids and solutes),
Aw (moisture),
pH, and anti-microbial content (natural or added)
greatly influence microbial destruction
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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.
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2. Nature of Microorganisms
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Factors that influence microbial sensitivity to heat is
inherent resistance, stage of growth, previous
exposure to heat and initial load.
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In general vegetative cells (moulds, yeasts and
bacteria) are more sensitive than spores
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thermoduric and thermophilic bacterial cells
(important in foods) are destroyed in 5 to 10
minutes at 75 to 80°C
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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
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Heating at 80 to 85°C for few minutes does
not kill.
But destroyed at 100°C in 30 min h/ever
some can withstand this
Destroyed at 121°C in 15min (sterilization
Temp /Time)
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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.
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3. Nature of Process
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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.
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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
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Food in a small container is heated more rapidly
than in a large container
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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”.
see the reason for heating food before eating
!!!!!!
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Methods using heat
Low-heat processing or pasteurization.
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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.
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Methods using heat
High-heat processing
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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.
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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 .
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Methods using heat
Microwave heating
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Heating of foods by microwave (quite common at home).
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Frozen foods can be thawed and heated rapidly in a few
minutes depending upon the size of the product.
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Microwave treatment is lethal to microorganisms and
destruction is caused by high temperature.
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If the food is not heated uniformly, some areas can remain
cold and if food harbors pathogens, there is chance of
their survival.
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CONTROL BY LOW
TEMPERATURE
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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.
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Mechanisms of microbial control
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Metabolic activities, enzymatic reactions and growth
rates of microorganisms are maximum at optimum
growth temperatures.
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When temperature is lowered, microbial activities
associated with growth slow down.
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Rate of catalytic activity of enzymes decreases with
reduced temperature.
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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.
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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.
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Methods using heat
Ice chilling
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Usually used Retail stores where foods are kept
over ice.
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The surface is in contact with ice, temperature
can reach between 0oC – 1oC.
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Temperature fluctuation, duration of storage and
cross contamination can cause microbiological
problems – food borne pathogens.
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Methods using heat
Refrigeration
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The temperature specification for refrigeration of
foods has changed from time to time.
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From 7oC, technological improvements have
made it economical to have domestic
refrigeration units at 4 to 5 oC.
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For perishable products, ≤ 4.4oC is considered
desirable refrigeration temperature.
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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.
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Methods using heat
Freezing
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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.
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Dry ice ( -78oC) and liquid nitrogen (- 196oC) can
also be used for instant rapid freezing, but not for
food.
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After freezing, the temperature of the food is
maintained around -20oC to -30oC.
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Microbial cells will die upon during frozen
storage, but survivors can multiply in the frozen
state.
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Accidental thawing or slow thawing can facilitate
growth of survivors.
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Enzymes released by dead microbial cells can
reduce the acceptance quality of food.
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CONTROL BY REDUCED
Aw
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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.
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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.
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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.
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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.
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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
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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.
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Depending on Temp & time of exposure, some microbial
cells die during drying, while some other cells can be
sublethally injured.
Freeze-Drying
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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
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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
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Many meat and fish products are exposed to low heat
and smoke for cooking and depositing smoke on the
surface at the same time.
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The heating process removes water from the products
lowering their Aw.
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Low heat processed meat products (dry and semidry
sausages) and smoked fish are produced this way.
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Heat kills many microorganisms.
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Growth of survivors is controlled by low Aw as well as
antimicrobial substances present in the smoke.
Intermediate Moisture Foods (IMF)
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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.
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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.