Control of Microbes
There are two main methods of controlling microbes one is to kill them
and the other is to inactivate (stop them growing) the microbes. Both methods
have their advantages and disadvantages.
Killing microbes can make a product safe in the short term but it is
only remains "safe" as long as it prevented form of re-contamination. i.e.
it must be sealed in some way to prevent new organisms entering and growing
in the product. A disadvantage of killing bacteria is that act if all the
microbes are killed a product can be more dangerous then before it was
processed if not stored correctly. This is because before processing the
products have had many different bacteria present. Then if it was kept
under the wrong conditions this mixture of y bacteria soon show obvious
deteriorate (food spoilage) thus preventing consumers from eating a potentially
dangerous product. In fact the food spoilage organisms will grow faster
and more obviously than the food poisoning bacteria and in some cases spoilage
organisms will actually inhibit the growth of dangerous microbes. Hence
unprocessed food is less likely to cause food borne disease than cooked
The spoilage bacteria act as an indicator of poor handling. So food
spoilage can protect the consumer from disease.
If all the microbes are killed this protection no longer exists. Once
a sterilized product becomes contaminated with a "food poisoning microbe"
it is possible that this microbe can grow to a dangerous level with out
any visible sign. This is why most food poisoning in western communities
come from cooked food not uncooked food.
The option of inactivating microbes can retain the protection provided
by spoilage organisms as well as the controlling the dangerous bacteria
by stopping them growing. It is possible to combine the two methods (killing
and inactivating) by using a minimal processing method (pasteurizing).
In pasteurization the pathogenic bacteria that don't have to grow to become
dangerous are killed then the spoilage bacteria survive to remain an indicator
of poor handling.
The food is however only safe as long as the inactivating action remains
in place. I.e. a died product must remain dry, a salted product must remain
salted and a frozen product must remain frozen. Once the product is reconstituted
the microbes can grow just like they had never been inhibited.
Microbes can be killed by heat, radiation, poisonous gases, chemicals,
In every method of killing there are two
things must be taken account of every time: Time and Intensity
E.g.. The temperature / time combination needed to kill TB bacteria
in milk is 63 C for 30 min. If the temperature is increased by only 10
C the exposure time is reduced by 99.2% i.e. 73C requires only 15
sec. To have the same killing effect as 63 C for 30 min.
The level of killing agent used (e.g. temperature. Concentration of
chemical, intensity of radiation etc.)
Time of exposure.
Generally a small increase in intensity produces a large reduction
in exposure time required to inactivate the proportion of bacteria
In every case the exposure time is inversely related to the Intensity.
Organisms vary in relation to their resistance to killing agents and
the Intensity/Time combination required is can be greater for resistant
The Exposure time required is also Greater for larger populations
than smaller populations. Once a killing process has been chosen the initial
population must be kept below expected initial numbers to make sure not
too many organism survive the treatment
Killing with High Temperature
The exposure time to kill organisms depends on the:
The type of microorganisms(s) that are in the food
Physical and chemical properties of the food.
Killing bacteria using a temperature less than 100 C
Pasteurization of Milk
The temperature / time combination needed to kill TB bacteria in milk
is 63 C for 30 min.
This requires holding large volumes of milk in vats for a long time.
The process is referred to as a batch process and is expensive. The low
temperature has less effect on flavour than pasteurization at higher temperatures
73C requires only 15 sec. To have the same killing effect
as LTLT but the very short time allow the heat treatment to be applied
while the milk passes between heated plates
The process can be run as a continuous method allowing large volumes
to be pasteurized quickly and economically but with a slightly inferior
Pasteurization of Other Foods
The composition of the food may limit temperature used.
Pasteurization temperature of eggs is limited so that the egg does
not get "hard Boiled"
Egg pulp maximum 600 C 3.5 min
Egg yolk maximum 600 C 6.2 min
The temperature needed for pasteurization of fruit juice is higher
than needed to kill pathogenic bacteria because it must also is inactivate
endogenous enzymes that would cause spoilage. Temperature may be up to
Boiling at 1000C
few seconds at 1000C is all that is need
to kill most vegetative cells however boiling for 30 min. may be needed
to kill more thermoduric organisms. Boiling for 5 min. is often recommended
to purify water. However some cysts of Giardia may survive
5 min. Endo spores can survive boiling for many hours. Sterilization is
the only practical way to control spores.
Intermittent Boiling (Tyndallization)
or "Fractional sterilization" This involves boiling the media at
1000C for 30 min. on three successive days.
and leaving the material at 370C during
the intervening times so that surviving spores can germinate. Each boil
kills of the newly germinated vegetative cells.
After three incubations there should be no more ungerminated spores
remaining. The initial cooking period acts as "heat activation" to initiate
sporulation. This was once a popular method for home preserving and is
better than nothing if a pressurized vessel is not available. However not
all spores are obliging enough to sporulate when expected. So this method
is not as reliable as sterilization.
Killing bacteria using a temperature over 1000C
At temperatures over 1000C most bacteria
found in food are killed instantaneously
However a small number produce heat resistant endospores that will withstand
boiling for many hours.
Temperature greater than 100 C must be used to reduce the exposure time
to a practical
The process was developed by Nicolas
Appert and published in 1810
All vegetative organisms that could grow in the food and cause spoilage
under normal handling and storage conditions are destroyed. However commercial
sterile foods may contain a small number of heat resistant bacterial
spores, but they will not multiply under normal handling and storage conditions
The problem with spore is that
they are not all killed by the same amount of heat.
Types of commercially sterile processes include canning,
bottling, and aseptic processing
Commercial sterilization must make sure the numbers of surviving spores
are at an acceptable level
The acceptable number of spores will depend on what type of damage they
are capable of causing if the start to grow. If the damage is in regard
to food spoilage and not a heath risk the acceptable number will depend
on what the company accepts as an acceptable number of consumer complaints.
A decision has to be made on the commercial damage caused by too many consumer
compared to the commercial damage done by heating the product longer and
causing an inferior product in relation to taste and appearance.
There are a few spores that do represent a health risk. The most significant
is from Clostridium botulinum. If botulinum spores germinate the
bacteria can produce a lethal toxin. The number of acceptable spores for
botulinum is 1 in 1,000,000 containers. Food that can support the growth
of Cl. botulinum must be given a 12D cook.
If it is assumed that a container had one million spores per can the heat
treatment needed to reduce the number to one in one million i.e. from 106
to 10-12 involves a reduction of twelve decimal places
i.e. from 1,000,000 to 0.0000001
This is called a 12D cook
A "d" value is the time needed to reduce the population of spores by one
I.e. from 100 to 10 or 100,000 to 10,000
The killing effect of a time / temperature combination is referred to as
the F value
F = 1 is heat killing effect equivalent to 1 min at 121 C
The F value required to achieve a 12D cook depends on the resistance of
the particular type of bacteria. One of the most resistant species is Bacillus
stearothermophilus which is 5 or 6 time more resistant than botulinum.
A 12-D cook for Cl. botulinum may require an F value of 2.52
A 12 D cook for B. stearothermophilus may require F = 18
The lethality of a thermal treatment will also be influenced by the composition
of the food
Moist Heat vs Dry Heat
Moist Heat kills bacteria by coagulating proteins whereas dry heat kills
by oxidation of cell contents.
Moist heat in requires less heat (temperature or time) than dry heat.
121 C for 10 min of moist heat is equivalent to about 30 min at 200 c dry
For this reason a lot of sterilization procedures use super heated steam
that provides moist heat.
Temperature over 100 C requires cooking under elevated pressure, (like
in a pressure cooker) 121 C require 100 kpa extra pressure.
It is important that no air pockets are allowed to develop when a product
being sterilized with steam. In air pockets food is exposed to dry heat
and not the time /temperature is not enough.
Three forms of radiation are common
1 UV light
2 Atomic radiation
How does irradiation work?
UV light of 260 nm wavelength is absorbed by DNA causing lethal mutations.
Microorganisms are killed or inactivated.
However UV light does not penetrate far though food or packaging and has
little use in the food industry
It can be used to control contamination of surfaces.
When food in containers is passed through a shielded area, radiation passes
through the food and breaks molecular bonds in the water producing ions
and free radicals. These Free radicals react with DNA and kill bacteria,
parasites, viruses, fungi, and insects
The food must be exposed to an irradiation source for an amount of time
that depends on the food and the type of radiation.
Atomic radiation includes alpha, beta and gamma rays. However only gamma
rays have a useable penetrating ability and can pass through most food
Irradiation can also delay ripening and prevent sprouting in fresh fruits
It is thought by some that the
food is left virtually unchanged. However it is possible that changes
can occur to the flavour and level of oxidation and may leave residual
radiation. Radiation is not allowed in Australia but it is permitted in
Radiation is a cheap and effective method of treating large volumes of
grain and in areas of the world where food is in short supply it is an